Entanglement Swapping and Action at a Distance

TL;DR

The paper critically examines the 2015 Delft experiment claiming to demonstrate quantum nonlocality, highlighting a potential loophole called the Collider Loophole that questions the conclusion of action at a distance.

Contribution

It introduces the Collider Loophole as a new concern in entanglement swapping experiments and analyzes its impact on interpreting Bell test results.

Findings

The Delft experiment's geometry may allow collider bias to mimic action at a distance.

The timing of measurements affects the vulnerability to the Collider Loophole.

Retrocausality considerations are discussed in relation to causal influence in entanglement swapping.

Abstract

A 2015 experiment by Hanson and Delft colleagues provided further 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 'spooky action at a distance'. 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 an additional loophole in the argument from violation of the Bell inequalities to action at a distance: the apparent action at a distance may be an artifact of 'collider bias'. In the absence of retrocausality, the sensitivity of such experiments to this 'Collider Loophole' (CL) depends on the temporal relation between the entanglement swapping measurement C and the two measurements A and B between which we seek to infer a causal connection. CL looms 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 CL, unable to establish conclusively that it avoids it. An Appendix discusses the implications of permitting retrocausality for the issue of causal influence across entanglement swapping measurements.