Wolfgang Sohler

If two parties were to exploit today’s quantum key distribution (QKD) systems, they would be limited to being at most ~100 km apart [1]. It is possible to overcome this limit with a quantum repeater that exploits quantum memories for qubit synchronization [1]. Among other criteria desired for quantum memories, simultaneous storage of multiple qubits (multiplexing) and recall of any desired qubit on-demand is required for a quantum repeater [1,2]. These properties are generally associated with the ability to trigger the re-emission of any previously stored qubit at a desired time [3]. We will argue that this view is too restricted, and that it is possible to build a quantum repeater using quantum memories that allow storage of frequency multiplexed qubits supplemented with frequency-selective read-out on demand. Furthermore we report on measurements exploiting the atomic frequency comb protocol in a Ti:Tm:LiNbO3 waveguide cooled to 3 K [4,5] that shows the required on-demand readout with average fidelities of 0.95 ± 0.03 thereby significantly violating the maximum fidelity of 0.67 possible using a classical memory. Our demonstration constitutes an important step towards the development of a quantum repeater. [1] N. Sangouard et al., Reviews of Modern Physics 83, 33 (2011). [2] A. I. Lvovsky, W. Tittel, and B.C. Sanders, Nature Photonics 3, 706 (2009). [3] C. Simon et al., Phys. Rev. Lett. 98, 190503 (2007). [4] M. Afzelius et al., Phys. Rev. A 79, 052329 (2009). [5] E. Saglamyurek et al., Nature 469, 512 (2011).