A spectral hole memory for light at the single photon level

TL;DR

This paper demonstrates a solid-state optical memory capable of storing and retrieving single-photon level light with high efficiency and low noise, using spectral hole burning and spin-wave techniques in a Pr$^{3+}$:Y$_2$SiO$_5$ crystal.

Contribution

It introduces a novel spectral hole memory that achieves the most efficient single-photon level storage in a solid state device to date.

Findings

Storage and retrieval efficiencies up to 39% for bright pulses.

Successful storage and retrieval of weak coherent pulses at the single-photon level.

Low noise level with a signal-to-noise ratio of 33 for single photons.

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 $3\,\mathrm{\mu s}$-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 $(9\pm1)\times 10^{-3}$ photons per pulse in a detection window of $4\,\mathrm{\mu s}$ 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.