Solid state optical quantum memories

Quantum information science (QIS) aims at controlling quantum coherence effects in light and matter in order to enable new information processing capabilities that are not possible with classical resources. Quantum information can be encoded in photons that are used as flying quantum bits that can be sent over long distances, and in atoms that are used as stationary quantum bits to store and process the information. The efficient transfer of quantum information between single photons and atomic systems is an important requirement for QIS. It enables the realization of a quantum memory for light, which is required for many applications in QIS. In particular, it is necessary for scalable implementations of quantum information networks. It is also a crucial element of quantum repeaters that have been proposed to overcome losses in the transport of photonic quantum information over long distances. In this talk, I will concentrate on our latest results (obtained at the University of Geneva), where we demonstrate the quantum storage of photonic entanglement in a Neodymium doped crystal. We have developed a narrow band non-degenerate photon pair source adapted for quantum repeater architectures. One photon is in the telecom band and the other one is compatible with the spectral requirement of a Nd doped crystal QM (35 MHz bandwidth). Entanglement between the photon at telecommunication wavelength and a collective atomic excitation stored in the crystal is demonstrated by mapping the twin photon onto the crystal, using an atomic frequency comb protocol. The successful entanglement mapping is demonstrated by the violation of a Bell inequality. These results represent an important step towards quantum communication technologies based on solid-state devices.