Atomic-ensemble-based quantum memory for sideband modulations

Interaction of a control and a signal field with an ensemble of three-level atoms allows direct mapping of the quantum state of the signal field into long-lived coherences of an atomic ground state. For a vapour of caesium atoms, using electromagnetically induced transparency (EIT) and Zeeman coherences, we compare the case where a tunable single sideband is stored independently of the other one to the case where the two symmetrical sidebands are stored using the same transparency window. We study the conditions in which simultaneous storage of two non-commuting variables carried by light and subsequent read-out is possible. We show that excess noise associated with spontaneous emission and spin relaxation is small, and we evaluate the quantum performance of our memory by measuring the signal transfer coefficient T and the conditional variance V and using the T–V criterion introduced by Hetet et al (2008 Phys. Rev. A 77), Roch et al (1997 Phys. Rev. Lett. 78) and Ralph and Lam (1998 Phys. Rev. Lett. 81) as a state-independent benchmark.