Nuala Timoney, Imam Usmani, Mikael Afzelius and Nicolas Gisin
Group of applied physics, University of Geneva, CH-1211 Geneva 4, Switzerland
Quantum communication provides a platform for provably secure communications. The fact that a quantum state cannot be copied is the reason why quantum communication is secure but also limits the distance over which quantum communication can be performed. This obstacle is overcome by a quantum repeater scheme. An essential part of this scheme is a quantum memory - where a photonic quantum state can be stored and retrieved at a time determined after the storage - a so called on demand memory .
A quantum memory based on a complete AFC scheme involves transferring the optical coherence to a spin coherence . To date such experiments have been hampered by the inhomogeneous spin broadening of the material, the longest times reported are of the order of 10 μs .
Here we report storage times almost an order of magnitude larger, using a 151Eu3+Y2S2O5 crystal. This is due to the smaller inhomogeneous spin linewidth (γIS) of the material. The inhomogeneous spin broadening does not cause a hard limit for the storage time in rare earth doped crystals. It is known that RF refocusing pulses will increase the storage time to the T2 of the spin transition . The T2 of a 151Eu3+Y2Si2O5 crystal has been measured to be 15.5 ms . The smaller inhomogeneous spin linewidth means that less bandwidth is required of future refocusing pulses, reducing the technical challenges of future experiments. In future experiments, we aim to perform this experiment at a single photon level.
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and Timoney et al., J Phys. B, To appear
 K. Heshami et al., Phys. Rev. A 83:032315, 2011
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