Quantum teleportation of optical coherent states was demonstrated experimentally using squeezed-state entanglement. The quantum nature of the achieved teleportation was verified by the experimentally determined fidelity Fexp = 0.58 +/- 0.02, which describes the match between input and output states. A fidelity greater than 0.5 is not possible for coherent states without the use of entanglement. This is the first realization of unconditional quantum teleportation where every state entering the device is actually teleported.

During the past decade the interaction of light with multi-atom ensembles has attracted a lot of attention as a basic building block for quantum information processing and quantum state engineering. The field started with the realization that optically thick free space ensembles can be efficiently interfaced with quantum optical fields. By now the atomic ensemble - light interfaces have become a powerful alternative to the cavity-enhanced interaction of light with single atoms. We discuss various mechanisms used for the quantum interface, including quantum nondemolition or Faraday interaction, quantum measurement and feedback, Raman interaction and electromagnetically induced transparency. The paper provides a common theoretical frame for these processes, describes basic experimental techniques and media used for quantum interfaces, and reviews several key experiments on quantum memory for light, quantum entanglement between atomic ensembles and light, and quantum teleportation with atomic ensembles. We discuss the two types of quantum measurements which are most important for the interface: homodyne detection and photon counting. The paper concludes with an outlook on the future of atomic ensembles as an enabling technology in quantum information processing.Comment: 57 pages, 22 figures, 4.6MB; published versio

Entanglement is considered to be one of the most profound features of quantum mechanics 1,2 . An entangled state of a system consisting of two subsystems cannot be described as a product of the quantum states of the two subsystems 9,10,16,17 . In this sense the entangled system is considered inseparable and nonlocal. It is generally believed that entanglement manifests itself mostly in systems consisting of a small number of microscopic particles. Here we demonstrate experimentally the entanglement of two objects, each consisting of about 10 12 atoms. Entanglement is generated via interaction of the two objects -more precisely, two gas samples of cesium atoms -with a pulse of light, which performs a non-local Bell measurement on collective spins of the samples 14 . The entangled spin state can be maintained for 0.5 millisecond. Besides being of fundamental interest, the robust, long-lived entanglement of material objects demonstrated here is expected to be useful in quantum information processing, including teleportation 3-5 of quantum states of matter and quantum memory. In this Letter we describe an experiment on the generation of entanglement between two separate samples of atoms containing 10 12 atoms each, along the lines of a recent proposal 14 . Besides the fact that we demonstrate a quantum entanglement at the level of macroscopic objects, our experiment proves feasible a new approach to the quantum interface between light and atoms suggested in 14,15 and paves the road towards the other protocols proposed there, such as the teleportation of atomic states and quantum memory. The entanglement is generated through a non-local Bell measurement on the two samples' spins performed by transmitting a pulse of light through the samples.The ideal EPR entangled state of two sub-systems described by continuous non- . Recently in 16,17 , the necessary and sufficient condition for the entanglement or inseparability for such Gaussian quantum variables has been cast in a form of an inequality involving only the variances of variables: