2016

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.
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.
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.
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.
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.
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.