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.
L Cohen, Pilnyak, Y, Istrati, D, Retzker, A, and Eisenberg, HS . 7/15/2016.
“Demonstration Of A Quantum Error Correction For Enhanced Sensitivity Of Photonic Measurements”. Physical Review A, 94, 1. .
Publisher's Version 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.
T Unden, Balasubramanian, P, Louzon, D, Vinkler, Y, Plenio, MB , Markham, M, Twitchen, D, Stacey, A, Lovchinsky, I, Sushkov, AO , Lukin, MD , Retzker, A, Naydenov, B, McGuinness, LP , and Jelezko, F. 6/9/2016.
“Quantum Metrology Enhanced By Repetitive Quantum Error Correction”. Physical Review Letters, 116, 23. .
Publisher's Version 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, Randall, J, Webster, SC , Lake, K, Webb, AE , Cohen, I, Navickas, T, Lekitsch, B, Retzker, A, and Hensinger, WK . 6/5/2016.
“Entangling Gate With Trapped Ions Using Long”. 2016 Conference On Lasers And Electro-Optics (Cleo). .
Publisher's Version 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.
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.
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, Schwarz, I, Jelezko, F, Retzker, A, and Plenio, MB . 2/24/2016.
“Resonance-Inclined Optical Nuclear Spin Polarization Of Liquids In Diamond Structures”. Physical Review B, 93, 6. .
Publisher's Version 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
R Nigmatullin, del Campo, A, De Chiara, G, Morigi, G, Plenio, MB , and Retzker, A. 1/25/2016.
“Formation Of Helical Ion Chains”. Physical Review B, 93, 1. .
Publisher's Version 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.
J Scheuer, Schwartz, I, Chen, Q, Schulze-Sunninghausen, D, Carl, P, Hofer, P, Retzker, A, Sumiya, H, Isoya, J, Luy, B, Plenio, MB , Naydenov, B, and Jelezko, F. 1/18/2016.
“Optically Induced Dynamic Nuclear Spin Polarisation In Diamond”. New Journal Of Physics, 18. .
Publisher's Version 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.
I Baumgart, Cai, JM , Retzker, A, Plenio, MB , and Wunderlich, C. 2016.
“Ultrasensitive Magnetometer Using A Single Atom”. Physical Review Letters, 116, 24.