delayedchoice

Delayed-choice is a class of experiments in quantum mechanics where the choice of measurement setting is delayed until after the system has entered the measurement apparatus. Proposed by John Archibald Wheeler in 1978 as a thought experiment to probe wave-particle duality and the role of measurement. It explores whether the behavior is determined at emission or by measurement.

In a typical setup, a particle such as a photon passes through a beam splitter into a Mach-Zehnder interferometer. If a second beam splitter is present, the paths interfere and detectors record an interference pattern (wave-like). If the second beam splitter is removed or a which-path detector inserted, the results reveal which path was taken (particle-like). In a delayed-choice version, the decision to insert or remove the second beam splitter is made after the photon has entered the apparatus.

The experiments do not imply retrocausal signaling; instead, they illustrate that quantum systems do not possess definite classical properties independent of measurement. The outcomes are consistent with quantum mechanics, where the experimental context defines the observed phenomenon, and the correlations reflect the whole setup rather than a hidden preexisting truth.

Variants include quantum-delayed-choice proposals in which the measurement device itself is placed in a quantum superposition, and delayed-choice quantum erasers where which-path information can be erased after detection to restore interference. Numerous photonic experiments and atom experiments have demonstrated the core predictions.

Delayed-choice remains a topic of interpretation rather than a technological requirement, highlighting the counterintuitive nature of quantum measurement and the interplay between measurement, information, and reality.