Delayed-choice Gedanken Experiment Research Articles

Encrypted quantum correlations: delayed choice of quantum statistics and other applications

In a three-particle extension of Wheeler’s delayed choice gedanken experiment, the quantum statistics of two particles is undetermined until a third particle is measured. As a function of the measurement result, the particles behave either as bosons or as fermions. The particles are distinguishable if no measurement is performed at all or when the measurement is performed in a rotated basis. The scheme is based on Greenberger–Horne–Zeilinger quantum correlations. It can be interpreted more generally as the encryption of maximally entangled states in a larger quantum superposition. The local quantum information is scrambled but can be decoded by the measurement result of a control particle. This can be extended to multiple particles and allows to develop quantum information protocols whose successful implementation depends on the collaboration of all parties.

Quantum wave–particle superposition in a delayed-choice experiment

Wave–particle duality epitomizes the counterintuitive character of quantum physics. A striking illustration is the quantum delayed-choice experiment, which is based on Wheeler’s classic delayed-choice gedanken experiment, but with the addition of a quantum-controlled device enabling wave-to-particle transitions. Here, we realize a quantum delayed-choice experiment in which we control the wave and the particle states of photons and particularly the phase between them, thus directly establishing the created quantum nature of the wave–particle. We generate three-photon entangled states and inject one photon into a Mach–Zehnder interferometer embedded in a 186-m-long two-photon Hong–Ou–Mandel interferometer. The third photon is sent 141 m away from the interferometers and remotely prepares a two-photon quantum gate according to independent active choices under Einstein locality conditions. We realize transitions between wave and particle states in both classical and quantum scenarios, and therefore tests of the complementarity principle that go fundamentally beyond earlier implementations. The quantum-delayed choice experiment is implemented with multiple entangled photons under Einstein’s locality condition. The wave–particle quantum superposition is realized by controlling the relative phase between the wave and particle states.

Wheeler's delayed-choice gedanken experiment with a single atom

In a delayed-choice experiment the decision to measure the particle or wave nature comes after the photon enters the interferometer. An atomic version of the experiment provides the same outcome despite the mass and internal structure of the atoms.

Entanglement-Enabled Delayed-Choice Experiment

Wave-particle complementarity is one of the most intriguing features of quantum physics. To emphasize this measurement apparatus-dependent nature, experiments have been performed in which the output beam splitter of a Mach-Zehnder interferometer is inserted or removed after a photon has already entered the device. A recent extension suggested using a quantum beam splitter at the interferometer's output; we achieve this using pairs of polarization-entangled photons. One photon is tested in the interferometer and is detected, whereas the other allows us to determine whether wave, particle, or intermediate behaviors have been observed. Furthermore, this experiment allows us to continuously morph the tested photon's behavior from wavelike to particle-like, which illustrates the inadequacy of a naive wave or particle description of light.

A Quantum Delayed-Choice Experiment

Quantum systems exhibit particle- or wavelike behavior depending on the experimental apparatus they are confronted by. This wave-particle duality is at the heart of quantum mechanics. Its paradoxical nature is best captured in the delayed-choice thought experiment, in which a photon is forced to choose a behavior before the observer decides what to measure. Here, we report on a quantum delayed-choice experiment in which both particle and wave behaviors are investigated simultaneously. The genuinely quantum nature of the photon's behavior is certified via nonlocality, which here replaces the delayed choice of the observer in the original experiment. We observed strong nonlocal correlations, which show that the photon must simultaneously behave both as a particle and as a wave.

On Wheeler's delayed-choice Gedankenexperiment and its laboratory realization

We present an analysis and interpretation of the experiment performed by Jacques et al (2007 Science315 966), which represents a realization of Wheeler's delayed-choice Gedankenexperiment. Our analysis is based on the evolution of the photon state, since the photon enters into the Mach–Zehnder interferometer (MZI) with a removable beam splitter until it exits. Given the same photon state incident on the output beam splitter BSoutput, the photon's state at the exit will be very different depending on whether BSoutput is on or off. Hence, the statistics of photon counts collected by the two detectors, positioned along orthogonal directions at the exit of the interferometer, will also be very different for each case. Therefore, it is not that the choice of inserting (on) or removing (off) a beam splitter leads to a delayed influence on the photon behavior before arriving there, but that such a choice influences the photon state at and after BSoutput, i.e. after it has exited the MZI. The random on/off choice at BSoutput has no delayed effect on the photon to behave as a wave or a corpuscle at the entrance and inside the interferometer, but influences the subsequent evolution of the photon state incident on BSoutput.

Causality and Bell-CHSH Inequalities Violations in Entanglement Swapping Schemes

An ample amount of evidence supporting the violation of locality has been presented in quantum theory. If such an instantaneous influencing is possible, it is worth considering the possibility of a causality violation in quantum theory, i.e., a future event influencing the past. Motivated by the delayed-choice gedanken experiment, we provide two protocols of entanglement swapping that are subtle in involving causality conditions. In particular, we present protocols in which locality constraints are identical to causality conditions and closely examine Bell-inequalities violation based on these protocols. These protocols will provide a clear picture of how quantum theory still preserves causality while locality is violated. We also discuss a potential threat to the entanglement-based key distribution schemes.

Projection evolution and delayed-choice experiments

A theoretical description of Wheeler's delayed-choice Gedankenexperiment is presented in the non-relativistic space-time in terms of quantum events. It turns out that the description, in which time is a dynamical variable, allows to explain the delayed-choice problem as a result of spread of quantum states in time.

Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment

Wave-particle duality is strikingly illustrated by Wheeler's delayed-choice gedanken experiment, where the configuration of a two-path interferometer is chosen after a single-photon pulse has entered it: Either the interferometer is closed (that is, the two paths are recombined) and the interference is observed, or the interferometer remains open and the path followed by the photon is measured. We report an almost ideal realization of that gedanken experiment with single photons allowing unambiguous which-way measurements. The choice between open and closed configurations, made by a quantum random number generator, is relativistically separated from the entry of the photon into the interferometer.

Bell's inequalities: II: Logical loophole in their interpretation

Assumed data streams from a delayed choice gedanken experiment must satisfy a Bell's identity independently of locality assumptions. The violation of Bell's inequality by assumed correlations of identical form among these data streams implies that they cannot all result from statistically equivalent variables of a homogeneous process. This is consistent with both the requirements of arithmetic and distinctions between commuting and noncommuting observables in quantum mechanics. Neglect of these distinctions implies a logical loophole in the conventional interpretation of Bell's inequalities.