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:

The information carrier of today's communications, a weak pulse of light, is an intrinsically quantum object. As a consequence, complete information about the pulse cannot, even in principle, be perfectly recorded in a classical memory. In the field of quantum information this has led to a long standing challenge: how to achieve a high-fidelity transfer of an independently prepared quantum state of light onto the atomic quantum state 1-4 ? Here we propose and experimentally demonstrate a protocol for such quantum memory based on atomic ensembles. We demonstrate for the first time a recording of an externally provided quantum state of light onto the atomic quantum memory with a fidelity up to 70%, significantly

Quantum teleportation is an important ingredient in distributed quantum networks, and can also serve as an elementary operation in quantum computers. Teleportation was first demonstrated as a transfer of a quantum state of light onto another light beam; later developments used optical relays and demonstrated entanglement swapping for continuous variables. The teleportation of a quantum state between two single material particles (trapped ions) has now also been achieved. Here we demonstrate teleportation between objects of a different nature--light and matter, which respectively represent 'flying' and 'stationary' media. A quantum state encoded in a light pulse is teleported onto a macroscopic object (an atomic ensemble containing 10 caesium atoms). Deterministic teleportation is achieved for sets of coherent states with mean photon number (n) up to a few hundred. The fidelities are 0.58 +/- 0.02 for n = 20 and 0.60 +/- 0.02 for n = 5--higher than any classical state transfer can possibly achieve. Besides being of fundamental interest, teleportation using a macroscopic atomic ensemble is relevant for the practical implementation of a quantum repeater. An important factor for the implementation of quantum networks is the teleportation distance between transmitter and receiver; this is 0.5 metres in the present experiment. As our experiment uses propagating light to achieve the entanglement of light and atoms required for teleportation, the present approach should be scalable to longer distances.

Entanglement is a striking feature of quantum mechanics and an essential ingredient in most applications in quantum information. Typically, coupling of a system to an environment inhibits entanglement, particularly in macroscopic systems. Here we report on an experiment, where dissipation continuously generates entanglement between two macroscopic objects. This is achieved by engineering the dissipation using laser-and magnetic fields, and leads to robust event-ready entanglement maintained for 0.04s at room temperature. Our system consists of two ensembles containing about 10 12 atoms and separated by 0.5m coupled to the environment composed of the vacuum modes of the electromagnetic field. By combining the dissipative mechanism with a continuous measurement, steady state entanglement is continuously generated and observed for up to an hour. PACS numbers:To date, experiments investigating quantum superpositions and entanglement are hampered by decoherence. Its effects have been studied in several systems [1]. However, it was recognized [2] that the engineered interaction with a reservoir can drive the system into a desired steady state. In particular, dissipation common for two systems can drive them into an entangled state [3]. The idea of using and engineering dissipation rather than relying on coherent evolutions only, represents a paradigm shift with potentially significant practical advantages. Contrary to other methods, entanglement generation by dissipation does not require the preparation of a system in a particular input state and exists, in principle, for an arbitrary long time, which is expected to play an important role in quantum information protocols [4][5][6][7]. These features make dissipative methods inherently stable against weak random perturbations, with the dissipative dynamics stabilizing the entanglement.We report on the first demonstration of purely dissipative entanglement generation [8]. In contrast to previous approaches [9][10][11], entanglement is obtained without using measurements on the quantum state of the environment (i.e. the light field). The dissipation-based method implemented here is deterministic and unconditional and therefore fundamentally different from standard approaches such as the QND-based method [9] or the DLCZ protocol [4], which yield a separable state if the emitted photons are not detected. Furthermore, we report the creation of a steady state atomic entanglement by combining the dissipative mechanism proposed in [12] with continuous measurements. The generated entanglement is of the EPR type, which plays a central role in continuous variable quantum information processing [6,13], quantum sensing [14] and metrology [11,15,16]. Fig. 1a presents the principles of engineered dissipation in our system consisting of two 133 Cs ensembles, interacting with a y-polarized laser field at ω L . A pair of twolevel systems is encoded in the 6S 1/2 ground state sublevels |↑ I ≡ |4, 4 I , |↓ I ≡ |4, 3 I and |↑ II ≡ |4, −3 II , |↓ II ≡ |4, −4 II . Operators J ± I/II with J ...

We report on the experimental observation of quantum-network-compatible light described by a nonpositive Wigner function. The state is generated by photon subtraction from a squeezed vacuum state produced by a continuous wave optical parametric amplifier. Ideally, the state is a coherent superposition of odd photon number states, closely resembling a superposition of weak coherent states |alpha > - |-alpha >. In the limit of low squeezing the state is basically a single photon state. Light is generated with about 10,000 and more events per second in a nearly perfect spatial mode with a Fourier-limited frequency bandwidth which matches well atomic quantum memory requirements. The generated state of light is an excellent input state for testing quantum memories, quantum repeaters, and linear optics quantum computers.

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