Quantum storage of photonic entanglement in a crystal
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 repeaters1, 2, 3 and quantum networks4). Here we report the demonstration of entanglement between a photon at a telecommunication wavelength (1,338nm) and a single collective atomic excitation stored in a crystal. One photon from an energy–time entangled pair5 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 inequality6 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 repeaters7 for long-distance quantum networks.