Quantum erasure with causally disconnected choice

TL;DR

This paper demonstrates a quantum eraser experiment where the choice to erase or not erase which-path information is space-like separated from the interference, challenging classical notions of causality and realism.

Contribution

It presents the first quantum eraser experiment enforcing Einstein locality with space-like separated choices, eliminating any possible communication influencing the results.

Findings

Quantum behavior depends on causally disconnected choices.

No classical realistic model can explain the results.

Entanglement enables space-like separated quantum erasure.

Abstract

The counterintuitive features of quantum physics challenge many common-sense assumptions. In an interferometric quantum eraser experiment, one can actively choose whether or not to erase which-path information, a particle feature, of one quantum system and thus observe its wave feature via interference or not by performing a suitable measurement on a distant quantum system entangled with it. In all experiments performed to date, this choice took place either in the past or, in some delayed-choice arrangements, in the future of the interference. Thus in principle, physical communications between choice and interference were not excluded. Here we report a quantum eraser experiment, in which by enforcing Einstein locality no such communication is possible. This is achieved by independent active choices, which are space-like separated from the interference. Our setup employs hybrid path-polarization entangled photon pairs which are distributed over an optical fiber link of 55 m in one experiment, or over a free-space link of 144 km in another. No naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether.