Experimental amplification of an entangled photon: what if the detection loophole is ignored?

TL;DR

This paper demonstrates that ignoring the detection loophole in Bell tests with amplified entangled photons can lead to false claims of micro-macro entanglement, highlighting the importance of proper detection and analysis methods.

Contribution

It shows that postselection and system losses can falsely indicate entanglement, emphasizing the need for careful interpretation in micro-macro quantum experiments.

Findings

Violation of CHSH inequality observed despite classical amplification

Postselection can mislead to claim micro-macro entanglement

Micro-micro entanglement confirmed by non-separability criterion

Abstract

The experimental verification of quantum features, such as entanglement, at large scales is extremely challenging because of environment-induced decoherence. Indeed, measurement techniques for demonstrating the quantumness of multiparticle systems in the presence of losses are difficult to define and, if not sufficiently accurate, they provide wrong conclusions. We present a Bell test where one photon of an entangled pair is amplified and then detected by threshold detectors, whose signals undergo postselection. The amplification is performed by a classical machine, which produces a fully separable micro-macro state. However, by adopting such a technique, one can surprisingly observe a violation of the CHSH inequality. This is due to the fact that ignoring the detection loophole, opened by the postselection and the system losses, can lead to misinterpretations, such as claiming the micro-macro entanglement in a setup where evidently there is not. By using threshold detectors and postselection, one can only infer the entanglement of the initial pair of photons, so micro-micro entanglement, as it is further confirmed by the violation of a non-separability criterion for bipartite systems. How to detect photonic micro-macro entanglement in the presence of losses with currently available technology remains an open question.