Entanglement between stationary systems at remote locations is a key resource for quantum networks. We report on the experimental generation of remote entanglement between a single atom inside an optical cavity and a Bose-Einstein condensate (BEC). To produce this, a single photon is created in the atom-cavity system, thereby generating atom-photon entanglement. The photon is transported to the BEC and converted into a collective excitation in the BEC, thus establishing matter-matter entanglement. After a variable delay, this entanglement is converted into photon-photon entanglement. The matter-matter entanglement lifetime of 100 μs exceeds the photon duration by 2 orders of magnitude. The total fidelity of all concatenated operations is 95%. This hybrid system opens up promising perspectives in the field of quantum information.
An experiment is performed where a single rubidium atom trapped within a high-finesse optical cavity emits two independently triggered entangled photons. The entanglement is mediated by the atom and is characterized both by a Bell inequality violation of S = 2.5, as well as full quantumstate tomography, resulting in a fidelity exceeding F = 90%. The combination of cavity-QED and trapped atom techniques makes our protocol inherently deterministic -an essential step for the generation of scalable entanglement between the nodes of a distributed quantum network.PACS numbers: 03.65. Ud, 03.67.Bg, 42.50.Pq, 42.50.Xa Of all the technologies currently being pursued for quantum information science, individually trapped atoms are among the most proven candidates for quantum information storage [1]. Photons, on the other hand, are the obvious choice for carriers of quantum information over large distances. Together, this naturally leads to an atom-photon interface as an ideal node for distributed quantum computing networks [1,2,3]. Progress towards the construction of such quantum networks has been recently achieved in experiments entangling single atoms trapped in a free-space radiation environment with their spontaneously emitted photons [4,5,6,7], however, high photon loss rates in the emission process severely limit their usefulness for quantum information processing protocols [8]. For scalable atom-photon based quantum information processing, it is necessary to increase this entanglement efficiency. The most promising method to accomplish this is to combine the advantages of trapped atom entanglement techniques with cavity quantum electrodynamics where both atomic and photonic qubits are under complete control [3,9,10,11,12].In this Letter, we demonstrate a deterministic entanglement protocol with a single atom trapped in an optical cavity and two subsequently emitted single photons. Compared to previous entanglement experiments with a probabilistic transit of atoms through a cavity [9], our results increase the atom-cavity interaction time, and therefore also the number of successful atom-photon entanglement events from a single atom, by a factor of 10 5 . The long trapping times shown here also allow us to ensure that exactly one atom is within the cavity at a given time. This is critical for the generation of high-fidelity entangled states, and is not possible with atoms randomly loaded into a cavity [9]. Furthermore, the highly efficient photon collection in the cavity output mode allows for photon detection efficiencies that are more than an order of magnitude greater than in free-space atom-photon entanglement experiments [6,7]. This also allows for the coherent mapping of the atomic quantum state onto the state of a second photon. The resulting entanglement is verified by a Bell inequality measurement between the two emitted photons [13], and is in convincing violation
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