Controlled rephasing of collective spin excitations in cold atom quantum memories for temporally multiplexed quantum repeaters

Pau Farrera^a, Boris Albrecht^a, Matteo Cristiani^a and Hugues de Riedmatten^a,b

^a ICFO-The Institute of Photonic Sciences

^b ICREA-Institució Catalana de Recerca i Estudis Avançats

Quantum memories (QM) for light, which allow a coherent and reversible transfer of quantum information between light and long lived matter quantum bits, are crucial devices in quantum information science [1]. In particular they enable to interface stationary quantum bits (encoded in atom-like systems) and flying qubits (encoded in photons). QMs are for example needed for the implementation of ultra-long distance quantum communication using quantum repeater architectures [2]. An important capability for quantum memories is the ability to store multiple qubits at the same time and retrieve them selectively. Laser cooled atomic gases are currently one of the best systems for QM applications, but so far temporal multiplexing with quantum information has not been achieved in these systems. In this contribution, we show a significant step towards achieving this goal. Our experiment is based on a type of QM that has proven to be one of the most advanced quantum repeater building blocks [3], and consists on a laser cooled Rubidium atomic ensemble where we create correlated photon pairs. One of the photons is directly emitted while the other one can be stored in the form of a single collective atomic spin excitation (spin wave) before being mapped into a single photon. Applying a controlled and reversible dephasing to the spin wave allows the creation of photon-spin pairs in different temporal modes and to map into a single photon only the desired spin excitation [4]. So far we have been able to observe the controlled dephasing and rephasing of single spin waves. This has allowed us to perform a selective mapping into photons between collective single spin excitations created at two different times in the same atomic ensemble. Our next goal is to use this system to demonstrate the operation of a building block for a temporally multiplexed quantum repeater protocol, which promises a significant rate enhancement.

[1] H. J. Kimble, “The quantum internet”, Nature 453, 1023–1030 (2008).

[2] N. Sangouard, C. Simon, H. de Riedmatten, and N. Gisin, “Quantum repeaters based on atomic ensembles and linear optics”, Rev. Mod. Phys. 83, 33–80 (2011).

[3] L.-M. Duan, M. D. Lukin, J. I. Cirac and P. Zoller, "Long distance quantum communication with atomic ensembles and linear optics", Nature 414, 413-418 (2001).

[4] C. Simon, H. de Riedmatten and M. l. Afzelius, "Temporally multiplexed quantum repeaters with atomic gases", Phys. Rev. A 82, 010304(R) (2010).