Optical quantum memories, the quantum counterpart of classical computer memories, are used to faithfully store quantum information encoded into photons. Such memories are an indispensable component of a quantum repeater, which will allow the distribution of entanglement, and hence quantum communication, in theory over arbitrarily long distances.
In our group, we employ cryogenically-cooled rare-earth ion-doped (REI-doped) crystals in conjunction with the so-called atomic frequency comb (AFC) protocol to develop quantum memories. This approach promises high storage fidelity, large storage bandwidth, long storage times, and high input-output efficiency – all of which are key properties for a quantum repeater. Our goals include the development of spectrally-multiplexed quantum memories with fidelities above 99%, around 100 spectral channels with bandwidths between 100 MHz and 1 GHz, storage times on the order of 100 microseconds, and efficiencies exceeding 90%.
Over the past years, we have specialized in the use of thulium and erbium-doped materials, and transitions that allow storing photons at 795 and 1532 nm, respectively. Our progress so far includes recall fidelities of more than 97%, particularly large storage bandwidths of several GHz, and the storage of photons in up to 26 spectrally-multiplexed channels of 100 MHz bandwidth.
Our current research focusses on developing quantum memories within impedance-matched optical cavities, which promises meeting the requirement of high efficiency, as well as on memories based on thulium-doped yttrium gallium garnet (Tm:YGG), a novel crystal that features exceptionally long optical coherence times and is therefore a strong candidate for achieving the targeted storage times. We anticipate achieving a performance level that allows integrating our quantum memories with our sources of entangled photon pairs into elementary quantum repeater links within the next year, an exciting development at the nexus between fundamental science and practical applications of quantum technology.