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.
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.
Bohr's complementarity notion is at the heart of quantum physics. It suggests that quantum systems are observed as waves or particles depending on the type of measurement, i.e., the experimental arrangement, they are subjected to. For instance, sending single photons to an open or closed Mach-Zehnder interferometer (MZI) leads to the observation of particle or wave behaviours, respectively. In 1984, Wheeler proposed a "Gedanken" delayed-choice experiment in which the interferometer configuration is chosen at will by the experimentalist only after the photon has already passed the input beam-splitter (BS_in) of the device. This experiment was realized using a single photon source and showed that Bohr's complementarity notion was still obeyed.It was recently proposed to take Wheeler's experiment one step further by employing an output "quantum beam-splitter" (QBS), i.e.,preparing BS_out in a coherent superposition of being absent and present. This allows choosing the type of measurement, i.e., wave or particle, only after having determined the state of the QBS, which can be (in principle) infinitely delayed.We realized such an experiment by exploiting the resource of entanglement, in this case pairs of polarization entangled photons. The behaviour of one of the paired photons, called the test photon, is analysed using a MZI having a QBS at its output. This QBS is enabled by entanglement and made of a polarization dependent beam-splitter. The other photon, called the corroborative photon, allows determining the state of the QBS, and consequently, which test photon behaviour (wave, particle, or both) is observed.By manipulating the corroborative photon polarization state, we demonstrate a continuous morphing of the test photon from wave to particle behaviour. This refutes simple models of single photons behaving exclusively as waves or particles. The state of the QBS is determined via the measurement of the corroborative photon only after the test photon has already been detected. The space-like separation between the two measurements invalidates local-hidden variable models associated with pre-existing information about the measurement outcomes. In other words, when the test photon is detected, no information is available about the type of measurement it underwent. These results still perfectly obey Bohr's complementarity notion and its extension.
Bohr's complementarity notion is at the heart of quantum physics, and suggests that in a quantum experiment, the observed phenomena are inherently linked to the type of measurement that is performed. For example, sending single photons to an open or closed Mach-Zehnder interferometer leads to the observation of particle or wave behaviours. In 1984, Wheeler proposed a 'Gedanken' experiment in which the experimentalist chooses the interferometer configuration, at will, only after the photon has already passed the input beam-splitter (BSin) of the device. This experiment was realized using a diamond-based single photon source, and it was shown that Bohr's complementarity notion was still obeyed, as expected. It was recently proposed theoretically to take Wheeler's experiment one step further by employing a 'quantum beam-splitter' (QBS) at the interferometer output. In this case, the output beam-splitter (BSout) is placed in a coherent superposition of being absent and present, which also allows to further delay the choice of the measurement type. The type of measurement is determined only after having measured the state of the QBS, which can be (in principle) infinitely delayed. We demonstrate the realization of such an experiment by exploiting entanglement. In particular, we take advantage of pairs of polarization entangled photons. The behaviour of one of the paired photons, called the test photon, is analysed by sending it to a Mach-Zehnder interferometer having an entanglement-enabled QBS at its output. This device is made of a polarization dependent beam-splitter followed by polarization state erasers. The other photon, called the corroborative photon, allows determining the actual state of the QBS, and consequently, which test photon behaviour (wave, particle, or both) is measured. By manipulating the corroborative photon, we demonstrate a continuous morphing from wave to particle behaviour, which refutes too simple views of single photons behaving exclusively as waves or particles. The measurements on the test photon and the interferometer configuration are performed in a strictly delayed manner. The state of the QBS is measured only after the test photon already left the interferometer, and is detected, which means that it is destroyed before the type of measurement it under- goes is determined. We find that even in this configuration, Bohr's complementarity notion, and its extension, are perfectly obeyed. The space-like separation between the measurements invalidates local-hidden variable models associated with pre-existing information about the measurement out- comes. In other words, when the test photon is detected, no information is available about the type of measurement that was performed. This assertion is confirmed by the violation of the Bell's inequalities with more than 10 standard deviations. The presence of entanglement shows furthermore that genuine quantum behaviour for the test photons has been observed. As a conclusion, we have demonstrated the experimental realization of the quantum version of Wheeler's experiment. Trying to explain our observations in classical terms causes severe contradictions, but the results are in perfect agreement with quantum physics, in which the measurement timing order does not matter. The beauty in such experiments is that space and time do not seem to play any role.
A quantum system can behave as a wave or as a particle, depending on the experimental arrangement. When, for example, measuring a photon using a Mach-Zehnder interferometer, the photon acts as a wave if the second beam splitter is inserted, but as a particle if this beam splitter is omitted. The decision of whether or not to insert this beam splitter can be made after the photon has entered the interferometer, as in Wheeler's famous delayed-choice thought experiment. In recent quantum versions of this experiment, this decision is controlled by a quantum ancilla, while the beam splitter is itself still a classical object. Here, we propose and realize a variant of the quantum delayed-choice experiment. We configure a superconducting quantum circuit as a Ramsey interferometer, where the element that acts as the first beam splitter can be put in a quantum superposition of its active and inactive states, as verified by the negative values of its Wigner function. We show that this enables the wave and particle aspects of the system to be observed with a single setup, without involving an ancilla that is not itself a part of the interferometer. We also study the transition of this quantum beam splitter from a quantum to a classical object due to decoherence, as observed by monitoring the interferometer output.
The chapter discusses the Mach–Zehnder interferometer (MZI), which uses beam splitters in combination with mirrors to place a photon into a superposition of paths. In a final beam splitter, the two paths may be recombined to interfere, showing wave behavior. If the final beam splitter is removed, the photon is definitely in one path or the other, showing particle behavior. The polarization of light allows additional applications—one can use a polarizing beam splitter to separate and reconstitute the wave functions of a photon polarized at an angle. In a delayed-choice experiment, one decides which kind of experiment one is to do, particle or wave, after the photon has already entered the arms of the MZI. The chapter discusses a version of this that puts the MZI and a second-detector photon in an entangled state of particle and wave results.