A Solid-State Multimode Long-Lived Optical Quantum Memory for High-Rate Quantum Repeaters

Antariksha Das

QuTech & Kavli Institute of Nanoscience, TU Delft

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. The most promising approach for achieving long-distance quantum communication is to use quantum repeaters, many of which employ optical quantum memories to store quantum information. Space-borne quantum memory-equipped satellites additionally offer solutions to overcome the limitation of entanglement distribution rates based on fiber-based repeaters over terrestrial links. Rare earth ion-doped crystals 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 a number of essential demonstrations. Such as the storage of optical pulses for up to 100 μs of optical storage time, frequency-multiplexed storage of several frequency modes, and a proof of principle demonstration of frequency-selective read-out of the stored frequency modes. Furthermore, I will also shed some light on the optical coherence and relaxation dynamics of Tm: YGG, which is crucial in order to build an efficient long-lived quantum memory. Our results suggest that Tm: YGG can be a potential candidate to be used as a long-lived multimode optical quantum memory in a frequency multiplexed quantum repeater architecture.