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Tunable ion–photon entanglement in an optical cavity



Proposed quantum networks require both a quantum interface between light and matter and the coherent control of quantum states1,2. A quantum interface can be realized by entangling the state of a single photon with the state of an atomic or solid-state quantum memory, as demonstrated in recent experiments with trapped ions3,4, neutral atoms5,6, atomic ensembles7,8 and nitrogen-vacancy spins9. The entangling interaction couples an initial quantum memory state to two possible light–matter states, and the atomic level structure of the memory determines the available coupling paths. In previous work, the transition parameters of these paths determined the phase and amplitude of the final entangled state, unless the memory was initially prepared in a superposition state4 (a step that requires coherent control). Here we report fully tunable entanglement between a single 40Ca+ ion and the polarization state of a single photon within an optical resonator. Our method, based on a bichromatic, cavity-mediated Raman transition, allows us to select two coupling paths and adjust their relative phase and amplitude. The cavity setting enables intrinsically deterministic, high-fidelity generation of any two-qubit entangled state. This approach is applicable to a broad range of candidate systems and thus is a promising method for distributing information within quantum networks.

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Figure 1: Experimental apparatus and entanglement sequence.
Figure 2: Quantum state tomography of the joint ion–photon state, containing 40,000 events.
Figure 3: State tomography as a function of Raman phase (340,000 events).
Figure 4: State tomography for three values of amplitude, cos α.

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We thank J. Barreiro, D. Nigg, K. Hammerer and W. Rosenfeld for discussions. This work was supported by the Austrian Science Fund (FWF), the European Commission (AQUTE), the Institut für Quanteninformation GmbH, and a Marie Curie International Incoming Fellowship within the 7th European Framework Program.

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Experiments were performed by A.S., B.C. and T.E.N., with contributions from P.S. to the set-up. Data analysis was performed by A.S., B.C. and T.M. The experiment was conceived by P.O.S. and R.B. and further developed in discussions with A.S., B.B., B.C. and T.E.N. All authors contributed to the discussion of results and participated in manuscript preparation.

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Correspondence to T. E. Northup.

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The authors declare no competing financial interests.

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Stute, A., Casabone, B., Schindler, P. et al. Tunable ion–photon entanglement in an optical cavity. Nature 485, 482–485 (2012).

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