Using macroscopic entanglement to close the detection loophole in Bell inequality

TL;DR

This paper explores how macroscopic entanglement can be used to mitigate detection loopholes in Bell tests by counteracting post-unitary losses, but not pre-unitary decoherence effects.

Contribution

It demonstrates that increasing the size of multi-photon entangled states enhances robustness against detection inefficiencies in Bell tests.

Findings

Macroscopic entanglement improves robustness against detection losses.

Losses after unitary transformations can be mitigated by larger entangled states.

Losses before transformations cause decoherence that cannot be compensated.

Abstract

We consider a Bell-like inequality performed using various instances of multi-photon entangled states to demonstrate that losses occurring after the unitary transformations used in the nonlocality test can be counteracted by enhancing the "size" of such entangled states. In turn, this feature can be used to overcome detection inefficiencies affecting the test itself: a slight increase in the size of such states, pushing them towards a more "macroscopic" form of entanglement, significantly improves the state robustness against detection inefficiency, thus easing the closing of the detection loophole. Differently, losses before the unitary transformations cause decoherence effects that cannot be compensated using macroscroscopic entanglement.