Quantum networks provide opportunities and challenges across a range of intellectual and technical frontiers, including quantum computation, communication and metrology. The realization of quantum networks composed of many nodes and channels requires new scientific capabilities for generating and characterizing quantum coherence and entanglement. Fundamental to this endeavour are quantum interconnects, which convert quantum states from one physical system to those of another in a reversible manner. Such quantum connectivity in networks can be achieved by the optical interactions of single photons and atoms, allowing the distribution of entanglement across the network and the teleportation of quantum states between nodes.

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.

We propose a scheme to utilize photons for ideal quantum transmission between atoms located at spatially separated nodes of a quantum network. The transmission protocol employs special laser pulses that excite an atom inside an optical cavity at the sending node so that its state is mapped into a time-symmetric photon wave packet that will enter a cavity at the receiving node and be absorbed by an atom there with unit probability. Implementation of our scheme would enable reliable transfer or sharing of entanglement among spatially distant atoms.[ S0031-9007(97) We consider a quantum network consisting of spatially separated nodes connected by quantum communication channels. Each node is a quantum system that stores quantum information in quantum bits and processes this information locally using quantum gates [1]. Exchange of information between the nodes of the network is accomplished via quantum channels. A physical implementation of such a network could consist, e.g., of clusters of trapped atoms or ions representing the nodes, with optical fibers or similar photon "conduits" providing the quantum channels. Atoms and ions are particularly well suited for storing qubits in long-lived internal states, and recently proposed schemes for performing quantum gates between trapped atoms or ions provide an attractive method for local processing within an atom͞ion node [2][3][4]. On the other hand, photons clearly represent the best qubit carrier for fast and reliable communication over long distances [5,6], since fast and internal-state-preserving transportation of atoms or ions seems to be technically intractable.To date, no process has actually been identified for using photons (or any other means) to achieve efficient quantum transmission between spatially distant atoms [7]. In this Letter we outline a scheme to implement this basic building block of communication in a distributed quantum network. Our scheme allows quantum transmission with (in principle) unit efficiency between distant atoms 1 and 2 (see Fig. 1). The possibility of combining local quantum processing with quantum transmission between the nodes of the network opens the possibility for a variety of novel applications ranging from entangled-state cryptography [8], teleportation [9], and purification [10], and is interesting from the perspective of distributed quantum computation [11].The basic idea of our scheme is to utilize strong coupling between a high-Q optical cavity and the atoms [5] forming a given node of the quantum network. By applying laser beams, one first transfers the internal state of an atom at the first node to the optical state of the cavity mode. The generated photons leak out of the cavity, propagate as a wave packet along the transmission line, and enter an optical cavity at the second node. Finally, the optical state of the second cavity is transferred to the internal state of an atom. Multiple-qubit transmissions can be achieved by sequentially addressing pairs of atoms (one at each node), as entanglements between arbitrarily l...