Publications

2022
Philipp J. Vetter, Alastair Marshall, Genko T. Genov, Tim F. Weiss, Nico Striegler, Eva F. Großmann, Santiago Oviedo-Casado, Javier Cerrillo, Javier Prior, Philipp Neumann, and Fedor Jelezko. 4/14/2022. “Zero- and Low-Field Sensing with Nitrogen-Vacancy Centers.” PHYSICAL REVIEW APPLIED, 17, 4, Pp. 044028. Publisher's Version Abstract
Over the years, an enormous effort has been made to establish nitrogen vacancy (N-V) centers in diamond as easily accessible and precise magnetic field sensors. However, most of their sensing protocols rely on the application of bias magnetic fields, preventing their usage in zero- or low-field experiments. We overcome this limitation by exploiting the full spin S=1 nature of the N-V center, allowing us to detect nuclear spin signals at zero and low field with a linearly polarized microwave field. As conventional dynamical decoupling protocols fail in this regime, we develop robust pulse sequences and optimize pulse pairs, which allow us to sense temperature and weak ac magnetic fields and achieve an efficient decoupling from environmental noise. Our work allows for much broader and simpler applications of N-V centers as magnetic field sensors in the zero- and low-field regime and can be further extended to three-level systems in ions and atoms.
Tim R. Eichhorn, Anna J. Parker, Felix Josten, Christoph Müller, Jochen Scheuer, Jakob M. Steiner, Martin Gierse, Jonas Handwerker, Michael Keim, Sebastian Lucas, Mohammad Usman Qureshi, Alastair Marshall, Alon Salhov, Yifan Quan, Jan Binder, Kay D. Jahnke, Philipp Neumann, Stephan Knecht, John W. Blanchard, Martin B. Plenio, Fedor Jelezko, Lyndon Emsley, Christophoros C. Vassiliou, Patrick Hautle, and Ilai Schwartz. 2/3/2022. “Hyperpolarized Solution-State NMR Spectroscopy with Optically Polarized Crystals.” Journal of the American Chemical Society, 144, Pp. 2511-2519. Publisher's Version Abstract
Nuclear spin hyperpolarization provides a promising route to overcome the challenges imposed by the limited sensitivity of nuclear magnetic resonance. Here we demonstrate that dissolution of spin-polarized pentacene-doped naphthalene crystals enables transfer of polarization to target molecules via intermolecular cross-relaxation at room temperature and moderate magnetic fields (1.45 T). This makes it possible to exploit the high spin polarization of optically polarized crystals, while mitigating the challenges of its transfer to external nuclei. With this method, we inject the highly polarized mixture into a benchtop NMR spectrometer and observe the polarization dynamics for target 1H nuclei. Although the spectra are radiation damped due to the high naphthalene magnetization, we describe a procedure to process the data to obtain more conventional NMR spectra and extract the target nuclei polarization. With the entire process occurring on a time scale of 1 min, we observe NMR signals enhanced by factors between −200 and −1730 at 1.45 T for a range of small molecules.
2021
L.Pelzer, K.Dietze, J.Kramer, F.Dawel, L.Krinner, N.Spethmann, V.Martinez, N.Aharon, A.Retzker, K.Hammerer, and P.O.Schmidt. 12/1/2021. “Tailored optical clock transition in 40Ca+.” Science Direct, 18. Publisher's Version Abstract
We engineer an artificial optical clock transition in 40Ca+ with a continuous dynamical decoupling scheme. It suppresses inhomogeneous tensor shifts as well as the linear Zeeman shift, making it suitable for multi-ion operation. Coherence times approaching the natural lifetime limit ensure low statistical uncertainties on the optical transition. We continuously apply the dressing fields, making the tailored transition robust against magnetic field fluctuations during the entire clock probe time.
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.
Simon Schmitt, Tuvia Gefen, Daniel Louzon, Christian Osterkamp, Nicolas Staudenmaier, Johannes Lang, Matthew Markham, and Alex Retzk. 4/1/2021. “Optimal frequency measurements with quantum probes.” npj quantum information, 7, Pp. 55. 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.
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.
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.
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.
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.
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.
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.
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.
2019
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.
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.
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.
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.
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.
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.