Cavity-Enhanced Spin-Wave Solid-State Quantum Memory

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mrow><a:msup><a:mrow><a:mi>Pr</a:mi></a:mrow><a:mrow><a:mn>3</a:mn><a:mo>+</a:mo></a:mrow></a:msup></a:mrow><a:mo>:</a:mo><a:msub><a:mrow><a:mi mathvariant="normal">Y</a:mi></a:mrow><a:mrow><a:mn>2</a:mn></a:mrow></a:msub><a:mrow><a:msub><a:mrow><a:mi>SiO</a:mi></a:mrow><a:mrow><a:mn>5</a:mn></a:mrow></a:msub></a:mrow></a:mrow></a:math> 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 <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mrow><d:mo stretchy="false">(</d:mo><d:mrow><d:mn>40</d:mn><d:mo>±</d:mo><d:mn>2</d:mn></d:mrow><d:mo stretchy="false">)</d:mo><d:mo>%</d:mo></d:mrow></d:math> 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.