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arXiv:quant-ph/0202025

Comment on ``Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping''

Eric Dennis

Published Feb 4, 2002• 2 citations
arXivHTMLPDF

Abstract

In quant-ph/0201134 Jennewein et al. report experiments demonstrating entanglement swapping under various conditions. In one instance, they claim to have exhibited Bell inequality violations by a pair of photons prior to performing any entangling operations on the pair. However, the Bell inequality violation results in this case from post-selection of the data, not from quantum non-locality.

DOI
10.48550/arxiv.quant-ph/0202025

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In quant-ph/0201134 Jennewein et al. report experiments demonstrating entanglement swapping under various conditions. In one instance, they claim to have exhibited Bell inequality violations by a pair of photons prior to performing any entangling operations on the pair. However, the Bell inequality violation results in this case from post-selection of the data, not from quantum non-locality.
Quantum teleportation strikingly underlines the peculiar features of the quantum world. We present an experimental proof of its quantum nature, teleporting an entangled photon with such high quality that the nonlocal quantum correlations with its original partner photon are preserved. This procedure is also known as entanglement swapping. The nonlocality is confirmed by observing a violation of Bell's inequality by 4.5 standard deviations. Thus, by demonstrating quantum nonlocality for photons that never interacted, our results directly confirm the quantum nature of teleportation.
A 2015 experiment by Hanson and Delft colleagues provided new confirmation that the quantum world violates the Bell inequalities, being the first Bell test to close two known experimental loopholes simultaneously. The experiment was also taken to provide new evidence of quantum nonlocality. Here we argue for caution about the latter claim. The Delft experiment relies on entanglement swapping, and our main claim is that this geometry introduces new loopholes in the argument from violation of the Bell inequalities to nonlocality. In the absence of retrocausality, the sensitivity of such experiments to these new loopholes depends on the temporal relation between the entanglement swapping measurement C and the two measurements A and B between which we seek to infer nonlocality. The loopholes loom large if the C is in the future of A and B, but not if C is in the past. The Delft experiment itself is the intermediate case, in which the separation is spacelike. We argue that this leaves it vulnerable to the new loopholes, unable to establish conclusively that it avoids them. We also discuss the implications of permitting retrocausality for the issue of causal influence across entanglement swapping measurements.
We present an overview of the method for generating entangled photon pairs via spontaneous parametric down conversion and the experimental method of detecting entangled states or Bell-states with the use of photon statistics, the two key systems required to realize quantum state teleportation and other related experiments. As a proof of the quantum nature of teleportation we performed an entanglement swapping experiment, where the teleported state itself was entangled. The experiment clearly showed that the entanglement survived the teleportation, as the quantum correlations of the teleported photon violated Bell's inequality.
Quantum systems that have never interacted can become nonlocally correlated through a process called entanglement swapping. To characterize nonlocality in this context, we introduce local models where quantum systems that are initially uncorrelated are described by uncorrelated local variables. This additional assumption leads to stronger tests of nonlocality. We show, in particular, that an entangled pair generated through entanglement swapping will already violate a Bell-type inequality for visibilities as low as 50% under our assumption.
Detecting non-locality requires almost ideal experimental conditions - ideal sources for entangled photon pairs and ideal detectors. Deviations from this ideal situation may critically hamper interpretations of experiments attempting to prove non-locality. Of two major known loopholes, the detector loophole is not yet unequivocally closed. Now another loophole emerges: When in such experiments down converting crystals with lasers as primary sources are used, exceeding threshold values in the Bell inequalities, i.e. their violation, can be caused by so far not fully recognized problems in the generation and detection of photon pairs. Taken together, nonlocality is still not yet unequivocally proven.