Entanglement of Remote Atomic Qubits

Abstract

We report observations of entanglement of two remote atomic qubits, achieved by generating an entangled state of an atomic qubit and a single photon at site A, transmitting the photon to site B in an adjacent laboratory through an optical fiber, and converting the photon into an atomic qubit. Entanglement of the two remote atomic qubits is inferred by performing, locally, quantum state transfer of each of the atomic qubits onto a photonic qubit and subsequent measurement of polarization correlations in violation of the Bell inequality jSj 2. We experimentally determine S exp 2:16 0:03. Entanglement of two remote atomic qubits, each qubit consisting of two independent spin wave excitations, and reversible, coherent transfer of entanglement between matter and light represent important advances in quantum information science. Realization of massive qubits, and their entanglement, is central to practical quantum information systems [1][2][3]. Remote entanglement of photons can now be achieved in a robust manner using the well-developed technology of spontaneous parametric down-conversion [4], with propagation to remote locations by means of optical fibers. Photons, however, are difficult to store for any appreciable period of time, whereas qubits based on ground-state atoms have long lifetimes. Local entanglement of massive qubits has been observed between adjacent trapped ions [5] and between pairs of Rydberg atoms in a collimated beam [6]. In order to entangle qubits at remote locations, the use of photons as an intermediary seems essential [7][8][9][10]. Photons also offer some flexibility as information carriers as they can propagate in optical fiber with low losses. The creation, transport, storage, and retrieval of single photons between remote atomic ensembles located in two different laboratories were recently observed [11] [see also a related work on electromagnetically induced transparency (EIT) with single photon pulses [12] ]. The first step in creating remote entanglement between massive qubits is to entangle one such qubit with the mediating light field, which is then directed towards the second qubit via an optical fiber. There have recently been important advances towards this goal by demonstrating entanglement of a photon with a trapped ion [13], with a collective atomic qubit [14,15], and with a single trapped atom [16].A promising route towards the creation and application of long-lived qubit entanglement in scalable quantum networks was proposed by Duan, Lukin, Cirac, and Zoller [2,10]. These atomic qubits rely on collective atomic states containing exactly one spin excitation. In two recent experiments, collective atomic qubits were generated using cold atomic ensembles [14,15] While remote entanglement of atomic qubits has not been previously demonstrated, Refs. [14,15] realized two basic primitives of a quantum network: (a) entanglement of photonic and atomic qubits, and (b) quantum state transfer from an atomic to a photonic qubit. The crucial additional ingredient is the reverse operation, t...

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