Experimental delayed-choice entanglement swapping

TL;DR

This paper reports the first experimental realization of Peres' delayed-choice entanglement swapping, demonstrating that entanglement can be effectively created after particles have been measured, challenging classical notions of causality.

Contribution

The work provides the first experimental demonstration of delayed-choice entanglement swapping using four photons with active measurement delay and quantum randomization.

Findings

Successfully delayed measurement choice into the future of other photons' detection

Demonstrated projection onto entangled or separable states after initial measurements

Showed the feasibility of 'quantum steering into the past'

Abstract

Motivated by the question, which kind of physical interactions and processes are needed for the production of quantum entanglement, Peres has put forward the radical idea of delayed-choice entanglement swapping. There, entanglement can be "produced a posteriori, after the entangled particles have been measured and may no longer exist". In this work we report the first realization of Peres' gedanken experiment. Using four photons, we can actively delay the choice of measurement-implemented via a high-speed tunable bipartite state analyzer and a quantum random number generator-on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one definite of two mutually exclusive quantum states in which either the photons are entangled (quantum correlations) or separable (classical correlations). This can also be viewed as "quantum steering into the past".