Spectral-hole memory for light at the single-photon level

Abstract

We demonstrate a solid-state spin-wave optical memory based on stopped light in a spectral hole. A long-lived narrow spectral hole is created by optical pumping in the inhomogeneous absorption profile of a ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ crystal. Optical pulses sent through the spectral hole experience a strong reduction of their group velocity and are spatially compressed in the crystal. A short Raman pulse transfers the optical excitation to the spin state before the light pulse exits the crystal, effectively stopping the light. After a controllable delay, a second Raman pulse is sent, which leads to the emission of the stored photons. We reach storage and retrieval efficiencies for bright pulses of up to $39%$ in a 5-mm-long crystal. We also show that our device works at the single-photon level by storing and retrieving $3\ensuremath{-}\ensuremath{\mu}\mathrm{s}$-long weak coherent pulses with efficiencies up to $31%$, demonstrating the most efficient spin-wave solid-state optical memory at the single-photon level so far. We reach an unconditional noise level of $(9\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ photons per pulse in a detection window of $4\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{s}$, leading to a signal-to-noise ratio of $33\ifmmode\pm\else\textpm\fi{}4$ for an average input photon number of 1, making our device promising for long-lived storage of nonclassical light.