Light's wave-particle duality is at the heart of quantum mechanics and can be well illustrated by Wheeler's delayed-choice experiment: The choice of inserting or removing the second classical (quantum) beam splitter in a Mach-Zehnder interferometer determines the classical (quantum) wave-particle behavior of a photon. In this paper, we report our experiment on directly observing simultaneous wave and particle behavior in a narrowband single-photon wave packet by classically inserting or removing the second beam splitter when part of the wave packet passes through it. Our experiment demonstrates that the produced wave-particle state can be utilized in encoding quantum information.
Light's wave-particle duality is at the heart of quantum mechanics and can be well illustrated by Wheeler's delayed-choice experiment: The choice of inserting or removing the second classical (quantum) beam splitter in a Mach-Zehnder interferometer determines the classical (quantum) wave-particle behavior of a photon. In this paper, we report our experiment on directly observing simultaneous wave and particle behavior in a narrowband single-photon wave packet by classically inserting or removing the second beam splitter when part of the wave packet passes through it. Our experiment demonstrates that the produced wave-particle state can be utilized in encoding quantum information.
Wheeler's delayed-choice experiment demonstrates wave-particle duality of particles using different experimental configurations of a Mach-Zehnder interferometer. In a quantum version of this experiment, the wave-particle behavior of photons can be observed by controlling presence or absence of the second beam splitter. Here, we implement a delayed-choice experiment with dual selections based on entangled photons, experimentally controlling both of two beam splitters in the Mach-Zehnder interferometer, where the presence and absence of the second beam splitter are simultaneously controlled through path encoding. Our experiment reveals the wave-particle behavior of single photons under different configurations of two beam splitters. We further discuss the scenario when two beam splitters are in a quantum superposition of being present and absent, photons will be in a wave-particle quantum superposition state.
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.
Wheeler’s gedanken experiment is the most fascinating and subtle illustration of wave-particle duality [1]. If we consider a single-photon light pulse in a Mach-Zehnder interferometer, one can either measure the interference at the output – evidencing the “wave-like” behavior of the photon- or one can detect the path followed by the photon – evidencing its “particle-like” behavior. In order to pinpoint how surprising this is, John Wheeler considered a “delayed-choice” regime where the choice of measuring either property is done only after the photon has entered the interferometer.
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.
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 wave-particle duality of light plays a fundamental role in introductory courses on quantum mechanics. Traditionally the wave and particle aspects of light are demonstrated in separate experiments which makes it difficult for students to understand their complementary nature. We present an experiment using a single apparatus that demonstrates the wave aspect, the particle aspect, and most importantly, their coexistence. The apparatus is based on a Mach-Zehnder interferometer in which a light beam is attenuated so that at each instant there is only a single photon in the interferometer. In this way the observation of single photon interference becomes possible. By integrating the single photon events in a storage oscilloscope the evolution toward classical interference fringes can be shown in real time. A second strong laser beam, derived from the same pointer, but slightly displaced, traverses the interferometer at the same time, allowing the simultaneous demonstration of wave aspects. Special features of the setup are low cost, simplicity, didactical power and suitability for presentations in large lecture halls using both multimedia projections and audible signals.