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.
Highlights
In 2015, Ronald Hanson and colleagues at Delft reported the first of an important new class of ‘Bell tests’—i.e., experimental confirmations that the quantum world violates the Bell inequalities [1]
The first argument turns on the observation that it is going to be difficult, probably impossible, to exclude the possibility that in the actual experiment, C is in the future of A and B with respect to the preferred frame
The use of entanglement swapping in Bell tests introduces an additional loophole into arguments from violation of the Bell inequalities to at a distance’ (AAD)
Summary
In 2015, Ronald Hanson and colleagues at Delft reported the first of an important new class of ‘Bell tests’—i.e., experimental confirmations that the quantum world violates the Bell inequalities [1]. We will argue that in the light of this, the case for AAD across the ∨∨ in the Delft experiment is weaker than in the subsequent similar experiments, in which the central vertex lies in the overlap of the past light cones of the outer vertices. The sensitivity of CL to the location of the central vertex depends on the assump‐ tion that there is no retrocausality in play This may seem uncontroversial, but it is has been challenged in this context, an option that has been held to provide a dif‐ ferent reason for questioning the inference from violation of the Bell inequalities to AAD. 6, with these variants as comparisons, we return to the actual Delft exper‐ iment In this case, as we noted, the entanglement swapping measurement occurs at spacelike separation from the two measurements between which the experiment claims to reveal AAD.
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