Delayed-choice gedanken experiments and their realizations

TL;DR

This paper reviews the history and experimental realizations of delayed-choice experiments, which challenge classical notions of wave-particle duality by allowing the observer to choose the nature of a quantum system at a late stage.

Contribution

It provides a comprehensive overview of delayed-choice experiments, including recent implementations like quantum erasure and entanglement swapping, highlighting their significance in understanding quantum duality.

Findings

Delayed-choice experiments challenge classical wave-particle notions.

Experimental realizations demonstrate the observer’s late choice affects quantum behavior.

Entanglement swapping links wave-particle duality to entanglement-separability duality.

Abstract

The wave-particle duality dates back to Einstein's explanation of the photoelectric effect through quanta of light and de Broglie's hypothesis of matter waves. Quantum mechanics uses an abstract description for the behavior of physical systems such as photons, electrons, or atoms. Whether quantum predictions for single systems in an interferometric experiment allow an intuitive understanding in terms of the particle or wave picture, depends on the specific configuration which is being used. In principle, this leaves open the possibility that quantum systems always either behave definitely as a particle or definitely as a wave in every experimental run by a priori adapting to the specific experimental situation. This is precisely what is tried to be excluded by delayed-choice experiments, in which the observer chooses to reveal the particle or wave character -- or even a continuous transformation between the two -- of a quantum system at a late stage of the experiment. We review the history of delayed-choice gedanken experiments, which can be traced back to the early days of quantum mechanics. Then we discuss their experimental realizations, in particular Wheeler's delayed choice in interferometric setups as well as delayed-choice quantum erasure and entanglement swapping. The latter is particularly interesting, because it elevates the wave-particle duality of a single quantum system to an entanglement-separability duality of multiple systems.