Quantum communication and memory with entangled atomic ensembles

Quantum information processing, a rapidly developing area of physics, involves operations with quantum states, in which information can be encoded. The number of degrees of freedom available for the encoding grows dramatically, as compared to classical encoding, due to the existence of quantum superpositions. One of the necessary components for quantum information processing is the quantum state transfer between distant nodes of a quantum network. Light is a natural agent for such a transfer, whereas atoms are natural agents for storing the information. Therefore, a problem of efficient quantum state interface between light and atoms has become one of the major problems in the field. Conceptually the simplest scenario for such an exchange involves an interaction of a single photon (a flying qubit) with a single atom (a stationary qubit). However, efficient interaction of this kind requires the atom and the photon be coupled to a high finesse cavity [1,2]. Although spectacular results have been achieved along these lines, the quantum interface between the flying and stationary qubits remains a challenge. Several years ago we have initiated a new approach to this problem involving atomic ensembles and multi-photon states. In the first attempt [3] the transfer of a squeezed state of light onto a spin squeezed state of a cold atomic sample via complete absorption of light has been demonstrated. Further theoretical development which makes use of Raman processes followed [4]. However, an off-resonant type of interaction as suggested in [5,6] proved to be more versatile and promising for the quantum interface. This interaction, which has been earlier used for detecting quantum noise of atomic states [7,3] and for generation of spin squeezed atomic state via a quantum nondemolition measurement [8] is the basis for the experiments discussed in this paper.