A 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 μs-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 \(9pm1\)times 10^-3 photons per pulse in a detection window of 4 μs 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.