Research ArticlePHYSICS

Experimental test of local observer independence

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Science Advances  20 Sep 2019:
Vol. 5, no. 9, eaaw9832
DOI: 10.1126/sciadv.aaw9832
  • Fig. 1 Wigner’s friend experiment.

    (A) A quantum system in an equal superposition of two possible states is measured by Wigner’s friend (inside the box). According to quantum theory, in each run, she will randomly obtain one of two possible measurement outcomes. This can be verified by directly looking into her laboratory and reading which result she recorded. (B) From outside the closed laboratory, however, Wigner must describe his friend and her quantum system as a joint entangled state. Wigner can also verify this state assignment through an interference experiment, concluding that his friend cannot have seen a definite outcome in the first place. (C) We consider an extended version of that experiment, where an entangled state is sent to two different laboratories, each involving an experimenter and their friend.

  • Fig. 2 Experimental setup.

    Pairs of entangled photons from the source S0, in modes a and b, respectively, are distributed to Alice’s and Bob’s friend, who locally measure their respective photon in the {∣h〉, ∣v〉}-basis using entangled sources SA, SB and type I fusion gates. These use nonclassical interference on a polarizing beam splitter (PBS) together with a set of half-wave (HWP) and quarter-wave plates (QWP). The photons in modes α′ and β′ are detected using superconducting nanowire single-photon detectors (SNSPDs) to herald the successful measurement, while the photons in modes α and β record the friends’ measurement results. Alice (Bob) then either performs a Bell-state measurement via nonclassical interference on a 50/50 beam splitter (BS) on modes a and α (b and β) to measure A1 (B1) and establish her (his) own fact or removes the BS to measure A0 (B0) to infer the fact recorded by their respective friend (see the Supplementary Materials for details).

  • Fig. 3 Experimental data.

    The outcome probabilities comprising each of the four expectation values 〈A0B0〉, 〈A0B1〉, 〈A1B0〉, and 〈A1B1〉 are obtained from the measured sixfold coincidence events for each set of 4 × 4 eigenvectors during a fixed time window. Shown here are only the data corresponding to nonzero eigenvalues labeled on the horizontal axes + and − for +1 and −1, respectively, with the full data shown in the Supplementary Materials. The theoretical predictions are shown as orange bars, and each measured expectation value is given above the corresponding subfigure. Uncertainties on the latter and error bars on the data represent 1σ statistical confidence intervals assuming Poissonian counting statistics (see the Supplementary Materials).

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Fig. S1. Detailed experimental setup.
    • Fig. S2. Full experimental data.
    • Fig. S3. Alternative protocol experimental data.
    • References (2931)

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