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Quantum 2.0 Conference and Exhibition

Cavity-Enhanced on-demand Spin-Wave Solid State Quantum Memory

Leo Feldmann, Sören Wengerowsky, Stefano Duranti, Hugues de Riedmatten

Published Jan 1, 2024• 0 citations
Publisher

Abstract

We report the first cavity-enhanced on-demand atomic frequency comb spin-wave quantum memory. We used a Pr3+ :Y2SiO5crystal embedded in an impedance-matched cavity to achieve a device efficiency of 38 % for weak coherent pulses.
DOI
10.1364/quantum.2024.qw4b.4

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We report on the realization of an efficient solid-state spin-wave quantum memory, with on-demand readout, using the full atomic frequency comb (AFC) scheme in a Pr^{3+}:Y_{2}SiO_{5} crystal embedded in an impedance-matched cavity. We demonstrate operation at the single-photon level by storing weak coherent states with an efficiency up to (40±2)% and a signal-to-noise ratio of 14 for an input photon number of 0.42 photons per pulse. We also investigated the enhancement of the incoherent noise due to the impedance-matched cavity and characterized the quantum memory performance, showing a two-way transfer from excited to spin states and back of up to 83%. Finally, we confirmed the quantum nature of our memory by storing nonclassical states of light, i.e., a heralded single photon from a nondegenerate spontaneous parametric down-conversion source, and achieved nonclassical correlations between the heralding and the stored-and-retrieved photon. These results demonstrate that impedance-matched AFC spin-wave quantum memories with on-demand readout can be used for experiments involving the storage of photonic quantum states. They also open the door to solid-state on-demand quantum memories with very high efficiencies, serving as a key resource for quantum networks and quantum repeaters.
We report on the experimental demonstration of an optical spin-wave memory, based on the atomic frequency comb (AFC) scheme, where the storage efficiency is strongly enhanced by an optical cavity. The cavity is of low finesse, but operated in an impedance matching regime to achieve high absorption in our intrinsically low-absorbing Eu 3+ :Y 2 SiO 5 crystal. For storage of optical pulses as an optical excitation (AFC echoes), we reach efficiencies of 53% and 28% for 2 μ s and 10 μ s delays, respectively. For a complete AFC spin-wave memory we reach an efficiency of 12%, including spin-wave dephasing, which is a 12-fold increase with respect to previous results in this material. This result is an important step towards the goal of making efficient and long-lived quantum memories based on spin waves, in the context of quantum repeaters and quantum networks.
We report on the high-efficiency storage and retrieval of weak coherent optical pulses and photonic qubits in a cavity-enhanced solid-state quantum memory. By using an atomic frequency comb (AFC) memory in a Pr 3+ :Y 2 SiO 5 crystal embedded in an impedance-matched cavity, we stored weak coherent pulses at the single photon level with up to 62% efficiency for a pre-determined storage time of 2 µs. We also confirmed that the impedance-matched cavity enhances the efficiency for longer storage times up to 70 µs. Harnessing the temporal multimodality of the AFC scheme, we stored weak coherent time-bin qubits with a record (51 ± 2%) efficiency and a fidelity over (94.8 ± 1.4)%, limited by imperfections in the qubits creation and measurement.
We demonstrate the first solid-state spin-wave optical quantum memory with on-demand read-out. Using the full atomic frequency comb scheme in a ${\mathrm{Pr}}^{3+}\ensuremath{\mathbin:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ crystal, we store weak coherent pulses at the single-photon level with a signal-to-noise ratio >10>10>10. Narrow-band spectral filtering based on spectral hole burning in a second ${\mathrm{Pr}}^{3+}\ensuremath{\mathbin:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ crystal is used to filter out the excess noise created by control pulses to reach an unconditional noise level of $(2.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ photons per pulse. We also report spin-wave storage of photonic time-bin qubits with conditional fidelities higher than achievable by a measure and prepare strategy, demonstrating that the spin-wave memory operates in the quantum regime. This makes our device the first demonstration of a quantum memory for time-bin qubits, with on-demand read-out of the stored quantum information. These results represent an important step for the use of solid-state quantum memories in scalable quantum networks.
We present an extension of the atomic frequency comb protocol that utilizes the Stark effect to perform noisefree, on-demand, control.An experimental realization of this protocol was implemented in the Pr 3+ :Y 2 SiO 5 solid-state system, and a recall efficiency of 38% for a 0.8 μs storage time was achieved.Experiments were performed with both bright pulses as well as weak-coherent states, the latter achieving a signal-to-noise ratio of 570 ± 120 using input pulses with an average photon number of ∼0.1.The principal limitation for a longer storage time was found to be the minimum peak width attainable for Pr 3+ :Y 2 SiO 5 .We employ an adaptation of an established atomic frequency comb model to investigate an on-demand, wide-bandwidth, memory based on Eu 3+ :Y 2 SiO 5 .From this, we determine that a storage time as long as 100 μs may be practical even without recourse to spin-wave storage.