Entanglement storage in atomic ensembles

– We propose to entangle macroscopic atomic ensembles in cavity using EPRcorrelated beams. We show how the field entanglement can be almost perfectly mapped onto the long-lived atomic spins associated with the ground states of the ensembles, and how it can be retrieved in the fields exiting the cavities after a variable storage time. Such a continuous variable quantum memory is of interest for manipulating entanglement in quantum networks. Entanglement is one of the most intriguing feature of quantum mechanics and, since the enunciation of the famous Einstein-Podolsky-Rosen paradox [1], has always attracted a lot of attention. In particular, it is at the heart of quantum communication and quantum information protocols such as quantum cryptography, teleportation, dense coding, quantum computing [2]. The past few years have seen many realizations of entangled beams in the continuous variable regime, using χ process in optical parametric amplifiers (OPAs) [3,4,5,6], Kerr effect in optical fibers [7, 8] or in cold atoms [9]. Efficient sources of entangled beams now exist and strong correlations have been achieved over rather broad bandwidth [6]. In order to build quantum communication networks in which light beams connect atomic ensembles, a major issue is to be able to store entanglement into the atoms [10, 11]. Entanglement between two atomic ensembles has been successfully demonstrated by Julsgaard et al. by sending pulses of coherent light through two atomic vapor cells [12] and measuring the outgoing field. However, the possibility to store entanglement between quantum-correlated beams into atoms remains to be demonstrated. In this Letter we propose a cw scheme to achieve entanglement between two cold atom ensembles placed in cavities by using EPR-entangled beams, as produced by OPAs for instance, and coherent control fields. The entanglement between the beams is mapped onto the ground state spins of the atoms and no measurement of the field is required. Given the long lifetime of the cold atoms spin the entanglement can thus be stored for a rather long time when the control field is switched off. It can then be retrieved in the vacuum modes exiting the cavities by switching on the control field again after a variable storage time. We then give a method to directly measure the entanglement of the outgoing beams and, consequently, the atomic entanglement in one simultaneous measurement of the EPR variances with two homodyne detections and a single local oscillator.