Towards a Long-Lived and Efficient Photonic Quantum Memory in a Thulium-Doped Crystal

Abstract

A future quantum network will allow distributing entanglement over in principle arbitrarily long distances. This topic holds importance for applications in quantum information science as well as for fundamental investigations, and currently receives significant attention around the world. It will enable a plethora of new applications such as secure communication, computation, and metrology. However, the task of building a globe-spanning quantum network is difficult due to the detrimental effect of loss on photons during transmission. The most promising approach for achieving long-distance quantum communication over terrestrial links is to use quantum repeaters, many of which employ optical quantum memories to store quantum information. Rare-earth ion-doped materials with long optical coherence lifetimes are arguably ideally suited for building such quantum memories. Towards this end, we investigate a thulium-doped yttrium gallium garnet crystal (Tm: YGG) at temperatures as low as 500 mK. This crystal offers an optical coherence time 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. Such as the storage of optical pulses for up to 100 μs of optical storage time, frequency-multiplexed storage of distinct frequency modes, and a proof of principle demonstration of frequency-selective read-out of the stored frequency modes. We characterize the optical coherence and relaxation dynamics of this crystal, which is crucial in order to build an efficient long-lived quantum memory. Furthermore, we also discuss our efforts towards building a monolithic and alignment-free (pre-aligned) highly-efficient cavity quantum memory in Tm: YGG. Our results suggest that Tm: YGG can be a potential candidate to be used as a long-lived multimode optical quantum memory in frequency multiplexed quantum repeater architecture.