Storage and Retrieval of Single Photons Transmitted Between Remote Quantum Memories

Two groups this week report a significant step on the long road to quantum computing: the storage and retrieval of single photons onto and from atomic quantum memories. Chanelière et al. produced single photons from an atomic quantum memory in one lab, transported them through a 100-metre-long optical fibre and stored them for a time in a second memory. The atomic excitation was then converted back into a single photon. Previously, weak coherent laser pulses have been stopped and retrieved in atomic media, but single photons are ideal for realizing quantum bits. Eisaman et al. report a similar approach, using the coherent control technique known as electromagnetically induced transparency for the generation, transmission and storage of single photons. A third paper reports progress in another technology critical for quantum communication and computation: the storage and distribution of entangled quantum states. Chou et al. have achieved entanglement between two samples of atoms separated by 2.8 metres that jointly store one quantum bit of information. An elementary quantum network operation involves storing a qubit state in an atomic quantum memory node, and then retrieving and transporting the information through a single photon excitation to a remote quantum memory node for further storage or analysis. Implementations of quantum network operations are thus conditioned on the ability to realize matter-to-light and/or light-to-matter quantum state mappings. Here we report the generation, transmission, storage and retrieval of single quanta using two remote atomic ensembles. A single photon is generated from a cold atomic ensemble at one site 1, and is directed to another site through 100 metres of optical fibre. The photon is then converted into a single collective atomic excitation using a dark-state polariton approach2. After a programmable storage time, the atomic excitation is converted back into a single photon. This is demonstrated experimentally, for a storage time of 0.5 microseconds, by measurement of an anti-correlation parameter. Storage times exceeding ten microseconds are observed by intensity cross-correlation measurements. This storage period is two orders of magnitude longer than the time required to achieve conversion between photonic and atomic quanta. The controlled transfer of single quanta between remote quantum memories constitutes an important step towards distributed quantum networks.