Towards an alignment-free, impedance-matched cavity quantum memory in a thulium-doped crystal

Photonic quantum memories are key constituents of quantum repeaters for long-distance quantum communications. Most of the practical implementations of quantum communication protocols require a quantum memory with a long storage time and high storage efficiencies. Rare earth ion-doped crystals are arguably ideally suited for building such quantum memories due to their exceptionally long coherence times and narrow optical line widths. Towards this end, we investigate 1% thulium-doped yttrium gallium garnet (Tm³⁺:YGG) crystal, cooled to temperatures as low as 500 mK. This crystal offers an optical coherence lifetime exceeding one millisecond and a ground-state Zeeman level lifetime as long as tens of seconds. By taking advantage of such exceptional features, we already have shown several key demonstrations: storage of optical pulses up to 100 μs storage time; frequency-multiplexed storage, and frequency-selective read-out of stored frequency modes. But for all of these experiments, the storage efficiency was a few percent, which was mainly limited by the low absorption of the crystal. To overcome this problem, we implement an atomic frequency comb (AFC) quantum memory protocol inside an impedance-matched asymmetric cavity-crystal of Tm³⁺:YGG. In this work, we will discuss the developments toward building highly-efficient memories that can be realized in a cavity-memory system with a weakly absorbing crystal, by using the impedance matching condition.