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Raman quantum memory based on an ensemble of nitrogen-vacancy centers coupled to a microcavity

Khabat Heshami, Charles Santori, Behzad Khanaliloo, Chris Healey, Víctor M. Acosta, Paul E. Barclay, Christoph Simon

Published Jan 1, 2014• 0 citations
Publisher

Abstract

We propose a scheme to realize quantum memories based on Raman coupling for storing photons in the electronic spin of NV–ensembles. We include all optical transitions in a 9-level configuration and evaluate the efficiencies.
DOI
10.1364/cleo_qels.2014.ff2a.4

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We propose a scheme to realize optical quantum memories in an ensemble of nitrogen-vacancy centers in diamond that are coupled to a microcavity. The scheme is based on off-resonant Raman coupling, which allows one to circumvent optical inhomogeneous broadening and store optical photons in the electronic spin coherence. This approach promises a storage time of order 1 s and a time-bandwidth product of order 107{10}^{7}107. We include all possible optical transitions in a nine-level configuration, numerically evaluate the efficiencies, and discuss the requirements for achieving high efficiency and fidelity.
Candidate platforms in the global endeavour to build long distance quantum networks need to satisfy a variety of properties to act as a network node. These include - interfacing between stationary spins and photonic flying qubits, long quantum memory times, indistinguishable photons and a large photon flux. Cavities operating in the weak coupling regime offer the advantages that the spontaneous emission rate of the emitter is enhanced while the loss rate from the cavity is also high. With optimized output couplers, this results in an increased photon extraction efficiency and the proportion of coherent photons is also improved. Cavity-coupled Nitrogen-vacancy centers in diamond (NV) fulfill most of these criteria with one weakness remaining - sensitivity to electronic noise [1]–[3].
This article reviews efforts to build a new type of quantum device, which combines an ensemble of electronic spins with long coherence times, and a small-scale superconducting quantum processor. The goal is to store over long times arbitrary qubit states in orthogonal collective modes of the spin-ensemble, and to retrieve them on-demand. We first present the protocol devised for such a multi-mode quantum memory. We then describe a series of experimental results using NV (as in nitrogen vacancy) center spins in diamond, which demonstrate its main building blocks: the transfer of arbitrary quantum states from a qubit into the spin ensemble, and the multi-mode retrieval of classical microwave pulses down to the single-photon level with a Hahn-echo like sequence. A reset of the spin memory is implemented in-between two successive sequences using optical repumping of the spins.
Quantum memories are a key technology enabling arbitrarily-scalable optical quantum information processing architectures via active synchronisation and multiplexing of quantum information. Optically active confined impurities, called colour centres, in a spin-free and highly transparent diamond host material are promising systems to realise such devices in the solid-state, suitable for integration in a scalable guided-wave on-chip photonic platform. In this thesis, the transition from optical Raman quantum memories in atomic vapours to solid-state systems is prepared, exploring two diamond-based systems: On the basis of ensembles of neutrally charged nitrogen vacancy centres, desirable properties and criteria of solid-state systems used for optical quantum memories are developed. These include non-vanishing off-resonant Raman coupling, variable acceptance bandwidth for broadband pulses and cross-linear polarised optical transitions to prevent the generation of noise during the Raman memory protocol, which have been realised experimentally by applying external uniaxial stress to the system. However, attempts to measure the very short orbital ground state lifetime affirmed unfavourable theoretical estimations, rendering this centre unsuitable for memory applications. As a potential alternative, the negatively charged silicon vacancy centre has been investigated, which offers a large ground state splitting with extended life and coherence times and narrow inhomogeneous spectral lines due to its inversion symmetry. In an ensemble of silicon vacancy centres, the ground state lifetime is measured and coherent manipulation via resonant Ramsey interference and Hahn echo is demonstrated. In a second step, off-resonant two-photon transitions are achieved implementing stimulated Raman adiabatic passage between the orbital ground states. Finally, strong light-matter interaction of a weak signal field with the ensemble via phase-sensitive four-wave mixing is shown forming the basis for integrated Raman-based memories and single photon nonlinearities in ensembles of silicon vacancy centres and closely related defects.
We have built a hybrid system composed of a superconducting flux qubit (the processor) and an ensemble of nitrogen-vacancy centers in diamond (the memory) that can be directly coupled to one another, and demonstrated how information can be transferred from the flux qubit to the memory, stored, and subsequently retrieved. We have established the coherence properties of the memory and succeeded in creating an entangled state between the processor and memory, demonstrating how the entangled state's coherence is preserved. Our results are a significant step towards using an electron spin ensemble as a quantum memory for superconducting qubits.
We propose a multimode quantum memory protocol able to store the quantum state of the field in a microwave resonator into an ensemble of electronic spins. The stored information is protected against inhomogeneous broadening of the spin ensemble by spin-echo techniques resulting in memory times orders of magnitude longer than previously achieved. By calculating the evolution of the first and second moments of the spin-cavity system variables for current experimental parameters, we show that a memory based on nitrogen vacancy center spins in diamond can store a qubit encoded on the |0> and |1> Fock states of the field with 80% fidelity and outperform classical memory strategies for storage times ≤69 μs.