A Quantum Delayed-Choice Experiment

Abstract

Quantum systems exhibit particle-like or wave-like behaviour depending on the experimental apparatus they are confronted by. This wave-particle duality is at the heart of quantum mechanics, and is fully captured in Wheeler's famous delayed choice gedanken experiment. In this variant of the double slit experiment, the observer chooses to test either the particle or wave nature of a photon after it has passed through the slits. Here we report on a quantum delayed choice experiment, based on a quantum controlled beam-splitter, in which both particle and wave behaviours can be investigated simultaneously. The genuinely quantum nature of the photon's behaviour is tested via a Bell inequality, which here replaces the delayed choice of the observer. We observe strong Bell inequality violations, thus showing that no model in which the photon knows in advance what type of experiment it will be confronted by, hence behaving either as a particle or as wave, can account for the experimental data.Quantum mechanics predicts with remarkable accuracy the result of experiments involving small objects, such as atoms and photons. However, when looking more closely at these predictions, we are forced to admit that they defy our intuition. Indeed, quantum mechanics tells us that a single particle can be in several places at the same time, and that distant entangled particles behave as a single physical object no matter how far apart they are [1].In trying to grasp the basic principles of the theory, in particular to understand more intuitively the behaviour of quantum particles, some of its pioneers introduced the notion of wave-particle duality [2]. A quantum system, for instance a photon, may behave either as a particle or a wave. However, the way in which it behaves depends on the kind of experimental apparatus with which it is measured. Hence, both aspects, particle and wave, which appear to be incompatible, are never observed simultaneously [3]. This is the notion of complementarity in quantum mechanics [4,5], which is central in the standard Copenhagen interpretation, and has been intensely debated in the past.In an effort to reconcile quantum predictions and common sense, it was suggested that quantum particles may in fact know in advance to which experiment they will be confronted, via a hidden variable, and could thus decide which behaviour to exhibit. This simplistic argument was however challenged by Wheeler in his elegant 'delayed choice' arrangement [6][7][8]. In this gedanken experiment, sketched in Fig. 1(a), a quantum particle is sent towards a Mach-Zender interferometer. The relative phase ϕ between the two arms of the interferometer can be adjusted such that the particle will emerge in output D 0 with certainty. That is, the interference is fully constructive in output D 0 , and fully destructive in output D 1 . This measurement thus clearly highlights the wave aspect of the quantum particle. However, the observer performing the experiment has the choice of modifying the above experiment, in particular by removing ...