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arXiv:1310.0028 · Journal of the Optical Society of America B

Ion–photon entanglement and Bell inequality violation with ^138Ba^+

Carolyn Auchter, Chen-Kuan Chou, Thomas W. Noel, B. B. Blinov

Published Jun 12, 2014• 17 citations
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Abstract

We report on the demonstration of ion–photon entanglement and Bell inequality violation in a system of trapped Ba138+ ions. Entanglement between the Zeeman sublevels of the ground state of a single Ba138+ ion and the polarization state of a single 493 nm photon emitted by the ion with a fidelity of 0.84±0.01 was achieved, along with a Bell signal of 2.3, exceeding the classical limit of 2 by more than eight standard deviations. This system is a promising candidate for a loophole-free Bell inequality violation test using long-range ion–ion entanglement as the wavelengths of the transitions of Ba138+ are in the visible region and thus suitable for long-range transmission over fiber optic cable.

DOI
10.1364/josab.31.001568

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We report on the demonstration of ion–photon entanglement and Bell inequality violation in a system of trapped Ba138+ ions. Entanglement between the Zeeman sublevels of the ground state of a single Ba138+ ion and the polarization state of a single 493 nm photon emitted by the ion with a fidelity of 0.84±0.01 was achieved, along with a Bell signal of 2.3, exceeding the classical limit of 2 by more than eight standard deviations. This system is a promising candidate for a loophole-free Bell inequality violation test using long-range ion–ion entanglement as the wavelengths of the transitions of Ba138+ are in the visible region and thus suitable for long-range transmission over fiber optic cable.
We present work toward remote entanglement of barium ions in traps separated by a few meters. A new version of an ion trap specialized for remote entanglement is introduced. The new trap allows for highly efficient collection of ion fluorescence while simultaneously minimizing ion micromotion and aligning the trap position precisely to the focus of an in-vacuum parabolic mirror by using a set of bias electrodes and a piezoelectric micro-positioning system. The success rate of the remote entanglement procedure depends strongly on the efficiency with which ion fluorescence can be coupled into an optical fiber. Characterization of our system in terms of ion fluorescence collection and fiber coupling efficiency is presented. Results demonstrating entanglement between a single barium ion and single spontaneously emitted photons are shown. The entanglement fidelity of the ion-photon state is measured to be 0.84(1) and a CHSH Bell signal of 2.303(36) finds violation of the CHSH version of the Bell inequality by over eight standard deviations. Barium’s relatively long wavelength transitions make it an ideal candidate for our longer term goal of remote entanglement of ions separated by a kilometer or more. Such long distance remote entanglement should allow for a loophole-free verification of the violation of the Bell inequality.
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Efficient ion-photon interface is critical for ion-photon and ion-ion entanglement generation, which are the fundamental building blocks for loophole-free Bell inequality violation tests based on ion qubits, and for the quantum computer architecture based on ion-photon networks. As one approach to address the problem, the feasibility of integrating reflective optics into ion trap to increase free-space photon interception is studied. The development and operation of two novel ion traps are demonstrated: one integrates a spherical mirror with a linear quadrapole trap, and the other combines the optical surface with the RF electrode. The possibility of using aspherical optics to improve ion images is studied and partially implemented. A continuously excited single photon source based on 138Ba+ ion is demonstrated and calibrated. Methods of scaling up current design are also discussed.
We demonstrate remote entanglement of trapped-ion qubits via a quantum-optical fiber link with fidelity and rate approaching those of local operations. Two ^{88}Sr^{+} qubits are entangled via the polarization degree of freedom of two spontaneously emitted 422 nm photons which are coupled by high-numerical-aperture lenses into single-mode optical fibers and interfere on a beam splitter. A novel geometry allows high-efficiency photon collection while maintaining unit fidelity for ion-photon entanglement. We generate heralded Bell pairs with fidelity 94% at an average rate 182 s^{-1} (success probability 2.18×10^{-4}).