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

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 Pr3+ : Y2SiO5 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-mu 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 +/- 1) x 10(-3) photons per pulse in a detection window of 4 mu s, leading to a signal-to-noise ratio of 33 +/- 4 for an average input photon number of 1, making our device promising for long-lived storage of nonclassical light.