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arXiv:1301.1500 · Physical Review Letters

Quantum Memory for Microwave Photons in an Inhomogeneously Broadened Spin Ensemble

Brian Julsgaard, Cécile Grèzes, Patrice Bertet, Klaus Mølmer

Published Jun 18, 2013• 159 citations
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Abstract

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.

DOI
10.1103/physrevlett.110.250503

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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.
A quantum memory at microwave frequencies, able to store the state of multiple superconducting qubits for long times, is a key element for quantum information processing. Electronic and nuclear spins are natural candidates for the storage medium as their coherence time can be well above one second. Benefiting from these long coherence times requires to apply the refocusing techniques used in magnetic resonance, a major challenge in the context of hybrid quantum circuits. Here we report the first implementation of such a scheme, using ensembles of nitrogen-vacancy (NV) centres in diamond coupled to a superconducting resonator, in a setup compatible with superconducting qubit technology. We implement the active reset of the NV spins into their ground state by optical pumping and their refocusing by Hahn echo sequences. This enables the storage of multiple microwave pulses at the picoWatt level and their retrieval after up to 35μ35 μ35μs, a three orders of magnitude improvement compared to previous experiments.
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.
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.
We report the storage of microwave pulses at the single-photon level in a spin-ensemble memory consisting of 1010{10}^{10}1010 nitrogen-vacancy centers in a diamond crystal coupled to a superconducting LCLCLC resonator. The energy of the signal, retrieved $100\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{s}$ later by spin-echo techniques, reaches 0.30.3%0.3 of the energy absorbed by the spins. This 0.30.3%0.3 storage efficiency is quantitatively accounted for by simulations. This figure of merit is sufficient to envision first implementations of a quantum memory for superconducting qubits.
Inhomogeneously broadened spin ensembles play an important role in present-day implementation of hybrid quantum processing architectures. When coupled to a cavity, such an ensemble may serve as a multimode quantum memory for the cavity field, and by employing spin-refocusing techniques the quantum-memory time can be extended to the coherence time of individual spins in the ensemble. In this paper, we investigate such a memory protocol capable of storing an unknown cavity-field state, and we examine separately the various constituents of the protocol: the storage and readout part, the memory hold time with the spin ensemble and cavity field decoupled, and the parts employing spin-refocusing techniques. Using both analytical and numerical methods, we derive how the obtainable memory performance scales with various physical parameters.