Event-Ready Bell Test Using Entangled Atoms Simultaneously Closing Detection and Locality Loopholes

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Event-Ready Bell Test Using Entangled Atoms Simultaneously Closing Detection and Locality Loopholes

Wenjamin Rosenfeld, Daniel Burchardt, Robert Garthoff, Kai Redeker, Norbert Ortegel, Markus Rau, and Harald Weinfurter

Phys. Rev. Lett. 119, 010402 – Published 6 July 2017

Abstract

An experimental test of Bell’s inequality allows ruling out any local-realistic description of nature by measuring correlations between distant systems. While such tests are conceptually simple, there are strict requirements concerning the detection efficiency of the involved measurements, as well as the enforcement of spacelike separation between the measurement events. Only very recently could both loopholes be closed simultaneously. Here we present a statistically significant, event-ready Bell test based on combining heralded entanglement of atoms separated by 398 m with fast and efficient measurements of the atomic spin states closing essential loopholes. We obtain a violation with $S = 2.221 \pm 0.033$ (compared to the maximal value of 2 achievable with models based on local hidden variables) which allows us to refute the hypothesis of local realism with a significance level $P < 2.57 \times 10^{- 9}$ .

Received 14 November 2016

DOI:https://doi.org/10.1103/PhysRevLett.119.010402

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Nonlocality Quantum entanglement

Atoms

Atomic, Molecular & OpticalQuantum Information, Science & TechnologyGeneral Physics

Authors & Affiliations

Wenjamin Rosenfeld^1,2,*, Daniel Burchardt¹, Robert Garthoff¹, Kai Redeker¹, Norbert Ortegel¹, Markus Rau¹, and Harald Weinfurter^1,2

¹Fakultät für Physik, Ludwig-Maximilians-Universität München, D-80799 München, Germany
²Max-Planck Institut für Quantenoptik, D-85748 Garching, Germany

^*Corresponding author. W.R@lmu.de

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Issue

Vol. 119, Iss. 1 — 7 July 2017

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Images

Figure 1
(a) Space-time diagram of the experiment. The two observers (trap 1 and trap 2) are separated by 398 m with the BSM setup being located close to trap 1. Single photons and all communication signals are transmitted via optical fibers (lengths vary around 700 m) laid in cable ducts connecting the two stations. Sending a photon from trap 2 to the BSM takes roughly $3.6 μ s$ (photons from both traps arrive within a window of 120 ns represented by two lines for earliest and latest emission). Another $3.7 μ s$ are needed for communicating the success of the BSM back to trap 2. The state measurements (including random choice of the measurement direction) are performed such that a result is obtained outside of the light cone of the other side. (b) Overview of the experimental location on the main campus of LMU. Trap 1 is located in the basement of the faculty of physics and trap 2 in the basement of the department of economics. Map data were provided by Ref. [31].
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Figure 2
(a) Scheme of the atomic levels involved in the entanglement between the spin state of the atom (subspace $5^{2} S_{1 / 2}$ , $F = 1$ , $| m_{F} = \pm 1 ⟩$ ) and polarization of the photon (left- and right-circular, $| L ⟩$ , $| R ⟩$ , respectively). Entanglement is generated in the spontaneous decay of the $5^{2} P_{3 / 2}$ , $F^{'} = 0$ state after optical excitation. (b) Scheme of the atomic state measurement. A selected superposition of the spin states is excited to the $5^{2} P_{1 / 2}$ , $F^{'} = 1$ level depending on the polarization of a 795 nm laser pulse and is ionized with a 473 nm laser. The atom can spontaneously decay to the $5^{2} S_{1 / 2}$ , $F = 1$ or $F = 2$ levels during this procedure (gray wavy arrows). While decays into the $F = 1$ level can reduce the fidelity of the measurement process, population in the $F = 2$ level is excited with an additional 780 nm laser and ionized as well. (c) Schematic of the experimental setup. In each trap spin-polarization entanglement is generated between the atom and a single photon that is guided to the BSM via a single-mode fiber. Polarization stability in the 700 m fiber connecting trap 2 and the BSM is ensured by automatic compensation [32] performed every 5 min using reference light and a polarization controller. The photons are overlapped on a fiber beam splitter (BS), their coincident detection heralds entanglement of the atomic spins. Local measurements are performed on the atomic spins according to settings selected by quantum random number generators (QRNGs). Acousto-optic modulator (AOM), avalanche photo diode (APD), channel electron multiplier (CEM), field programmable gate array (FPGA), polarizing beam-splitter (PBS).
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Figure 3
Measured correlators $⟨ σ_{a} σ_{b} ⟩$ for the run started on April 15, 2016 for the atom-atom state $| Ψ^{-} ⟩$ (a) and $| Ψ^{+} ⟩$ (b). Displayed errors are equal to 1 standard deviation.
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