A 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 Pr${}^{3+}$:Y${}_2$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 $5\mathrm{mm}$-long crystal. We also show that our device works at the single photon level by storing and retrieving $3\mu \mathrm{s}$-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 $(9\pm 1)\times 10^{-3}$ photons per pulse in a detection window of $4\mu \mathrm{s}$ leading to a signal-to-noise ratio of $33\pm 4$ for an average input photon number of 1, making our device promising for long-lived storage of non-classical light.

5 pages, 4 figures

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