Long-Lived Spectrally-Multiplexed Quantum Memory In A Thulium-Doped Crystal*

Rare-earth ion-doped crystals with long optical coherence lifetimes can serve as frequency-multiplexed, long-lived optical quantum memories, which are of an essential requirement towards building frequency multiplexed quantum repeaters for long-distance quantum communications. Towards this end, we investigate a thulium-doped crystal (Tm: YGG) at temperatures as low as 500 mK and low magnetic fields. This crystal offers an optical coherence lifetime exceeding one millisecond and a ground-state Zeeman level lifetime as long as tens of seconds. We take advantage of such exceptional features to show several key demonstrations; storage of optical pulses for up to 100 μs of storage time over a few MHz-wide of storage bandwidth, frequency-selective read-out of 11 distinct stored frequency modes, storage of heralded single photons confirming the quantum nature of our thulium quantum memory. Our results suggest that Tm: YGG can be a potential candidate to be used as an optical quantum memory in frequency multiplexed quantum repeater architecture.