Publications

2021

J. Cerrillo, S. Oviedo Casado, and J. Prior. 6/3/2021. “Low Field Nano-NMR via Three-Level System Control.” PHYSICAL REVIEW LETTERS, 126, 22. Publisher's Version Abstract
Conventional control strategies for nitrogen-vacancy centers in quantum sensing are based on a two-level model of their triplet ground state. However, this approach fails in regimes of weak bias magnetic fields or strong microwave pulses, as we demonstrate. To overcome this limitation, we propose a novel control sequence that exploits all three levels by addressing a hidden Raman configuration with microwave pulses tuned to the zero-field transition. We report excellent performance in typical dynamical decoupling sequences, opening up the possibility for nano-NMR operation in low field environments.
Yaoming Chu, Pengcheng Yang, Musang Gong, Min Yu, Baiyi Yu, Martin B. Plenio, Alex Retzker, and Jianming Cai. 1/19/2021. “Precise Spectroscopy of High-Frequency Oscillating Fields with a Single-Qubit Sensor.” PHYSICAL REVIEW APPLIED, 15, 1. Publisher's Version Abstract
Precise spectroscopy of oscillating fields plays a significant role in many fields. Here, we propose an experimentally feasible scheme to measure the frequency of a fast-oscillating field using a single-qubit sensor. By invoking a stable classical clock, the signal phase correlations between successive measurements enable us to extract the target frequency with extremely high precision. In addition, we integrate dynamical decoupling technique into the framework to suppress the influence of slow environmental noise. Our framework is feasible with a variety of atomic and single solid-state spin systems within the state-of-the-art experimental capabilities as a versatile tool for quantum spectroscopy.
Simon Schmitt, Tuvia Gefen, Daniel Louzon, Christian Osterkamp, Nicolas Staudenmaier, Johannes Lang, Matthew Markham, Alex Retzker, Liam P. McGuinness, and Fedor Jelezko. 4/1/2021. “Optimal frequency measurements with quantum probes.” npj quantum information . Publisher's Version Abstract
Precise frequency measurements are important in applications ranging from navigation and imaging to computation and communication. Here we outline the optimal quantum strategies for frequency discrimination and estimation in the context of quantum spectroscopy, and we compare the effectiveness of different readout strategies. Using a single NV center in diamond, we implement the optimal frequency discrimination protocol to discriminate two frequencies separated by 2 kHz with a single 44 μs measurement, a factor of ten below the Fourier limit. For frequency estimation, we achieve a frequency sensitivity of 1.6 µHz/Hz2 for a 1.7 µT amplitude signal, which is within a factor of 2 from the quantum limit. Our results are foundational for discrimination and estimation problems in nanoscale nuclear magnetic resonance spectroscopy.

2020

Santiago Oviedo-Casado, Amit Rotem, Ramil Nigmatullin, Javier Prior, and Alex Retzker. 11/12/2020. “Correlated noise in Brownian motion allows for super resolution.” Scientific Reports . Publisher's Version Abstract
Diffusion broadening of spectral lines is the main limitation to frequency resolution in non-polarized liquid state nano-NMR. This problem arises from the limited amount of information that can be extracted from the signal before losing coherence. For liquid state NMR as with most generic sensing experiments, the signal is thought to decay exponentially, severely limiting resolution. However, there is theoretical evidence that predicts a power law decay of the signal’s correlations due to diffusion noise in the non-polarized nano-NMR scenario. In this work we show that in the NV based nano-NMR setup such diffusion noise results in high spectral resolution.
Genko T. Genov, Yachel Ben-Shalom, Fedor Jelezko, Alex Retzker, and Nir Bar-Gill. 8/7/2020. “Efficient and robust signal sensing by sequences of adiabatic chirped pulses.” Phys. Rev. Research , 2, 3. Publisher's Version Abstract
We propose a scheme for sensing of an oscillating field in systems with large inhomogeneous broadening and driving field variation by applying sequences of phased, adiabatic, chirped pulses. These act as a double filter for dynamical decoupling, where the adiabatic changes of the mixing angle during the pulses rectify the signal and partially remove frequency noise. The sudden changes between the pulses act as instantaneous π pulses in the adiabatic basis for additional noise suppression. We also use the pulses' phases to correct for other errors, e.g., due to nonadiabatic couplings. Our technique improves significantly the coherence time in comparison to standard XY8 dynamical decoupling in realistic simulations in NV centers with large inhomogeneous broadening. Beyond the theoretical proposal, we also present proof-of-principle experimental results for quantum sensing of an oscillating field in NV centers in diamond, demonstrating superior performance compared to the standard technique.
D. Cohen, R.Nigmatullin, M.Eldar, and A.Retzker. 6/15/2020. “Confined Nano‐NMR Spectroscopy Using NV Centers.” Advanced Quantum Technologies. Publisher's Version Abstract
Nano nuclear magnetic resonance (nano‐NMR) spectroscopy with nitrogen‐vacancy (NV) centers holds the potential to provide high‐resolution spectra of minute samples. This is likely to have important implications for chemistry, medicine, and pharmaceutical engineering. One of the main hurdles facing the technology is that diffusion of unpolarized liquid samples broadens the spectral lines thus limiting resolution. Experiments in the field are therefore impeded by the efforts involved in achieving high polarization of the sample which is a challenging endeavor. Here, a scenario where the liquid is confined to a small volume is examined. It is shown that the confinement “counteracts” the effect of diffusion, thus overcoming a major obstacle to the resolving abilities of the NV‐NMR spectrometer.
D. Cohen, T.Gefen, L.Ortiz, and A.Retzker. 9/18/2020. “Achieving the ultimate precision limit with a weakly interacting quantum probe.” npj Quantum Information , 6, 83. Publisher's Version Abstract
The ultimate precision limit in estimating the Larmor frequency of N unentangled qubits is well established, and is highly important for magnetometers, gyroscopes, and other types of quantum sensors. However, this limit assumes perfect projective measurements of the quantum registers. This requirement is not practical in many physical systems, such as NMR spectroscopy, where a weakly interacting external probe is used as a measurement device. Here, we show that in the framework of quantum nano-NMR spectroscopy, in which these limitations are inherent, the ultimate precision limit is still achievable using control and a finely tuned measurement.
Yotam V, Benedikt T, Tuvia G, Ilai S, Martin P, and Alex R. 9/8/2020. “Robustness of the NV-NMR Spectrometer Setup to Magnetic Field Inhomogeneities.” Physical Review Letters, 125, 110502. Publisher's Version Abstract
The NV-NMR spectrometer is a promising candidate for detection of NMR signals at the nanoscale. Field inhomogeneities, however, are a major source of noise that limits spectral resolution in state of the art NV-NMR experiments and constitutes a major bottleneck in the development of nanoscale NMR. Here we propose, a route in which this limitation could be circumvented in NV-NMR spectrometer experiments, by utilizing the nanometric scale and the quantumness of the detector.
Daniel Cohen, Ramil Nigmatullin, Oded Kenneth, Fedor Jelezko, Maxim Khodas, and Alex Retzker. 2020. “Utilising NV based quantum sensing for velocimetry at the nanoscale.” Scientific Reports, 10, 1. Publisher's Version Abstract
Nitrogen-Vacancy (NV) centers in diamonds have been shown in recent years to be excellent magnetometers on the nanoscale. One of the recent applications of the quantum sensor is retrieving the Nuclear Magnetic Resonance (NMR) spectrum of a minute sample, whose net polarization is well below the Signal-to-Noise Ratio (SNR) of classic devices. The information in the magnetic noise of diffusing particles has also been shown in decoherence spectroscopy approaches to provide a method for measuring different physical parameters. Similar noise is induced on the NV center by a flowing liquid. However, when the noise created by diffusion effects is more dominant than the noise of the drift, it is unclear whether the velocity can be efficiently estimated. Here we propose a non-intrusive setup for measuring the drift velocity near the surface of a flow channel based on magnetic field quantum sensing using NV centers. We provide a detailed analysis of the sensitivity for different measurement protocols, and we show that our nanoscale velocimetry scheme outperforms current fluorescence based approaches even when diffusion noise is dominant. Our scheme can be applied for the investigation of microfluidic channels, where the drift velocity is usually low and the flow properties are currently unclear. A better understanding of these properties is essential for the future development of microfluidic and nanofluidic infrastructures.
QY Cao, PC Yang, MS Gong, M Yu, A Retzker, MB Plenio, C Muller, N Tomek, B Naydenov, LP McGuinness, F Jelezko, and JM Cai. 2020. “Protecting Quantum Spin Coherence of Nanodiamonds in Living Cells.” Physical Review Applied, 13, 2. Publisher's Version Abstract
Because of its superior coherent and optical properties at room temperature, the nitrogen-vacancy (N-V) center in diamond has become a promising quantum probe for nanoscale quantum sensing. However, the application of N-V-containing nanodiamonds to quantum sensing suffers from their relatively short spin coherence times. Here we demonstrate energy-efficient protection of N-V spin coherence in nanodiamonds using concatenated continuous dynamical decoupling, which exhibits excellent performance with a less-stringent microwave-power requirement. When this is applied to nanodiamonds in living cells, we are able to extend the spin coherence time by an order of magnitude to the T1 limit of 30μs. Further analysis demonstrates concomitant improvements of sensing performance, which shows that our results provide an important step toward in vivo quantum sensing using N-V centers in nanodiamond.

2019

Amit Rotem, Tuvia Gefen, Santiago Oviedo-Casado, Javier Prior, Simon Schmitt, Yoram Burak, Liam McGuiness, Fedor Jelezko, and Alex Retzker. 2019. “Limits on Spectral Resolution Measurements by Quantum Probes.” Physical Review Letters, 122, 6. Publisher's Version Abstract
The limits of frequency resolution in nano-NMR experiments have been discussed extensively in recent years. It is believed that there is a crucial difference between the ability to resolve a few frequencies and the precision of estimating a single one. Whereas the efficiency of single frequency estimation gradually increases with the square root of the number of measurements, the ability to resolve two frequencies is limited by the specific timescale of the signal and cannot be compensated for by extra measurements. Here we show theoretically and demonstrate experimentally that the relationship between these quantities is more subtle and both are only limited by the Cramér-Rao bound of a single frequency estimation.
Genko T Genov, Nati Aharon, Fedor Jelezko, and Alex Retzker. 2019. “Mixed dynamical decoupling.” Quantum Science and Technology, 4, 3. Publisher's Version Abstract
We propose a scheme for mixed dynamical decoupling (MDD), where we combine continuous dynamical decoupling with robust sequences of phased pulses. Specifically, we use two fields for decoupling, where the first continuous driving field creates dressed states that are robust to environmental noise. Then, a second field implements a robust sequence of phased pulses to perform inversions of the dressed qubits, thus achieving robustness to amplitude fluctuations of both fields. We show that MDD outperforms standard concatenated continuous dynamical decoupling in realistic numerical simulations for dynamical decoupling in NV centers in diamond. Finally, we also demonstrate how our technique can be utilized for improved sensing.
T Gefen, A Rotem, and A Retzker. 11/1/2019. “Overcoming resolution limits with quantum sensing.” Nature Communications, 10. Publisher's Version Abstract
The field of quantum sensing explores the use of quantum phenomena to measure a broad range of physical quantities, of both static and time-dependent types. While for static signals the main figure of merit is sensitivity, for time dependent signals it is spectral resolution, i.e. the ability to resolve two different frequencies. Here we study this problem, and develop new superresolution methods that rely on quantum features. We first formulate a general criterion for superresolution in quantum problems. Inspired by this, we show that quantum detectors can resolve two frequencies from incoherent segments of the signal, irrespective of their separation, in contrast to what is known about classical detection schemes. The main idea behind these methods is to overcome the vanishing distinguishability in resolution problems by nullifying the projection noise.
Daniel Cohen, Maxim Khodas, Oded Keneth, and Alex Retzker. 3/1/2019. “Sensing nano-NMR diffusion spectra using a nitrogen vacancy center.” Optical, Opto-Atomic, and Entanglement-Enhanced Precision Metrology, 10934. Publisher's Version Abstract
Nano nuclear magnetic resonance (nano‐NMR) spectroscopy with nitrogen‐vacancy (NV) centers holds the potential to provide high‐resolution spectra of minute samples. This is likely to have important implications for chemistry, medicine, and pharmaceutical engineering. One of the main hurdles facing the technology is that diffusion of unpolarized liquid samples broadens the spectral lines thus limiting resolution. Experiments in the field are therefore impeded by the efforts involved in achieving high polarization of the sample which is a challenging endeavor. Here, a scenario where the liquid is confined to a small volume is examined. It is shown that the confinement “counteracts” the effect of diffusion, thus overcoming a major obstacle to the resolving abilities of the NV‐NMR spectrometer.
Nati Aharon, Ilai Schwartz, and Alex Retzker. 3/29/2019. “Quantum Control and Sensing of Nuclear Spins by Electron Spins under Power Limitations.” Physical Review Letters, 122, 12. Publisher's Version Abstract
State of the art quantum sensing experiments targeting frequency measurements or frequency addressing of nuclear spins require one to drive the probe system at the targeted frequency. In addition, there is a substantial advantage to performing these experiments in the regime of high magnetic fields, in which the Larmor frequency of the measured spins is large. In this scenario we are confronted with a natural challenge of controlling a target system with a very high frequency when the probe system cannot be set to resonance with the target frequency. In this contribution we present a set of protocols that are capable of confronting this challenge, even at large frequency mismatches between the probe system and the target system, both for polarization and for quantum sensing.
Nati Aharon, Amit Rotem, Liam P. McGuinness, Fedor Jelezko, Alex Retzker, and Zohar Ringel. 11/28/2019. “NV center based nano-NMR enhanced by deep learning.” Scientific Reports, 9. Publisher's Version Abstract
The growing field of nano nuclear magnetic resonance (nano-NMR) seeks to estimate spectra or discriminate between spectra of minuscule amounts of complex molecules. While this field holds great promise, nano-NMR experiments suffer from detrimental inherent noise. This strong noise masks to the weak signal and results in a very low signal-to-noise ratio. Moreover, the noise model is usually complex and unknown, which renders the data processing of the measurement results very complicated. Hence, spectra discrimination is hard to achieve and in particular, it is difficult to reach the optimal discrimination. In this work we present strong indications that this difficulty can be overcome by deep learning (DL) algorithms. The DL algorithms can mitigate the adversarial effects of the noise efficiently by effectively learning the noise model. We show that in the case of frequency discrimination DL algorithms reach the optimal discrimination without having any pre-knowledge of the physical model. Moreover, the DL discrimination scheme outperform Bayesian methods when verified on noisy experimental data obtained by a single Nitrogen-Vacancy (NV) center. In the case of frequency resolution we show that this approach outperforms Bayesian methods even when the latter have full pre-knowledge of the noise model and the former has none. These DL algorithms also emerge as much more efficient in terms of computational resources and run times. Since in many real-world scenarios the noise is complex and difficult to model, we argue that DL is likely to become a dominant tool in the field.
Nati Aharon, Nicolas Spethmann, Ian D Leroux, Piet O Schmidt, and Alex Retzker. 8/2019. “Robust optical clock transitions in trapped ions using dynamical decoupling.” New Journal of Physics, 21. Publisher's Version Abstract
We present a novel method for engineering an optical clock transition that is robust against external field fluctuations and is able to overcome limits resulting from field inhomogeneities. The technique is based on the application of continuous driving fields to form a pair of dressed states essentially free of all relevant shifts. Specifically, the clock transition is robust to magnetic field shifts, quadrupole and other tensor shifts, and amplitude fluctuations of the driving fields. The scheme is applicable to either a single ion or an ensemble of ions, and is relevant for several types of ions, such as ${}^{40}{\mathrm{Ca}}^{+}$ , ${}^{88}{\mathrm{Sr}}^{+}$ , ${}^{138}{\mathrm{Ba}}^{+}$ and ${}^{176}{\mathrm{Lu}}^{+}$ . Taking a spherically symmetric Coulomb crystal formed by 400 ${}^{40}{\mathrm{Ca}}^{+}$ ions as an example, we show through numerical simulations that the inhomogeneous linewidth of tens of Hertz in such a crystal together with linear Zeeman shifts of order 10 MHz are reduced to form a linewidth of around 1 Hz. We estimate a two-order-of-magnitude reduction in averaging time compared to state-of-the art single ion frequency references, assuming a probe laser fractional instability of ${10}^{-15}$. Furthermore, a statistical uncertainty reaching 2.9 × 10−16 in 1 s is estimated for a cascaded clock scheme in which the dynamically decoupled Coulomb crystal clock stabilizes the interrogation laser for an ${}^{27}{\mathrm{Al}}^{+}$ clock.

2018

Alexander Stark, Nati Aharon, Alexander Huck, Haitham A. R El-Ella, Alex Retzker, Fedor Jelezko, and Ulrik L. Andersen. 10/4/2018. “Clock transition by continuous dynamical decoupling of a three-level system.” Scientific Reports, 8. Publisher's Version Abstract
We present a novel continuous dynamical decoupling scheme for the construction of a robust qubit in a three-level system. By means of a clock transition adjustment, we first show how robustness to environmental noise is achieved, while eliminating drive-noise, to first-order. We demonstrate this scheme with the spin sub-levels of the NV-centre’s electronic ground state. By applying drive fields with moderate Rabi frequencies, the drive noise is eliminated and an improvement of 2 orders of magnitude in the coherence time is obtained compared to the pure dephasing time. We then show how the clock transition adjustment can be tuned to eliminate also the second-order effect of the environmental noise with moderate drive fields. A further detailed theoretical investigation suggests an additional improvement of more than 1 order of magnitude in the coherence time which is supported by simulations. Hence, our scheme predicts that the coherence time may be prolonged towards the lifetime-limit using a relatively simple experimental setup.
Nils Scharnhorst, Javier Cerrillo, Johannes Kramer, Ian D. Leroux, Jannes B. Wübbena, Alex Retzker, and Piet O. Schmidt. 8/27/2018. “Experimental and theoretical investigation of a multimode cooling scheme using multiple electromagnetically-induced-transparency resonances.” Physical Review a, 98, 2. Publisher's Version Abstract
We introduce and demonstrate double-bright electromagnetically-induced-transparency (D-EIT) cooling as an extension to EIT cooling. By involving an additional ground state, two bright states can be shifted individually into resonance for cooling of motional modes of frequencies that may be separated by more than the width of a single EIT cooling resonance. This allows three-dimensional ground-state cooling of a 40Ca+ ion trapped in a linear Paul trap with a single cooling pulse. Measured cooling rates and steady-state mean motional quantum numbers for this D-EIT cooling are compared with those of standard EIT cooling as well as concatenated standard EIT cooling pulses for multimode cooling. Experimental results are compared to full-density matrix calculations. We observe a failure of the theoretical description within the Lamb-Dicke regime that can be overcome by a time-dependent rate theory. Limitations of the different cooling techniques and possible extensions to multi-ion crystals are discussed.
P Fernandez-Acebal, O Rosolio, J Scheuer, C Muller, S Muller, S Schmitt, LP McGuinness, I Schwarz, Q Chen, A Retzker, B Naydenov, F Jelezko, and MB Plenio. 2/22/2018. “Toward Hyperpolarization of Oil Molecules via Single Nitrogen Vacancy Centers in Diamond.” Nano Letters, 18, 3, Pp. 1882-1887. Publisher's Version Abstract
Efficient polarization of organic molecules is of extraordinary relevance when performing nuclear magnetic resonance (NMR) and imaging. Commercially available routes to dynamical nuclear polarization (DNP) work at extremely low temperatures, relying on the solidification of organic samples and thus bringing the molecules out of their ambient thermal conditions. In this work, we investigate polarization transfer from optically pumped nitrogen vacancy centers in diamond to external molecules at room temperature. This polarization transfer is described by both an extensive analytical analysis and numerical simulations based on spin bath bosonization and is supported by experimental data in excellent agreement. These results set the route to hyperpolarization of diffusive molecules in different scenarios and consequently, due to an increased signal, to high-resolution NMR.
Tuvia Gefen, Maxim Khodas, Liam P. McGuinness, Fedor Jelezko, and Alex Retzker. 7/27/2018. “Quantum spectroscopy of single spins assisted by a classical clock.” Physical Review a, 98, 1. Publisher's Version Abstract
Quantum spectroscopy with single two-level systems has considerably improved our ability to detect weak signals. Recently it was realized that for classical signals, precision and resolution of quantum spectroscopy is limited mainly by coherence of the signal and the stability of the clock used to measure time. The coherence time of the quantum probe, which can be significantly shorter, is not a major limiting factor in resolution measurements. Here, we address a similar question for spectroscopy of quantum signals, for example, a quantum sensor is used to detect a single nuclear spin. We present and analyze a novel correlation spectroscopy technique with performance that is limited by the coherence time of the target spins and the stability of the clock.
J Cerrillo, A Retzker, and MB Plenio. 7/30/2018. “Double-path dark-state laser cooling in a three-level system.” Physical Review a, 98, 1. Publisher's Version Abstract
We present a detailed analysis of a robust and fast laser cooling scheme [J. Cerrillo et al., Phys. Rev. Lett. 104, 043003 (2010)] on a three-level system. A special laser configuration, applicable to trapped ions, atoms, or cantilevers, designs a double-path quantum interference that eliminates the blue sideband in addition to the carrier transition, thus excluding any heating process involving up to one-phonon interactions. As a consequence, cooling achieves vanishing phonon occupation up to first order in the Lamb-Dicke parameter expansion. Underlying this scheme is a combined action of two cooling schemes which makes the proposal very flexible under constraints of the physical parameters such as laser intensity, detuning, or optical access, making it a viable candidate for experimental implementation. Furthermore, it is considerably faster than existing ground state cooling schemes. Its suitability as a cooling scheme for several ions in a trap and three-dimensional cooling is shown.

2017

Alexander Stark, Nati Aharon, Thomas Unden, Daniel Louzon, Alexander Huck, Alex Retzker, Ulrik L. Andersen, and Fedor Jelezko. 2017. “Narrow-bandwidth sensing of high-frequency fields with continuous dynamical decoupling.” Nature Communications, 8. Publisher's Version Abstract
State-of-the-art methods for sensing weak AC fields are only efficient in the low frequency domain (<10 MHz). The inefficiency of sensing high-frequency signals is due to the lack of ability to use dynamical decoupling. In this paper we show that dynamical decoupling can be incorporated into high-frequency sensing schemes and by this we demonstrate that the high sensitivity achieved for low frequency can be extended to the whole spectrum. While our scheme is general and suitable to a variety of atomic and solid-state systems, we experimentally demonstrate it with the nitrogen-vacancy center in diamond. For a diamond with natural abundance of 13C, we achieve coherence times up to 1.43 ms resulting in a smallest detectable magnetic field strength of 4 nT at 1.6 GHz. Attributed to the inherent nature of our scheme, we observe an additional increase in coherence time due to the signal itself.
Simon Schmitt, Tuvia Gefen, Felix M. Stürner, Thomas Unden, Gerhard Wolff, Christoph Müller, Jochen Scheuer, Boris Naydenov, Matthew Markham, Sebastien Pezzagna, Jan Meijer, Ilai Schwarz, Martin Plenio, Alex Retzker, Liam P. McGuinness, and Fedor Jelezko. 2017. “Submillihertz magnetic spectroscopy performed with a nanoscale quantum sensor.” Science, 356, 6340, Pp. 832-836. Publisher's Version Abstract
Precise timekeeping is critical to metrology, forming the basis by which standards of time, length, and fundamental constants are determined. Stable clocks are particularly valuable in spectroscopy because they define the ultimate frequency precision that can be reached. In quantum metrology, the qubit coherence time defines the clock stability, from which the spectral linewidth and frequency precision are determined. We demonstrate a quantum sensing protocol in which the spectral precision goes beyond the sensor coherence time and is limited by the stability of a classical clock. Using this technique, we observed a precision in frequency estimation scaling in time T as T–3/2 for classical oscillating fields. The narrow linewidth magnetometer based on single spins in diamond is used to sense nanoscale magnetic fields with an intrinsic frequency resolution of 607 microhertz, which is eight orders of magnitude narrower than the qubit coherence time.
Y Pilnyak, N Aharon, D Istrati, E Megidish, A Retzker, and HS Eisenberg. 2017. “Simple source for large linear cluster photonic states.” Physical Review a, 95, 2. Publisher's Version Abstract
The experimental realization of many-body entangled states is one of the main goals of quantum technology as these states are a key resource for quantum computation and quantum sensing. However, increasing the number of photons in an entangled state has been proved to be a painstakingly hard task. This is a result of the nondeterministic emission of current photon sources and the distinguishability between photons from different sources. Moreover, the generation rate and the complexity of the optical setups hinder scalability. Here we present a scheme that is compact, requires a very modest number of components, and avoids the distinguishability issues by using only one single-photon source. States of any number of photons are generated with the same configuration, with no need for increasing the optical setup. The basic operation of this scheme is experimentally demonstrated, and its sensitivity to imperfections is considered.
T Manovitz, A Rotem, R Shaniv, I Cohen, Y Shapira, N Akerman, A Retzker, and R Ozeri. 11/29/2017. “Fast Dynamical Decoupling of the Molmer-Sorensen Entangling Gate.” Physical Review Letters, 119, 22. Publisher's Version Abstract
Engineering entanglement between quantum systems often involves coupling through a bosonic mediator, which should be disentangled from the systems at the operation’s end. The quality of such an operation is generally limited by environmental and control noise. One of the prime techniques for suppressing noise is by dynamical decoupling, where one actively applies pulses at a rate that is faster than the typical time scale of the noise. However, for boson-mediated gates, current dynamical decoupling schemes require executing the pulses only when the boson and the quantum systems are disentangled. This restriction implies an increase of the gate time by a factor of √N, with N being the number of pulses applied. Here we propose and realize a method that enables dynamical decoupling in a boson-mediated system where the pulses can be applied while spin-boson entanglement persists, resulting in an increase in time that is at most a factor of π/2, independently of the number of pulses applied. We experimentally demonstrate the robustness of our entangling gate with fast dynamical decoupling to σz noise using ions in a Paul trap.
T Gefen, F Jelezko, and A Retzker. 9/8/2017. “Control methods for improved Fisher information with quantum sensing.” Physical Review a, 96, 3. Publisher's Version Abstract
Recently new approaches for sensing the frequency of time dependent Hamiltonians have been presented, and it was shown that the optimal Fisher information scales as T4. We present here our interpretation of this new scaling, where the relative phase is accumulated quadratically with time, and show that this can be produced by a variety of simple pulse sequences. Interestingly, this scaling has a limited duration, and we show that certain pulse sequences prolong the effect. The performance of these schemes is analyzed and we examine their relevance to state-of-the-art experiments. We analyze the T3 scaling of the Fisher information which appears when multiple synchronized measurements are performed, and is the optimal scaling in the case of a finite coherence time.
T Gefen, D Cohen, I Cohen, and A Retzker. 3/9/2017. “Enhancing the fidelity of two-qubit gates by measurements.” Physical Review a, 95, 3. Publisher's Version Abstract
Dynamical decoupling techniques are the method of choice for increasing gate fidelities. While these methods have produced very impressive results in terms of decreasing local noise and increasing the fidelities of single-qubit operations, dealing with the noise of two-qubit gates has proven more challenging. The main obstacle is that the noise time scale is shorter than the two-qubit gate itself, so that refocusing methods do not work. We present a measurement- and feedback-based method to suppress two-qubit-gate noise, which cannot be suppressed by conventional methods. We analyze in detail this method for an error model, which is relevant for trapped-ion quantum information.
D Farfurnik, N Aharon, I Cohen, Y Hovav, A Retzker, and N Bar-Gill. 7/25/2017. “Experimental realization of time-dependent phase-modulated continuous dynamical decoupling.” Physical Review a, 96, 1. Publisher's Version Abstract
The coherence times achieved with continuous dynamical decoupling techniques are often limited by fluctuations in the driving amplitude. In this work, we use time-dependent phase-modulated continuous driving to increase the robustness against such fluctuations in a dense ensemble of nitrogen-vacancy centers in diamond. Considering realistic experimental errors in the system, we identify the optimal modulation strength and demonstrate an improvement of an order of magnitude in the spin preservation of arbitrary states over conventional single continuous driving. The phase-modulated driving exhibits results comparable to those found with previously examined amplitude-modulated techniques and is expected to outperform them in experimental systems having higher phase accuracy. The proposed technique could open new avenues for quantum information processing and many-body physics in systems dominated by high-frequency spin-bath noise, for which pulsed dynamical decoupling is less effective.
I Cohen, N Aharon, and A Retzker. 11/15/2016. “Continuous dynamical decoupling utilizing time-dependent detuning.” Fortschritte Der Physik-Progress of Physics, 65, 6-8. Publisher's Version Abstract
Resilience to noise and to decoherence processes is an important ingredient for the implementation of quantum information processing, and quantum technologies. To this end, techniques such as pulsed and continuous dynamical decoupling have been proposed to reduce noise effects. In this paper, we suggest a new approach to implementing continuous dynamical decoupling techniques, that uses an extra control parameter; namely, the ability to shape the time dependence of the detuning. This approach reduces the complexity of the experimental setup, such that we are only left with noise originating from the frequency of the driving field, which is much more robust than the amplitude (Rabi frequency) noise. As an example, we show that our technique can be utilized for improved sensing.
N Bar-Gill and A Retzker. 7/7/2017. “Observing chemical shifts from nanosamples.” Science, 357, 6346, Pp. 38-38. Publisher's Version Abstract
Nuclear magnetic resonance (NMR) is a highly versatile spectroscopy method widely used in diverse disciplines, but its sensitivity and spatial resolution are limited by the inductive measurement of magnetic nuclei. Nano-NMR methods are emerging that aim to measure a single nuclear spin—an improvement in sensitivity of 13 orders of magnitude. The main workhorse of these methods has been atomic defects in diamond, which have distinctive optical and magnetic properties. On page 67 of this issue, Aslam et al. (1) demonstrate a modified sensing scheme based on diamond defects that achieves spectral resolutions sufficient for measuring chemical shifts.
JP Chou, A Retzker, and A Gali. 3/24/2017. “Nitrogen-Terminated Diamond (111) Surface for Room-Temperature Quantum Sensing and Simulation.” Nano Letters, 17, 4, Pp. 2294-2298. Publisher's Version Abstract
The nitrogen-vacancy (NV) center in diamond has shown great promise of nanoscale sensing applications, however, near-surface NV suffer from relatively short spin coherence time that limits its sensitivity. This is presumably caused by improper surface termination. Using first-principles calculations, we propose that nitrogen-terminated (111) diamond provides electrical inactivity and surface spin noise free properties. We anticipate that the nitrogen-terminated (111) surface can be fabricated by nitrogen plasma treatment. Our findings pave the way toward an improved NV-based quantum sensing and quantum simulation operating at room temperature.
I Baumgart, JM Cai, SS Ivanov, C Piltz, MB Plenio, A Retzker, T Sriarunothai, S Wolk, and C Wunderlich. 4/5/2017. “Coherent Quantum Fourier Transform Using 3-Qubit Conditional Gates and Ultrasensitive Magnetometry with RF-Driven Trapped Ions.” 2017 Conference on Lasers and Electro-Optics (Cleo). Publisher's Version Abstract
Using long-range magnetic gradient induced coupling between three effective spins, a coherent QFT is efficiently realized with trapped Yb+ ions. With a single Yb+ ion, RF magnetic fields are measured close to the quantum limit.
N Aharon, M Drewsen, and A Retzker. 7/7/2017. “Enhanced quantum sensing with multi-level structures of trapped ions.” Quantum Science and Technology, 2, 3. Publisher's Version Abstract
We present a method of enhanced sensing of AC magnetic fields. The method is based on the construction of a robust qubit by the application of continuous driving fields. Specifically, magnetic noise and power fluctuations of the driving fields do not operate within the robust qubit subspace, hence robustness to both external and controller noise is achieved. The scheme is applicable to either a single ion or an ensemble of ions. We consider trapped-ion based implementation via the dipole transitions, which is relevant for several types of ions, such as the ${}^{40}{\mathrm{Ca}}^{+}$${}^{88}{\mathrm{Sr}}^{+}$ and the ${}^{138}{\mathrm{Ba}}^{+}$ ions. Taking experimental errors into account, we conclude that the coherence time of the robust qubit can be improved by up to ~4 orders of magnitude compared to the coherence time of the bare states. We show how the robust qubit can be utilised for the task of sensing AC magnetic fields in the range $\sim 0.1\,-\,100\,\mathrm{MHz}$ with an improvement of ~2 orders of magnitude of the sensitivity. In addition, we present a microwave-based sensing scheme that is suitable for ions with a hyperfine structure, such as the ${}^{9}{\mathrm{Be}}^{+}$,${}^{25}{\mathrm{Mg}}^{+}$,${}^{43}{\mathrm{Ca}}^{+}$,${}^{87}{\mathrm{Sr}}^{+}$,${}^{137}{\mathrm{Ba}}^{+}$,${}^{111}{\mathrm{Cd}}^{+}$,${}^{171}{\mathrm{Yb}}^{+}$ and the ${}^{199}{\mathrm{Hg}}^{+}$ ions. This scheme enables the enhanced sensing of high-frequency fields at the GHz level.

2016

S Weidt, J Randall, SC Webster, K Lake, AE Webb, I Cohen, T Navickas, B Lekitsch, A Retzker, and WK Hensinger. 11/23/2016. “Trapped-Ion Quantum Logic with Global Radiation Fields.” Physical Review Letters, 117, 22. Publisher's Version Abstract
Trapped ions are a promising tool for building a large-scale quantum computer. However, the number of required radiation fields for the realization of quantum gates in any proposed ion-based architecture scales with the number of ions within the quantum computer, posing a major obstacle when imagining a device with millions of ions. Here, we present a fundamentally different approach for trapped-ion quantum computing where this detrimental scaling vanishes. The method is based on individually controlled voltages applied to each logic gate location to facilitate the actual gate operation analogous to a traditional transistor architecture within a classical computer processor. To demonstrate the key principle of this approach we implement a versatile quantum gate method based on long-wavelength radiation and use this method to generate a maximally entangled state of two quantum engineered clock qubits with fidelity 0.985(12). This quantum gate also constitutes a simple-to-implement tool for quantum metrology, sensing, and simulation.
S Weidt, J Randall, SC Webster, K Lake, AE Webb, I Cohen, T Navickas, B Lekitsch, A Retzker, and WK Hensinger. 6/5/2016. “Entangling gate with trapped ions using long.” 2016 Conference on Lasers and Electro-Optics (Cleo). Publisher's Version Abstract
The use of long-wavelength radiation for gate operations is a promising approach for trapped-ion quantum computation. We demonstrate the key principle of this approach by generating a maximally entangled two-qubit Bell-state with fidelity of 0.985.
T Unden, P Balasubramanian, D Louzon, Y Vinkler, MB Plenio, M Markham, D Twitchen, A Stacey, I Lovchinsky, AO Sushkov, MD Lukin, A Retzker, B Naydenov, LP McGuinness, and F Jelezko. 6/9/2016. “Quantum Metrology Enhanced by Repetitive Quantum Error Correction.” Physical Review Letters, 116, 23. Publisher's Version Abstract
We experimentally demonstrate the protection of a room-temperature hybrid spin register against environmental decoherence by performing repeated quantum error correction whilst maintaining sensitivity to signal fields. We use a long-lived nuclear spin to correct multiple phase errors on a sensitive electron spin in diamond and realize magnetic field sensing beyond the time scales set by natural decoherence. The universal extension of sensing time, robust to noise at any frequency, demonstrates the definitive advantage entangled multiqubit systems provide for quantum sensing and offers an important complement to quantum control techniques.
J Scheuer, I Schwartz, Q Chen, D Schulze-Sunninghausen, P Carl, P Hofer, A Retzker, H Sumiya, J Isoya, B Luy, MB Plenio, B Naydenov, and F Jelezko. 1/18/2016. “Optically induced dynamic nuclear spin polarisation in diamond.” New Journal of Physics, 18. Publisher's Version Abstract
The sensitivity of magnetic resonance imaging (MRI) depends strongly on nuclear spin polarisation and, motivated by this observation, dynamical nuclear spin polarisation has recently been applied to enhance MRI protocols (Kurhanewicz et al 2011 Neoplasia 13 81). Nuclear spins associated with the 13C carbon isotope (nuclear spin I = 1/2) in diamond possess uniquely long spin lattice relaxation times (Reynhardt and High 2011 Prog. Nucl. Magn. Reson. Spectrosc. 38 37). If they are present in diamond nanocrystals, especially when strongly polarised, they form a promising contrast agent for MRI. Current schemes for achieving nuclear polarisation, however, require cryogenic temperatures. Here we demonstrate an efficient scheme that realises optically induced 13C nuclear spin hyperpolarisation in diamond at room temperature and low ambient magnetic field. Optical pumping of a nitrogen-vacancy centre creates a continuously renewable electron spin polarisation which can be transferred to surrounding 13C nuclear spins. Importantly for future applications we also realise polarisation protocols that are robust against an unknown misalignment between magnetic field and crystal axis.
R Nigmatullin, A del Campo, G De Chiara, G Morigi, MB Plenio, and A Retzker. 1/25/2016. “Formation of helical ion chains.” Physical Review B, 93, 1. Publisher's Version Abstract
We study the nonequilibrium dynamics of the linear-to-zigzag structural phase transition exhibited by an ion chain confined in a trap with periodic boundary conditions. The transition is driven by reducing the transverse confinement at a finite quench rate, which can be accurately controlled. This results in the formation of zigzag domains oriented along different transverse planes. The twists between different domains can be stabilized by the topology of the trap, and under laser cooling the system has a chance to relax to a helical chain with nonzero winding number. Molecular dynamics simulations are used to obtain a large sample of possible trajectories for different quench rates. The scaling of the average winding number with different quench rates is compared to the prediction of the Kibble-Zurek theory, and a good quantitative agreement is found.
T Gefen, DA Herrera-Marti, and A Retzker. 3/28/2016. “Parameter estimation with efficient photodetectors.” Physical Review a, 93, 3. Publisher's Version Abstract
Current parameter estimation techniques rely on photodetectors which have a low efficiency and thus are based on gathering averaged statistics. Recently it was claimed that perfect photodetction will change the nature of sensing algorithms and will increase the sensing efficiency beyond the immediate effect of a higher collection efficiency. In this paper we bring up the observation that perfect photodetection implies Heisenberg scaling (1T) for parameter estimations. We analyze a specific example in detail.
L Cohen, Y Pilnyak, D Istrati, A Retzker, and HS Eisenberg. 7/15/2016. “Demonstration of a quantum error correction for enhanced sensitivity of photonic measurements.” Physical Review a, 94, 1. Publisher's Version Abstract
The sensitivity of classical and quantum sensing is impaired in a noisy environment. Thus, one of the main challenges facing sensing protocols is to reduce the noise while preserving the signal. State-of-the-art quantum sensing protocols that rely on dynamical decoupling achieve this goal under the restriction of long noise correlation times. We implement a proof-of-principle experiment of a protocol to recover sensitivity by using an error correction for photonic systems that does not have this restriction. The protocol uses a protected entangled qubit to correct a single error. Our results show a recovery of about 87% of the sensitivity, independent of the noise probability.
I Cohen, A Rotem, and A Retzker. 3/28/2016. “Refocusing two-qubit-gate noise for trapped ions by composite pulses.” Physical Review a, 93, 3. Publisher's Version Abstract
Amplitude noise, which inflicts a random two-qubit term, is one of the main obstacles preventing the implementation of a high-fidelity two-body gate below the fault-tolerance threshold. This noise is difficult to refocus as any refocusing technique could only tackle noise with frequency below the operation rate. Since the two-qubit-gate speed is normally the slowest rate in the system, it constitutes the last bottleneck toward an implementation of a gate below the fault-tolerant threshold. Here we propose to use composite pulses as a dynamical decoupling approach in order to reduce two-qubit-gate noise for trapped-ion systems. This is done by refocusing the building blocks of ultrafast entangling gates, where the amplitude noise is reduced to shot-to-shot noise. We present detailed simulations showing that the fault-tolerance threshold could be achieved using the proposed approach.
Q Chen, I Schwarz, F Jelezko, A Retzker, and MB Plenio. 2/24/2016. “Resonance-inclined optical nuclear spin polarization of liquids in diamond structures.” Physical Review B, 93, 6. Publisher's Version Abstract
Dynamic nuclear polarization (DNP) of molecules in a solution at room temperature has the potential to revolutionize nuclear magnetic resonance spectroscopy and imaging. The prevalent methods for achieving DNP in solutions are typically most effective in the regime of small interaction correlation times between the electron and nuclear spins, limiting the size of accessible molecules. To solve this limitation, we design a mechanism for DNP in the liquid phase that is applicable for large interaction correlation times. Importantly, while this mechanism makes use of a resonance condition similar to solid-state DNP, the polarization transfer is robust to a relatively large detuning from the resonance due to molecular motion. We combine this scheme with optically polarized nitrogen-vacancy (NV) center spins in nanodiamonds to design a setup that employs optical pumping and is therefore not limited by room temperature electron thermal polarization. We illustrate numerically the effectiveness of the model in a flow cell containing nanodiamonds immobilized in a hydrogel, polarizing flowing water molecules 4700-fold above thermal polarization in a magnetic field of 0.35 T, in volumes detectable by current NMR scanners
N Aharon, I Cohen, F Jelezko, and A Retzker. 12/12/2016. “Fully robust qubit in atomic and molecular three-level systems.” New Journal of Physics, 18. Publisher's Version Abstract
We present a new method of constructing a fully robust qubit in a three-level system. By the application of continuous driving fields, robustness to both external and controller noise is achieved. Specifically, magnetic noise and power fluctuations do not operate within the robust qubit subspace. Whereas all the continuous driving based constructions of such a fully robust qubit considered so far have required at least four levels, we show that in fact only three levels are necessary. This paves the way for simple constructions of a fully robust qubit in many atomic and solid state systems that are controlled by either microwave or optical fields. We focus on the NV-center in diamond and analyze the implementation of the scheme, by utilizing the electronic spin sub-levels of its ground state. In current state-of-the-art experimental setups the scheme leads to improvement of more than two orders of magnitude in coherence time, pushing it towards the lifetime limit. We show how the fully robust qubit can be used to implement quantum sensing, and in particular, the sensing of high frequency signals.

2015

C Senko, P Richerme, J Smith, A Lee, I Cohen, A Retzker, and C Monroe. 6/17/2015. “Realization of a Quantum Integer-Spin Chain with Controllable Interactions.” Physical Review X, 5, 2. Publisher's Version Abstract
The physics of interacting integer-spin chains has been a topic of intense theoretical interest, particularly in the context of symmetry-protected topological phases. However, there has not been a controllable model system to study this physics experimentally. We demonstrate how spin-dependent forces on trapped ions can be used to engineer an effective system of interacting spin-1 particles. Our system evolves coherently under an applied spin-1 XY Hamiltonian with tunable, long-range couplings, and all three quantum levels at each site participate in the dynamics. We observe the time evolution of the system and verify its coherence by entangling a pair of effective three-level particles (“qutrits”) with 86% fidelity. By adiabatically ramping a global field, we produce ground states of the XY model, and we demonstrate an instance where the ground state cannot be created without breaking the same symmetries that protect the topological Haldane phase. This experimental platform enables future studies of symmetry-protected order in spin-1 systems and their use in quantum applications.
HL Partner, R Nigmatullin, T Burgermeister, J Keller, K Pyka, MB Plenio, A Retzker, WH Zurek, A del Campog, and TE Mehlstaubler. 3/1/2015. “Structural phase transitions and topological defects in ion Coulomb crystals.” Physica B-Condensed Matter, 460, Pp. 114-118. Publisher's Version Abstract
We use laser-cooled ion Coulomb crystals in the well-controlled environment of a harmonic radiofrequency ion trap to investigate phase transitions and defect formation. Topological defects in ion Coulomb crystals (kinks) have been recently proposed for studies of nonlinear physics with solitons and as carriers of quantum information. Defects form when a symmetry breaking phase transition is crossed nonadiabatically. For a second order phase transition, the Kibble–Zurek mechanism predicts that the formation of these defects follows a power law scaling in the rate of the transition. We demonstrate a scaling of defect density and describe kink dynamics and stability. We further discuss the implementation of mass defects and electric fields as first steps toward controlled kink preparation and manipulation.
G Mikelsons, I Cohen, A Retzker, and MB Plenio. 5/22/2015. “Universal set of gates for microwave dressed-state quantum computing.” New Journal of Physics, 17. Publisher's Version Abstract
We propose a set of techniques that enable universal quantum computing to be carried out using dressed states. This applies in particular to the effort of realizing quantum computation in trapped ions using long-wavelength radiation, where coupling enhancement is achieved by means of static magnetic-field gradient. We show how the presence of dressing fields enables the construction of robust single and multi-qubit gates despite the unavoidable presence of magnetic noise, an approach that can be generalized to provide shielding in any analogous quantum system that relies on the coupling of electronic degrees of freedom via bosonic modes.
DA Herrera-Marti, T Gefen, D Aharonov, N Katz, and A Retzker. 11/9/2015. “Quantum Error-Correction-Enhanced Magnetometer Overcoming the Limit Imposed by Relaxation.” Physical Review Letters, 115, 20. Publisher's Version Abstract
When incorporated in quantum sensing protocols, quantum error correction can be used to correct for high frequency noise, as the correction procedure does not depend on the actual shape of the noise spectrum. As such, it provides a powerful way to complement usual refocusing techniques. Relaxation imposes a fundamental limit on the sensitivity of state of the art quantum sensors which cannot be overcome by dynamical decoupling. The only way to overcome this is to utilize quantum error correcting codes. We present a superconducting magnetometry design that incorporates approximate quantum error correction, in which the signal is generated by a two qubit Hamiltonian term. This two-qubit term is provided by the dynamics of a tunable coupler between two transmon qubits. For fast enough correction, it is possible to lengthen the coherence time of the device beyond the relaxation limit.
I Cohen, S Weidt, WK Hensinger, and A Retzker. 4/8/2015. “Multi-qubit gate with trapped ions for microwave and laser-based implementation.” New Journal of Physics, 17. Publisher's Version Abstract
A proposal for a phase gate and a Mølmer–Sørensen gate in the dressed state basis is presented. In order to perform the multi-qubit interaction, a strong magnetic field gradient is required to couple the phonon-bus to the qubit states. The gate is performed using resonant microwave driving fields together with either a radio-frequency (RF) driving field, or additional detuned microwave driving fields. The gate is robust to ambient magnetic field fluctuations due to an applied resonant microwave driving field. Furthermore, the gate is robust to fluctuations in the microwave Rabi frequency and is decoupled from phonon dephasing due to a resonant RF or a detuned microwave driving field. This makes this new gate an attractive candidate for the implementation of high-fidelity microwave based multi-qubit gates. The proposal can also be realized in laser-based set-ups.
I Cohen, P Richerme, ZX Gong, C Monroe, and A Retzker. 7/30/2015. “Simulating the Haldane phase in trapped-ion spins using optical fields.” Physical Review a, 92, 1. Publisher's Version Abstract
We propose to experimentally explore the Haldane phase in spin-one XXZ antiferromagnetic chains using trapped ions. We show how to adiabatically prepare the ground states of the Haldane phase, demonstrate their robustness against sources of experimental noise, and propose ways to detect the Haldane ground states based on their excitation gap and exponentially decaying correlations, nonvanishing nonlocal string order, and doubly degenerate entanglement spectrum.
JM Cai, I Cohen, A Retzker, and MB Plenio. 10/16/2015. “Proposal for High-Fidelity Quantum Simulation Using a Hybrid Dressed State.” Physical Review Letters, 115, 16. Publisher's Version Abstract
A fundamental goal of quantum technologies concerns the exploitation of quantum coherent dynamics for the realization of novel quantum applications such as quantum computing, quantum simulation, and quantum metrology. A key challenge on the way towards these goals remains the protection of quantum coherent dynamics from environmental noise. Here, we propose a concept of a hybrid dressed state from a pair of continuously driven systems. It allows sufficiently strong driving fields to suppress the effect of environmental noise while at the same time being insusceptible to both the amplitude and phase noise in the continuous driving fields. This combination of robust features significantly enhances coherence times under realistic conditions and at the same time provides new flexibility in Hamiltonian engineering that otherwise is not achievable. We demonstrate theoretically applications of our scheme for a noise-resistant analog quantum simulation in the well-studied physical systems of nitrogen-vacancy centers in diamond and of trapped ions. The scheme may also be exploited for quantum computation and quantum metrology.
JE Avron, O Kenneth, A Retzker, and M Shalyt. 4/8/2015. “Lindbladians for controlled stochastic Hamiltonians.” New Journal of Physics, 17. Publisher's Version Abstract
We construct Lindbladians associated with controlled stochastic Hamiltonians in the weak coupling regime. This construction allows us to determine the power spectrum of the noise from measurements of dephasing rates. Moreover, by studying the derived equation it is possible to optimize the control as well as to test numerical algorithms that solve controlled stochastic Schrödinger equations. A few examples are worked out in detail.
Q Chen, I Schwarz, F Jelezko, A Retzker, and MB Plenio. 11/18/2015. “Optical hyperpolarization of C-13 nuclear spins in nanodiamond ensembles.” Physical Review B, 92, 18. Publisher's Version Abstract
Dynamical nuclear polarization holds the key for orders of magnitude enhancements of nuclear magnetic resonance signals which, in turn, would enable a wide range of novel applications in biomedical sciences. However, current implementations of DNP require cryogenic temperatures and long times for achieving high polarization. Here we propose and analyze in detail protocols that can achieve rapid hyperpolarization of 13C nuclear spins in randomly oriented ensembles of nanodiamonds at room temperature. Our protocols exploit a combination of optical polarization of electron spins in nitrogen-vacancy centers and the transfer of this polarization to 13C nuclei by means of microwave control to overcome the severe challenges that are posed by the random orientation of the nanodiamonds and their nitrogen-vacancy centers. Specifically, these random orientations result in exceedingly large energy variations of the electron spin levels that render the polarization and coherent control of the nitrogen-vacancy center electron spins as well as the control of their coherent interaction with the surrounding 13C nuclear spins highly inefficient. We address these challenges by a combination of an off-resonant microwave double resonance scheme in conjunction with a realization of the integrated solid effect which, together with adiabatic rotations of external magnetic fields or rotations of nanodiamonds, leads to a protocol that achieves high levels of hyperpolarization of the entire nuclear-spin bath in a randomly oriented ensemble of nanodiamonds even at room temperature. This hyperpolarization together with the long nuclear-spin polarization lifetimes in nanodiamonds and the relatively high density of 13C nuclei has the potential to result in a major signal enhancement in 13C nuclear magnetic resonance imaging and suggests functionalized and hyperpolarized nanodiamonds as a unique probe for molecular imaging both in vitro and in vivo.

2014

ZY Wang, JM Cai, A Retzker, and MB Plenio. 8/14/2014. “All-optical magnetic resonance of high spectral resolution using a nitrogen-vacancy spin in diamond.” New Journal of Physics, 16. Publisher's Version Abstract
We propose an all-optical scheme to prolong the quantum coherence of a negatively charged nitrogen-vacancy (NV) center in diamond at cryogenic temperatures. Optical control of the NV spin suppresses energy fluctuations of the $^{3}{{{\rm A}}_{2}}$ ground states and forms an energy gap protected subspace. By optical control, the spectral linewidth of magnetic resonance is much narrower and the measurement of the frequencies of magnetic field sources has higher resolution. The optical control also improves the sensitivity of the magnetic field detection and can provide measurement of the directions of signal sources.
H Landa, A Retzker, T Schaetz, and B Reznik. 7/30/2014. “Entanglement Generation Using Discrete Solitons in Coulomb Crystals.” Physical Review Letters, 113, 5. Publisher's Version Abstract
Laser-cooled and trapped ions can crystallize and feature discrete solitons that are nonlinear, topologically protected configurations of the Coulomb crystal. Such solitons, as their continuum counterparts, can move within the crystal, while their discreteness leads to the existence of a gap-separated, spatially localized motional mode of oscillation above the spectrum. Suggesting that these unique properties of discrete solitons can be used for generating entanglement between different sites of the crystal, we study a detailed proposal in the context of state-of-the-art experimental techniques. We analyze the interaction of periodically driven planar ion crystals with optical forces, revealing the effects of micromotion in radio-frequency traps inherent to such structures, as opposed to linear ion chains. The proposed method requires Doppler cooling of the crystal and sideband cooling of the soliton’s localized modes alone. Since the gap separation of the latter is nearly independent of the crystal size, this approach could be particularly useful for producing entanglement and studying system-environment interactions in large, two- and possibly three-dimensional systems.
I Cohen and A Retzker. 1/31/2014. “Proposal for Verification of the Haldane Phase Using Trapped Ions.” Physical Review Letters, 112, 4. Publisher's Version Abstract
A proposal to use trapped ions to implement spin-one XXZ antiferromagnetic chains as an experimental tool to explore the Haldane phase is presented. We explain how to reach the Haldane phase adiabatically, demonstrate the robustness of the ground states to noise in the magnetic field and Rabi frequencies, and propose a way to detect them using their characteristics: an excitation gap and exponentially decaying correlations, a nonvanishing nonlocal string order, and a double degenerate entanglement spectrum. Scaling up to higher dimensions and more frustrated lattices, we obtain richer phase diagrams, and we can reach spin liquid phase, which can be detected by its entanglement entropy which obeys the boundary law.
G Arrad, Y Vinkler, D Aharonov, and A Retzker. 4/16/2014. “Increasing Sensing Resolution with Error Correction.” Physical Review Letters, 112, 15. Publisher's Version Abstract
The signal to noise ratio of quantum sensing protocols scales with the square root of the coherence time. Thus, increasing this time is a key goal in the field. By utilizing quantum error correction, we present a novel way of prolonging such coherence times beyond the fundamental limits of current techniques. We develop an implementable sensing protocol that incorporates error correction, and discuss the characteristics of these protocols in different noise and measurement scenarios. We examine the use of entangled versue untangled states, and error correction’s reach of the Heisenberg limit. The effects of error correction on coherence times are calculated and we show that measurement precision can be enhanced for both one-directional and general noise.
A Albrecht, A Retzker, and MB Plenio. 9/22/2014. “Testing quantum gravity by nanodiamond interferometry with nitrogen-vacancy centers.” Physical Review a, 90, 3. Publisher's Version Abstract
Interferometry with massive particles may have the potential to explore the limitations of standard quantum mechanics, in particular where it concerns its boundary with general relativity and the yet to be developed theory of quantum gravity. This development is hindered considerably by the lack of experimental evidence and testable predictions. Analyzing effects that appear to be common to many of such theories, such as a modification of the energy dispersion and of the canonical commutation relation within the standard framework of quantum mechanics, has been proposed as a possible way forward. Here we analyze in some detail the impact of a modified energy-momentum dispersion in a Ramsey-Bordé setup and provide achievable bounds of these correcting terms when operating such an interferometer with nanodiamonds. Thus, taking thermal and gravitational disturbances into account will show that without specific prerequisites, quantum gravity modifications may in general be suppressed requiring a revision of previously estimated bounds. As a possible solution we propose a stable setup which is rather insensitive to these effects. Finally, we address the problems of decoherence and pulse errors in such setups and discuss the scalings and advantages with increasing particle mass.
A Albrecht, G Koplovitz, A Retzker, F Jelezko, S Yochelis, D Porath, Y Nevo, O Shoseyov, Y Paltiel, and MB Plenio. 9/4/2014. “Self-assembling hybrid diamond-biological quantum devices.” New Journal of Physics, 16. Publisher's Version Abstract
The realization of scalable arrangements of nitrogen vacancy (NV) centers in diamond remains a key challenge on the way towards efficient quantum information processing, quantum simulation and quantum sensing applications. Although technologies based on implanting NV-centers in bulk diamond crystals or hybrid device approaches have been developed, they are limited by the achievable spatial resolution and by the intricate technological complexities involved in achieving scalability. We propose and demonstrate a novel approach for creating an arrangement of NV-centers, based on the self-assembling capabilities of biological systems and their beneficial nanometer spatial resolution. Here, a self-assembled protein structure serves as a structural scaffold for surface functionalized nanodiamonds, in this way allowing for the controlled creation of NV-structures on the nanoscale and providing a new avenue towards bridging the bio–nano interface. One-, two- as well as three-dimensional structures are within the scope of biological structural assembling techniques. We realized experimentally the formation of regular structures by interconnecting nanodiamonds using biological protein scaffolds. Based on the achievable NV-center distances of 11 nm, we evaluate the expected dipolar coupling interaction with neighboring NV-centers as well as the expected decoherence time. Moreover, by exploiting these couplings, we provide a detailed theoretical analysis on the viability of multiqubit quantum operations, suggest the possibility of individual addressing based on the random distribution of the NV intrinsic symmetry axes and address the challenges posed by decoherence and imperfect couplings. We then demonstrate in the last part that our scheme allows for the high-fidelity creation of entanglement, cluster states and quantum simulation applications.

2013

S Ulm, J Rossnagel, G Jacob, C Degunther, ST Dawkins, UG Poschinger, R Nigmatullin, A Retzker, MB Plenio, F Schmidt-Kaler, and K Singer. 8/7/2013. “Observation of the Kibble-Zurek scaling law for defect formation in ion crystals.” Nature Communications, 4. Publisher's Version Abstract
Traversal of a symmetry-breaking phase transition at finite rates can lead to causally separated regions with incompatible symmetries and the formation of defects at their boundaries, which has a crucial role in quantum and statistical mechanics, cosmology and condensed matter physics. This mechanism is conjectured to follow universal scaling laws prescribed by the Kibble–Zurek mechanism. Here we determine the scaling law for defect formation in a crystal of 16 laser-cooled trapped ions, which are conducive to the precise control of structural phases and the detection of defects. The experiment reveals an exponential scaling of defect formation γβ, where γ is the rate of traversal of the critical point and β=2.68±0.06. This supports the prediction of β=8/3≈2.67 for finite inhomogeneous systems. Our result demonstrates that the scaling laws also apply in the mesoscopic regime and emphasizes the potential for further tests of non-equilibrium thermodynamics with ion crystals.
D Shwa, RD Cohen, A Retzker, and N Katz. 8/7/2013. “Heralded generation of Bell states using atomic ensembles.” Physical Review a, 88, 6. Publisher's Version Abstract
Traversal of a symmetry-breaking phase transition at finite rates can lead to causally separated regions with incompatible symmetries and the formation of defects at their boundaries, which has a crucial role in quantum and statistical mechanics, cosmology and condensed matter physics. This mechanism is conjectured to follow universal scaling laws prescribed by the Kibble–Zurek mechanism. Here we determine the scaling law for defect formation in a crystal of 16 laser-cooled trapped ions, which are conducive to the precise control of structural phases and the detection of defects. The experiment reveals an exponential scaling of defect formation γβ, where γ is the rate of traversal of the critical point and β=2.68±0.06. This supports the prediction of β=8/3≈2.67 for finite inhomogeneous systems. Our result demonstrates that the scaling laws also apply in the mesoscopic regime and emphasizes the potential for further tests of non-equilibrium thermodynamics with ion crystals.
K Pyka, J Keller, HL Partner, R Nigmatullin, T Burgermeister, DM Meier, K Kuhlmann, A Retzker, MB Plenio, WH Zurek, A del Campo, and TE Mehlstaubler. 8/7/2013. “Topological defect formation and spontaneous symmetry breaking in ion Coulomb crystals.” Nature Communications, 4. Publisher's Version Abstract
Symmetry breaking phase transitions play an important role in nature. When a system traverses such a transition at a finite rate, its causally disconnected regions choose the new broken symmetry state independently. Where such local choices are incompatible, topological defects can form. The Kibble–Zurek mechanism predicts the defect densities to follow a power law that scales with the rate of the transition. Owing to its ubiquitous nature, this theory finds application in a wide field of systems ranging from cosmology to condensed matter. Here we present the successful creation of defects in ion Coulomb crystals by a controlled quench of the confining potential, and observe an enhanced power law scaling in accordance with numerical simulations and recent predictions. This simple system with well-defined critical exponents opens up ways to investigate the physics of non-equilibrium dynamics from the classical to the quantum regime.
MB Plenio and A Retzker. 11/4/2013. “Ion Traps as a testbed of classical and quantum statistical mechanics.” Annalen Der Physik, 525, 10-11, Pp. A159-A162. Publisher's Version Abstract
The development of statistical mechanics over the last century represents a major advance in the Natural Sciences. It is capable of describing a wide range of phenomena including the properties of phase transitions by reducing the complexity of quantum-many-body systems to simple general principles rooted in statistical methods that hold true in macroscopic systems. These principles apply to a broad range of physical theories, encompassing phenomena that are for example governed by classical, relativistic or quantum physics. While many of these predictions have been confirmed experimentally, the exploration, under highly controlled conditions, of the statistical mechanics of mesoscopic systems and the theoretical prediction of their behavior does still represent a challenge for both theory and experiment. A new avenue towards the controlled exploration of the classical and quantum statistical mechanics of mesoscopic systems has opened up in recent years thanks to the remarkable achievements in ion trap physics, which is pursued mainly for the realization of quantum information processing and precision metrology, in controlling and manipulating ion crystals.
HL Partner, R Nigmatullin, T Burgermeister, K Pyka, J Keller, A Retzker, MB Plenio, and TE Mehlstaubler. 10/16/2013. “Dynamics of topological defects in ion Coulomb crystals.” New Journal of Physics, 15. Publisher's Version Abstract
We study experimentally and theoretically the properties of structural defects (kink solitons) in two-dimensional ion Coulomb crystals. We show how different types of kink solitons with different physical properties can be realized, and transformed from one type into another by varying the aspect ratio of the trap confinement. Further, we discuss how impurities in ion Coulomb crystals, such as mass defects, can modify the dynamics of kink creation and their stability. For both pure and impure crystals, the experimentally observed kink dynamics are analysed in detail and explained theoretically by numerical simulations and calculations of the Peierls–Nabarro potential. Finally, we demonstrate that static electric fields provide a handle to vary the influence of mass defects on kinks in a controlled way and allow for deterministic manipulation and creation of kinks.
P London, J Scheuer, JM Cai, I Schwarz, A Retzker, MB Plenio, M Katagiri, T Teraji, S Koizumi, J Isoya, R Fischer, LP McGuinness, B Naydenov, and F Jelezko. 8/5/2013. “Detecting and Polarizing Nuclear Spins with Double Resonance on a Single Electron Spin.” Physical Review Letters, 111, 6. Publisher's Version Abstract
We report the detection and polarization of nuclear spins in diamond at room temperature by using a single nitrogen-vacancy (NV) center. We use Hartmann-Hahn double resonance to coherently enhance the signal from a single nuclear spin while decoupling from the noisy spin bath, which otherwise limits the detection sensitivity. As a proof of principle, we (i) observe coherent oscillations between the NV center and a weakly coupled nuclear spin and (ii) demonstrate nuclear-bath cooling, which prolongs the coherence time of the NV sensor by more than a factor of 5. Our results provide a route to nanometer scale magnetic resonance imaging and novel quantum information processing protocols.
JM Cai, A Retzker, F Jelezko, and MB Plenio. 1/20/2013. “A large-scale quantum simulator on a diamond surface at room temperature.” Nature Physics, 9, 3, Pp. 168-173. Publisher's Version Abstract
Strongly correlated quantum many-body systems may exhibit exotic phases, such as spin liquids and supersolids. Although their numerical simulation becomes intractable for as few as 50 particles, quantum simulators offer a route to overcome this computational barrier. However, proposed realizations either require stringent conditions such as low temperature/ultra-high vacuum, or are extremely hard to scale. Here, we propose a new solid-state architecture for a scalable quantum simulator that consists of strongly interacting nuclear spins attached to the diamond surface. Initialization, control and read-out of this quantum simulator can be accomplished with nitrogen-vacancy centers implanted in diamond. The system can be engineered to simulate a wide variety of strongly correlated spin models. Owing to the superior coherence time of nuclear spins and nitrogen-vacancy centers in diamond, our proposal offers new opportunities towards large-scale quantum simulation at ambient conditions of temperature and pressure.
JM Cai, F Jelezko, MB Plenio, and A Retzker. 1/11/2013. “Diamond-based single-molecule magnetic resonance spectroscopy.” New Journal of Physics, 15. Publisher's Version Abstract
The detection of a nuclear spin in an individual molecule represents a key challenge in physics and biology whose solution has been pursued for many years. The small magnetic moment of a single nucleus and the unavoidable environmental noise present the key obstacles for its realization. In this paper, we demonstrate theoretically that a single nitrogen-vacancy center in diamond can be used to construct a nano-scale single-molecule spectrometer that is capable of detecting the position and spin state of a single nucleus and can determine the distance and alignment of a nuclear or electron spin pair. The proposed device would find applications in single-molecule spectroscopy in chemistry and biology, for example in determining the protein structure or in monitoring macromolecular motions, and can thus provide a tool to help unravel the microscopic mechanisms underlying bio-molecular function.
A Albrecht, A Retzker, F Jelezko, and MB Plenio. 8/6/2013. “Coupling of nitrogen vacancy centres in nanodiamonds by means of phonons.” New Journal of Physics, 15. Publisher's Version Abstract
Realizing controlled quantum dynamics via the magnetic interactions between colour centres in diamond remains a challenge despite recent demonstrations for nanometre separated pairs. Here we propose to use the intrinsic acoustical phonons in diamond as a data bus for accomplishing this task. We show that for nanodiamonds the electron–phonon coupling can take significant values that together with mode frequencies in the THz range can serve as a resource for conditional gate operations. Based on these results, we analyse how to use this phonon-induced interaction for constructing quantum gates among the electron-spin triplet ground states, introducing the phonon dependence via Raman transitions. Combined with decoupling pulses this offers the possibility for creating entangled states within nanodiamonds on the scale of several tens of nanometres, a promising prerequisite for quantum sensing applications.
N Aharon, M Drewsen, and A Retzker. 12/6/2013. “General Scheme for the Construction of a Protected Qubit Subspace.” Physical Review Letters, 111, 23. Publisher's Version Abstract
We present a new robust decoupling scheme suitable for levels with either half-integer or integer angular momentum states. Through continuous dynamical decoupling techniques, we create a protected qubit subspace, utilizing a multistate qubit construction. Remarkably, the multistate system can also be composed of multiple substates within a single level. Our scheme can be realized with state-of-the-art experimental setups and thus has immediate applications for quantum information science. While the scheme is general and relevant for a multitude of solid-state and atomic systems, we analyze its performance for the case composed of trapped ions. Explicitly, we show how single qubit gates and an ensemble coupling to a cavity mode can be implemented efficiently. The scheme predicts a coherence time of ∼1  s, as compared to typically a few milliseconds for the bare states.

2012

T Tufarelli, A Retzker, MB Plenio, and A Serafini. 9/26/2012. “Input-output Gaussian channels: theory and application.” New Journal of Physics, 14. Publisher's Version Abstract
Setting off from the classic input–output formalism, we develop a theoretical framework to characterize the Gaussian quantum channels relating the initial correlations of an open bosonic system to those of properly identified output modes. We then proceed to apply our formalism to the case of quantum harmonic oscillators, such as the motional degrees of freedom of trapped ions or nanomechanical oscillators, interacting with travelling electromagnetic modes through cavity fields and subject to external white noise. We thus determine the degree of squeezing that can be transferred from an intra-cavity oscillator to light and show that the intra-cavity squeezing can be transformed into distributed optical entanglement if one can access both output fields of a two-sided cavity.
S Machnes, J Cerrillo, M Aspelmeyer, W Wieczorek, MB Plenio, and A Retzker. 4/10/2012. “Pulsed Laser Cooling for Cavity Optomechanical Resonators.” Physical Review Letters, 108, 15. Publisher's Version Abstract
A pulsed cooling scheme for optomechanical systems is presented that is capable of cooling at much faster rates, shorter overall cooling times, and for a wider set of experimental scenarios than is possible by conventional methods. The proposed scheme can be implemented for both strongly and weakly coupled optomechanical systems in both weakly and highly dissipative cavities. We study analytically its underlying working mechanism, which is based on interferometric control of optomechanical interactions, and we demonstrate its efficiency with pulse sequences that are obtained by using methods from optimal control. The short time in which our scheme approaches the optomechanical ground state allows for a significant relaxation of current experimental constraints. Finally, the framework presented here can be used to create a rich variety of optomechanical interactions and hence offers a novel, readily available toolbox for fast optomechanical quantum control.
H Landa, M Drewsen, B Reznik, and A Retzker. 9/13/2012. “Modes of oscillation in radiofrequency Paul traps.” New Journal of Physics, 14. Publisher's Version Abstract
We examine the time-dependent dynamics of ion crystals in radiofrequency traps. The problem of stable trapping of general three-dimensional crystals is considered and the validity of the pseudopotential approximation is discussed. We analytically derive the micromotion amplitude of the ions, rigorously proving well-known experimental observations. We use a recently proposed method to find the modes that diagonalize the linearized time-dependent dynamical problem. This allows one to obtain explicitly the ('Floquet–Lyapunov') transformation to coordinates of decoupled linear oscillators. We demonstrate the utility of the method by analyzing the modes of a small 'peculiar' crystal in a linear Paul trap. The calculations can be readily generalized to multispecies ion crystals in general multipole traps, and time-dependent quantum wavefunctions of ion oscillations in such traps can be obtained.
H Landa, M Drewsen, B Reznik, and A Retzker. 10/23/2012. “Classical and quantum modes of coupled Mathieu equations.” Journal of Physics a-Mathematical and Theoretical, 45, 45. Publisher's Version Abstract
We expand the solutions of linearly coupled Mathieu equations in terms of infinite-continued matrix inversions, and use it to find the modes which diagonalize the dynamical problem. This allows obtaining explicitly the (Floquet–Lyapunov) transformation to coordinates in which the motion is that of decoupled linear oscillators. We use this transformation to solve the Heisenberg equations of the corresponding quantum-mechanical problem, and find the quantum wavefunctions for stable oscillations, expressed in configuration space. The obtained transformation and quantum solutions can be applied to more general linear systems with periodic coefficients (coupled Hill equations, periodically driven parametric oscillators), and to nonlinear systems as a starting point for convenient perturbative treatment of the nonlinearity.
H Kaufmann, S Ulm, G Jacob, U Poschinger, H Landa, A Retzker, MB Plenio, and F Schmidt-Kaler. 12/28/2012. “Precise Experimental Investigation of Eigenmodes in a Planar Ion Crystal.” Physical Review Letters, 109, 26. Publisher's Version Abstract
The accurate characterization of eigenmodes and eigenfrequencies of two-dimensional ion crystals provides the foundation for the use of such structures for quantum simulation purposes. We present a combined experimental and theoretical study of two-dimensional ion crystals. We demonstrate that standard pseudopotential theory accurately predicts the positions of the ions and the location of structural transitions between different crystal configurations. However, pseudopotential theory is insufficient to determine eigenfrequencies of the two-dimensional ion crystals accurately but shows significant deviations from the experimental data obtained from resolved sideband spectroscopy. Agreement at the level of 2.5×10−3 is found with the full time-dependent Coulomb theory using the Floquet-Lyapunov approach and the effect is understood from the dynamics of two-dimensional ion crystals in the Paul trap. The results represent initial steps towards an exploitation of these structures for quantum simulation schemes.
A Bermudez, PO Schmidt, MB Plenio, and A Retzker. 4/4/2012. “Robust trapped-ion quantum logic gates by continuous dynamical decoupling.” Physical Review a, 85, 4. Publisher's Version Abstract
We introduce a scheme that combines phonon-mediated quantum logic gates in trapped ions with the benefits of continuous dynamical decoupling. We demonstrate theoretically that a strong driving of the qubit decouples it from external magnetic-field noise, enhancing the fidelity of two-qubit quantum gates. Moreover, the scheme does not require ground-state cooling, and is inherently robust to undesired ac Stark shifts. The underlying mechanism can be extended to a variety of other systems where a strong driving protects the quantum coherence of the qubits without compromising the two-qubit couplings.
A Bermudez, J Almeida, K Ott, H Kaufmann, S Ulm, U Poschinger, F Schmidt-Kaler, A Retzker, and MB Plenio. 9/25/2012. “Quantum magnetism of spin-ladder compounds with trapped-ion crystals.” New Journal of Physics, 14. Publisher's Version Abstract
The quest for experimental platforms that allow for the exploration, and even control, of the interplay of low dimensionality and frustration is a fundamental challenge in several fields of quantum many-body physics, such as quantum magnetism. Here, we propose the use of cold crystals of trapped ions to study a variety of frustrated quantum spin ladders. By optimizing the trap geometry, we show how to tailor the low dimensionality of the models by changing the number of legs of the ladders. Combined with a method for selectively hiding ions provided by laser addressing, it becomes possible to synthesize stripes of both triangular and Kagome lattices. Besides, the degree of frustration of the phonon-mediated spin interactions can be controlled by shaping the trap frequencies. We support our theoretical considerations by initial experiments with planar ion crystals, where a high and tunable anisotropy of the radial trap frequencies is demonstrated. We take into account an extensive list of possible error sources under typical experimental conditions, and describe explicit regimes that guarantee the validity of our scheme.
JM Cai, B Naydenov, R Pfeiffer, LP McGuinness, KD Jahnke, F Jelezko, MB Plenio, and A Retzker. 11/20/2012. “Robust dynamical decoupling with concatenated continuous driving.” New Journal of Physics, 14. Publisher's Version Abstract
The loss of coherence is one of the main obstacles for the implementation of quantum information processing. The efficiency of dynamical decoupling schemes, which have been introduced to address this problem, is limited itself by the fluctuations in the driving fields which will themselves introduce noise. We address this challenge by introducing the concept of concatenated continuous dynamical decoupling, which can overcome not only external magnetic noise but also noise due to fluctuations in driving fields. We show theoretically that this approach can achieve relaxation limited coherence times, and demonstrate experimentally that already the most basic implementation of this concept yields an order of magnitude improvement to the decoherence time for the electron spin of nitrogen vacancy centers in diamond. The proposed scheme can be applied to a wide variety of other physical systems, including trapped atoms and ions and quantum dots, and may be combined with other quantum technologies challenges such as quantum sensing and quantum information processing.
JM Cai, F Jelezko, N Katz, A Retzker, and MB Plenio. 9/17/2012. “Long-lived driven solid-state quantum memory.” New Journal of Physics, 14. Publisher's Version Abstract
We investigate the performance of inhomogeneously broadened spin ensembles as quantum memories under continuous dynamical decoupling. The role of the continuous driving field is twofold: firstly, it decouples individual spins from magnetic noise; secondly, and more importantly, it suppresses and reshapes the spectral inhomogeneity of spin ensembles. We show that a continuous driving field, which itself may also be inhomogeneous over the ensemble, can considerably enhance the decay of the tails of the inhomogeneous broadening distribution. This fact enables a spin-ensemble-based quantum memory to exploit the effect of cavity protection and achieve a much longer storage time. In particular, for a spin ensemble with a Lorentzian spectral distribution, our calculations demonstrate that continuous dynamical decoupling has the potential to improve its storage time by orders of magnitude for the state-of-the-art experimental parameters.

2011

N Timoney, I Baumgart, M Johanning, AF Varon, MB Plenio, A Retzker, and C Wunderlich. 8/10/2011. “Quantum gates and memory using microwave-dressed states.” Nature, 476, 7359, Pp. 185-U83. Publisher's Version Abstract
Trapped atomic ions have been used successfully to demonstrate1 basic elements of universal quantum information processing. Nevertheless, scaling up such methods to achieve large-scale, universal quantum information processing (or more specialized quantum simulations2,3,4,5) remains challenging. The use of easily controllable and stable microwave sources, rather than complex laser systems6,7, could remove obstacles to scalability. However, the microwave approach has drawbacks: it involves the use of magnetic-field-sensitive states, which shorten coherence times considerably, and requires large, stable magnetic field gradients. Here we show how to overcome both problems by using stationary atomic quantum states as qubits that are induced by microwave fields (that is, by dressing magnetic-field-sensitive states with microwave fields). This permits fast quantum logic, even in the presence of a small (effective) Lamb–Dicke parameter (and, therefore, moderate magnetic field gradients). We experimentally demonstrate the basic building blocks of this scheme, showing that the dressed states are long lived and that coherence times are increased by more than two orders of magnitude relative to those of bare magnetic-field-sensitive states. This improves the prospects of microwave-driven ion trap quantum information processing, and offers a route to extending coherence times in all systems that suffer from magnetic noise, such as neutral atoms, nitrogen-vacancy centres, quantum dots or circuit quantum electrodynamic systems.
A del Campo, A Retzker, and MB Plenio. 8/16/2011. “The inhomogeneous Kibble-Zurek mechanism: vortex nucleation during Bose-Einstein condensation.” New Journal of Physics, 13. Publisher's Version Abstract
The Kibble–Zurek mechanism is applied to the spontaneous formation of vortices in a harmonically trapped thermal gas following a temperature quench through the critical value for Bose–Einstein condensation. Whereas in the homogeneous scenario, vortex nucleation is always expected, we show that it can be completely suppressed in the presence of the confinement potential whenever the speed of the spatial front undergoing condensation is lower than a threshold velocity. Otherwise, the interplay between the geometry and the causality leads to different scaling laws for the density of vortices as a function of the quench rate, as we also illustrate for the case of a toroidal trapping potential.
A Bermudez, J Almeida, F Schmidt-Kaler, A Retzker, and MB Plenio. 11/11/2011. “Frustrated Quantum Spin Models with Cold Coulomb Crystals.” Physical Review Letters, 107, 20. Publisher's Version Abstract
We exploit the geometry of a zigzag cold-ion crystal in a linear trap to propose the quantum simulation of a paradigmatic model of long-ranged magnetic frustration. Such a quantum simulation would clarify the complex features of a rich phase diagram that presents ferromagnetic, dimerized-antiferromagnetic, paramagnetic, and floating phases, together with previously unnoticed features that are hard to assess by numerics. We analyze in detail its experimental feasibility, and provide supporting numerical evidence on the basis of realistic parameters in current ion-trap technology.
A Bermudez, F Jelezko, MB Plenio, and A Retzker. 10/3/2011. “Electron-Mediated Nuclear-Spin Interactions between Distant Nitrogen-Vacancy Centers.” Physical Review Letters, 107, 15. Publisher's Version Abstract
We propose a scheme enabling controlled quantum coherent interactions between separated nitrogen-vacancy centers in diamond in the presence of strong magnetic fluctuations. The proposed scheme couples nuclear qubits employing the magnetic dipole-dipole interaction between the electron spins and, crucially, benefits from the suppression of the effect of environmental magnetic field fluctuations thanks to a strong microwave driving. This scheme provides a basic building block for a full-scale quantum-information processor or quantum simulator based on solid-state technology.
A Albrecht, A Retzker, C Wunderlich, and MB Plenio. 3/8/2011. “Enhancement of laser cooling by the use of magnetic gradients.” New Journal of Physics, 13. Publisher's Version Abstract
We present a laser cooling scheme for trapped ions and atoms using a combination of laser couplings and a magnetic gradient field. In a Schrieffer–Wolff transformed picture, this setup cancels the carrier and blue sideband terms completely (up to first order in the Lamb–Dicke parameter), resulting in an improved cooling behaviour compared to standard cooling schemes in the Lamb–Dicke regime (e.g. sideband cooling) and allowing cooling to the vibrational ground state. A condition for optimal cooling rates is presented and the cooling behaviour for different Lamb–Dicke parameters and spontaneous decay rates is discussed. Cooling rates of one order of magnitude less than the trapping frequency are achieved using the new cooling method. Furthermore, the scheme exhibits fast rates and low final populations, even for significant deviations from the optimal parameters, and provides good cooling rates also in the multi-particle case.

2010 and before

L Vaidman, N Erez, and A Retzker. 2006. “Another look at quantum teleportation.” International Journal of Quantum Information, 4, 1, Pp. 197-208. Publisher's Version Abstract
A dialog with Asher Peres regarding the meaning of quantum teleportation is briefly reviewed. The Braunstein–Kimble method for teleportation of light is analyzed in the language of quantum wave functions. A pictorial example of continuous variable teleportation is presented using computer simulation.
A Retzker, E Solano, and B Reznik. 2/13/2007. “Tavis-Cummings model and collective multiqubit entanglement in trapped ions.” Physical Review a, 75, 2. Publisher's Version Abstract
We present a method of generating collective multiqubit entanglement via global addressing of an ion chain performing blue and red Tavis-Cummings interactions, where several qubits are coupled to a collective motional mode. We show that a wide family of Dicke states and irradiant states can be generated by single global laser pulses, unitarily or helped with suitable postselection techniques.
A Retzker and B Shapiro. 2/2002. “Infinite range correlations of intensity in random media.” Pramana-Journal of Physics, 58, 2, Pp. 225-231. Publisher's Version Abstract
We study a new type of long-range correlations for waves propagating in a random medium. These correlations originate from scattering events which take place close to a point source. The scattered waves propagate by diffusion to distant regions. In this way long range correlations, between any pair of distant points, are established.
We present a detailed study on the possibility of manipulating quantum information encoded in the 'radial' modes of arrays of trapped ions (i.e. in the ions' oscillations orthogonal to the trap's main axis). In such systems, because of the tightness of transverse confinement, the radial modes pertaining to different ions can be addressed individually. In the first part of the paper we show that, if local control of the radial trapping frequencies is available, any linear optical and squeezing operation on the locally defined modes—on single as well as on many modes—can be reproduced by manipulating the frequencies. Then, we proceed to describe schemes apt to generate unprecedented degrees of bipartite and multipartite continuous variable (CV) entanglement under realistic noisy working conditions and even restricting only to a global control of the trapping frequencies. Furthermore, we consider the transmission of the quantum information encoded in the radial modes along the array of ions, and show it to be possible to a remarkable degree of accuracy, for both finite-dimensional and CV quantum states. Finally, as an application, we show that the states which can be generated in this setting allow for the violation of multipartite non-locality tests, by feasible displaced parity measurements. Such a demonstration would be a first test of quantum non-locality for 'massive' degrees of freedom (i.e. for degrees of freedom describing the motion of massive particles).
A Retzker, MB Plenio, TE Simos, and G Maroulis. 2007. “Fast cooling of trapped ions using the dynamical stark shift.” Computation in Modern Science and Engineering Vol 2, Pts a and B, 2, Pp. 792-795. Publisher's Version Abstract
A laser cooling scheme for trapped ions is presented which is based on the fast dynamical Stark shift gate, described in (Jonathan et al 2000 Phys. Rev. A 62 042307). Since this cooling method does not contain an off resonant carrier transition, low final temperatures are achieved even in a traveling wave light field. The proposed method may operate in either pulsed or continuous mode and is also suitable for ion traps using microwave addressing in strong magnetic field gradients.
A Serafini, A Retzker, and MB Plenio. 10/2/2009. “Generation of continuous variable squeezing and entanglement of trapped ions in time-varying potentials.” Quantum Information Processing, 8, 6, Pp. 619-630. Publisher's Version Abstract
We investigate the generation of squeezing and entanglement for the motional degrees of freedom of ions in linear traps, confined by time-varying and oscillating potentials, comprised of a DC and an AC component. We show that high degrees of squeezing and entanglement can be obtained by controlling either the DC or the AC trapping component (or both), and by exploiting transient dynamics in regions where the ions’ motion is unstable, without any added optical control. Furthermore, we investigate the time-scales over which the potentials should be switched in order for the manipulations to be most effective.
A Retzker and MB Plenio. 8/23/2007. “Fast cooling of trapped ions using the dynamical Stark shift.” New Journal of Physics, 9. Publisher's Version Abstract
A laser cooling scheme for trapped ions is presented which is based on the fast dynamical Stark shift gate, described in (Jonathan et al 2000 Phys. Rev. A 62 042307). Since this cooling method does not contain an off resonant carrier transition, low final temperatures are achieved even in a traveling wave light field. The proposed method may operate in either pulsed or continuous mode and is also suitable for ion traps using microwave addressing in strong magnetic field gradients.
B Reznik, A Retzker, and J Silman. 4/14/2005. “Violating Bell's inequalities in vacuum.” Physical Review a, 71, 4. Publisher's Version Abstract
We employ an approach wherein the ground state entanglement of a relativistic free scalar field is directly probed in a controlled manner. The approach consists of having a pair of initially nonentangled detectors locally interact with the vacuum for a finite duration T, such that the two detectors remain causally disconnected, and then analyzing the resulting detector mixed state. We show that the correlations between arbitrarily far-apart regions of the vacuum cannot be reproduced by a local hidden-variable model, and that as a function of the distance L between the regions, the entanglement decreases at a slower rate than ∼exp[−(L∕cT)3].
A Retzker, JI Cirac, and B Reznik. 2/9/2005. “Detecting vacuum entanglement in a linear ion trap.” Physical Review Letters, 94, 5. Publisher's Version Abstract
We propose and study a method for detecting ground-state entanglement in a chain of trapped ions. We show that the entanglement between single ions or groups of ions can be swapped to the internal levels of two ions by sending laser pulses that couple the internal and motional degrees of freedom. This allows us to entangle two ions without actually performing gate operations. A proof of principle of the effect can be realized with two trapped ions and is feasible with current technology.
B Reznik, A Retzker, and J Silman. 7/6/2004. “A lower bound on ground state entanglement between two regions for a free field.” Journal of Modern Optics, 51, 6-7, Pp. 833-840. Publisher's Version Abstract
In discrete models, such as spin chains, the entanglement between a pair of particles in a chain has been shown to vanish beyond a certain separation. In the continuum, a quantum field ⊘(x) at a point represents a single degree of freedom, thus at a region of finite size there are infinite separate degrees of freedom. We show that as a consequence, in contrast to discrete models, the ground state of a free, quantized and relativistic field exhibits entanglement between any pair of arbitrarily separated finite regions. We also provide a lower bound on the decay rate of the entanglement as a function of the separation length between the regions and briefly discuss the physical reasons behind this different behaviour of discrete and continuous systems.
A Retzker, JI Cirac, MB Plenio, and B Reznik. 9/11/2008. “Methods for detecting acceleration radiation in a Bose-Einstein condensate.” Physical Review Letters, 101, 11. Publisher's Version Abstract
We propose and study methods for detecting Unruh-like acceleration radiation effects in a Bose-Einstein condensate in a (1+1)-dimensional setup. The Bogoliubov vacuum of a Bose-Einstein condensate is used to simulate a scalar field theory, and accelerated atom dots or optical lattices serve as detectors of phonon radiation due to acceleration effects. In particular, we study the dispersive effects of the Bogoliubov spectrum on the ideal case of exact thermalization. Our results suggest that acceleration radiation effects can be observed using currently accessible experimental methods.
A Retzker, RC Thompson, DM Segal, and MB Plenio. 12/30/2008. “Double Well Potentials and Quantum Phase Transitions in Ion Traps.” Physical Review Letters, 101, 26. Publisher's Version Abstract
We demonstrate that the radial degree of freedom of strings of trapped ions in the quantum regime may be prepared and controlled accurately through the variation of the external trapping potential while at the same time its properties are measurable with high spatial and temporal resolution. This provides a new testbed giving access to static and dynamical properties of the physics of quantum-many-body systems and quantum phase transitions that are hard to simulate on classical computers. Furthermore, it allows for the creation of double well potentials with experimentally accessible tunneling rates, with applications in testing the foundations of quantum physics and precision sensing.
S Marcovitch, A Retzker, MB Plenio, and B Reznik. 7/21/2009. “Critical and noncritical long-range entanglement in Klein-Gordon fields.” Physical Review a, 80, 1. Publisher's Version Abstract
We investigate the entanglement between two spatially separated intervals in the vacuum state of a free one-dimensional Klein-Gordon field by means of explicit computations in the continuum limit of the linear harmonic chain. We demonstrate that the entanglement, which we quantify by the logarithmic negativity, is finite with no further need for renormalization. We find that in the critical regime, the quantum correlations are scale invariant as they depend only on the ratio of distance to length. They decay much faster than the classical correlations as in the critical limit long-range entanglement decays exponentially for separations larger than the size of the blocks, while classical correlations follow a power-law decay. With decreasing distance of the blocks, the entanglement diverges as a power law in the distance. The noncritical regime manifests richer behavior, as the entanglement depends both on the size of the blocks and on their separation. In correspondence with the von Neumann entropy also long-range entanglement distinguishes critical from noncritical systems.
S Machnes, MB Plenio, B Reznik, AM Steane, and A Retzker. 5/6/2010. “Superfast Laser Cooling.” Physical Review Letters, 104, 18. Publisher's Version Abstract
Currently, laser cooling schemes are fundamentally based on the weak coupling regime. This requirement sets the trap frequency as an upper bound to the cooling rate. In this work we present a numerical study that shows the feasibility of cooling in the strong-coupling regime which then allows cooling rates that are faster than the trap frequency with experimentally feasible parameters. The scheme presented here can be applied to trapped atoms or ions as well as to mechanical oscillators. It can also cool medium sized ion chains close to the ground state.
H Landa, S Marcovitch, A Retzker, MB Plenio, and B Reznik. 1/29/2010. “Quantum Coherence of Discrete Kink Solitons in Ion Traps.” Physical Review Letters, 104, 4. Publisher's Version Abstract
We propose to realize quantized discrete kinks with cold trapped ions. We show that long-lived solitonlike configurations are manifested as deformations of the zigzag structure in the linear Paul trap, and are topologically protected in a circular trap with an odd number of ions. We study the quantum-mechanical time evolution of a high-frequency, gap separated internal mode of a static kink and find long coherence times when the system is cooled to the Doppler limit. The spectral properties of the internal modes make them ideally suited for manipulation using current technology. This suggests that ion traps can be used to test quantum-mechanical effects with solitons and explore ideas for the utilization of the solitonic internal modes as carriers of quantum information.
I Katz, A Retzker, R Straub, and R Lifshitz. 7/26/2007. “Signatures for a classical to quantum transition of a driven nonlinear nanomechanical resonator.” Physical Review Letters, 99, 4. Publisher's Version Abstract
We seek the first indications that a nanoelectromechanical system (NEMS) is entering the quantum domain as its mass and temperature are decreased. We find them by studying the transition from classical to quantum behavior of a driven nonlinear Duffing resonator. Numerical solutions of the equations of motion, operating in the bistable regime of the resonator, demonstrate that the quantum Wigner function gradually deviates from the corresponding classical phase-space probability density. These clear differences that develop due to nonlinearity can serve as experimental signatures, in the near future, that NEMS resonators are entering the quantum domain.
I Katz, R Lifshitz, A Retzker, and R Straub. 12/8/2008. “Classical to quantum transition of a driven nonlinear nanomechanical resonator.” New Journal of Physics, 10. Publisher's Version Abstract
Much experimental effort is invested these days in fabricating nanoelectromechanical systems (NEMS) that are sufficiently small, cold and clean, so as to approach quantum mechanical behavior as their typical quantum energy scale  becomes comparable with that of the ambient thermal energy kBT. Such systems will hopefully enable one to observe the quantum behavior of human-made objects, and test some of the basic principles of quantum mechanics. Here, we expand and elaborate on our recent suggestion (Katz et al 2007 Phys. Rev. Lett. 99 040404) to exploit the nonlinear nature of a nanoresonator in order to observe its transition into the quantum regime. We study this transition for an isolated resonator, as well as one that is coupled to a heat bath at either zero or finite temperature. We argue that by exploiting nonlinearities, quantum dynamics can be probed using technology that is almost within reach. Numerical solutions of the equations of motion display the first quantum corrections to classical dynamics that appear as the classical-to-quantum transition occurs. This provides practical signatures to look for in future experiments with NEMS resonators.
A Retzker, Y Aharonov, A Botero, S Nussinov, and B Reznik. 3/15/2006. “Aharonov-Bohm effect without closing a loop.” Physical Review a, 73, 3. Publisher's Version Abstract
We discuss the consequences of the Aharonov-Bohm (AB) effect in setups involving several charged particles, wherein none of the charged particles encloses a closed loop around the magnetic flux. We show that in such setups, the AB phase is encoded either in the relative phase of a bipartite or multipartite entangled photons states, or alternatively, gives rise to an overall AB phase that can be measured relative to another reference system. These setups involve processes of annihilation or creation of electron-hole pairs. We discuss the relevance of such effects in “vacuum birefringence" in QED, and comment on their connection to other known effects.
G De Chiara, A del Campo, G Morigi, MB Plenio, and A Retzker. 11/29/2010. “Spontaneous nucleation of structural defects in inhomogeneous ion chains.” New Journal of Physics, 12. Publisher's Version Abstract
Structural defects in ion crystals can be formed during a linear quench of the transverse trapping frequency across the mechanical instability from a linear chain to a zigzag structure. The density of defects after the sweep can be conveniently described by the Kibble–Zurek mechanism (KZM). In particular, the number of kinks in the zigzag ordering can be derived from a time-dependent Ginzburg–Landau equation for the order parameter, here the zigzag transverse size, under the assumption that the ions are continuously laser cooled. In a linear Paul trap, the transition becomes inhomogeneous, since the charge density is larger in the center and more rarefied at the edges. During the linear quench, the mechanical instability is first crossed in the center of the chain, and a front, at which the mechanical instability is crossed during the quench, is identified that propagates along the chain from the center to the edges. If the velocity of this front is smaller than the sound velocity, the dynamics become adiabatic even in the thermodynamic limit and no defect is produced. Otherwise, the nucleation of kinks is reduced with respect to the case in which the charges are homogeneously distributed, leading to a new scaling of the density of kinks with the quenching rate. The analytical predictions are verified numerically by integrating the Langevin equations of motion of the ions, in the presence of a time-dependent transverse confinement. We argue that the non-equilibrium dynamics of an ion chain in a Paul trap constitutes an ideal scenario to test the inhomogeneous extension of the KZM, which lacks experimental evidence to date.
A del Campo, G De Chiara, G Morigi, MB Plenio, and A Retzker. 8/11/2010. “Structural Defects in Ion Chains by Quenching the External Potential: The Inhomogeneous Kibble-Zurek Mechanism.” Physical Review Letters, 105, 7. Publisher's Version Abstract
The nonequilibrium dynamics of an ion chain in a highly anisotropic trap is studied when the transverse trap frequency is quenched across the value at which the chain undergoes a continuous phase transition from a linear to a zigzag structure. Within Landau theory, an equation for the order parameter, corresponding to the transverse size of the zigzag structure, is determined when the vibrational motion is damped via laser cooling. The number of structural defects produced during a linear quench of the transverse trapping frequency is predicted and verified numerically. It is shown to obey the scaling predicted by the Kibble-Zurek mechanism, when extended to take into account the spatial inhomogeneities of the ion chain in a linear Paul trap.
J Cerrillo, A Retzker, and MB Plenio. 1/27/2010. “Fast and Robust Laser Cooling of Trapped Systems.” Physical Review Letters, 104, 4. Publisher's Version Abstract
We present a robust and fast laser cooling scheme suitable for trapped ions, atoms, or cantilevers. Based on quantum interference, generated by a special laser configuration, it is able to rapidly cool the system such that the final phonon occupation vanishes to zeroth order in the Lamb-Dicke parameter in contrast to existing cooling schemes. Furthermore, it is robust under conditions of fluctuating laser intensity and frequency, thus making it a viable candidate for experimental applications.