Quantum storage of photonic entanglement in a crystal

Solid quantum memory progress Harnessing entanglement between light and material systems is of interest for future quantum information technologies. Two groups report advances in the development of the light–matter quantum interface that could pave the way for the construction of multiplexed quantum repeaters for long-distance quantum networks. Clausen et al . demonstrate entanglement between a photon at the telecommunication wavelength (1,338 nanometres) and a single collective atomic excitation stored in a neodymium-doped Y 2 SiO 5 crystal. Saglamyurek et al . use a thulium-doped LiNbO 3 waveguide to achieve a similar entanglement. Harnessing entanglement between light and material systems is of interest for future quantum information technologies. Here, entanglement is demonstrated between a photon at the telecommunication wavelength (1,338 nm) and a single collective atomic excitation stored in a crystal. These resources pave the way for building multiplexed quantum repeaters for long-distance quantum networks. Entanglement is the fundamental characteristic of quantum physics—much experimental effort is devoted to harnessing it between various physical systems. In particular, entanglement between light and material systems is interesting owing to their anticipated respective roles as ‘flying’ and stationary qubits in quantum information technologies (such as quantum repeaters 1 , 2 , 3 and quantum networks 4 ). Here we report the demonstration of entanglement between a photon at a telecommunication wavelength (1,338 nm) and a single collective atomic excitation stored in a crystal. One photon from an energy–time entangled pair 5 is mapped onto the crystal and then released into a well-defined spatial mode after a predetermined storage time. The other (telecommunication wavelength) photon is sent directly through a 50-metre fibre link to an analyser. Successful storage of entanglement in the crystal is proved by a violation of the Clauser–Horne–Shimony–Holt inequality 6 by almost three standard deviations (S = 2.64 ± 0.23). These results represent an important step towards quantum communication technologies based on solid-state devices. In particular, our resources pave the way for building multiplexed quantum repeaters 7 for long-distance quantum networks.