It is well-known that photons can act like waves or particles, depending on how they are measured experimentally. But they have never been seen exhibiting both behaviors at the same time — until now. Artist's impression of the behavior of single photons when passing through an interferometer having a quantum beamsplitter at its output. In the back of the picture, sinusoidal oscillations are observed, indicating single-photon interference, and therefore a wavelike phenomenon. In the front of the picture, no oscillations are observed, which is the signature of particle behavior. Between these two extremes, the single photons' behavior is continuously morphed from wavelike to particlelike, indicating superposition of these two states, and illustrating the inadequacy of a naive wave or particle description of light. This image relates to the paper by Kaiser et al. Courtesy of S. Tanzilli, CNRS. The particle-wave theories of light were debated from the earliest days of science. Sir Isaac Newton advocated for particle theory, while James Clerk Maxwell and his theory of electromagnetism gave credit to wave theory. But the debate changed dramatically in 1905, when Einstein showed that it was possible to explain the photoelectric effect using the idea that light is made of particles called photons. This discovery had a huge impact on physics, as it greatly contributed to the development of quantum mechanics. Artist’s impression, inspired by the work of the artist Maurits Cornelis Escher, of the continuous morphing between particle- and wavelike behavior of light achieved in the Bristol experiment. This image relates to the paper by Peruzzo et al. Courtesy of Alberto Peruzzo, Peter Shadbolt, Nicolas Brunner, and Jamie Simmonds. Quantum mechanics predicts with remarkable accuracy the behavior of small objects such as atoms and photons; however, these predictions are strikingly counterintuitive. For instance, quantum theory estimates that a particle such as a photon can be in different places at the same time, or it can even be in infinitely many places at the same time, exactly as a wave is. This notion of wave-particle duality, dubbed the "one real mystery of quantum mechanics" by Nobel Prize laureate Richard Feynman, is fundamental to all quantum systems. Two new independent studies appearing in Science use different approaches to measuring the morphing of light from wave to particle, something that could reveal light's true nature. Both approaches are derived from a classic experiment first proposed by theoretical physicist John Wheeler in the 1980s. Wheeler's experiment theorized that the act of observing a photon is what ultimately determines whether it will behave as a particle or a wave. Alberto Peruzzo (left) and Peter Shadbolt (right), joint lead authors of the experiment. This image relates to the paper by Peruzzo et al. Courtesy of Fernando Traquino. University of Bristol physicists and quantum theorists led by Alberto Peruzzo, research fellow at the Center for Quantum Photonics, devised a new test based on Wheeler's original experiment to measure the particle and wave nature of light at the same time (doi: 10.1126/science.1226719). They used a quantum beamsplitter to entangle a single second photon with a single first photon. Performing a measurement on the second photon can decide what kind of measurement is done on the first, in the process allowing the researchers to explore the continuous passage of light from wavelike behavior to particlelike action. The quantum photonic chip used to test wave-particle duality in the Bristol experiment. Single photons are sent into the circuit using optical fibers, and are detected at the output using extremely sensitive detectors. This image relates to the paper by Peruzzo et al. Courtesy of Peter Shadbolt. “The measurement apparatus detected strong nonlocality, which certified that the photon behaved simultaneously as a wave and a particle in our experiment,” Peruzzo said. “This represents a strong refutation of models in which the photon is either a wave or a particle.” “To conduct this research, we used a quantum photonic chip, a novel technology pioneered in Bristol. The chip is reconfigurable so it can be programmed and controlled to implement different circuits,” said professor Jeremy O’Brien, director of the Center for Quantum Photonics. “Today this technology is a leading approach in the quest to build a quantum computer and in the future will allow for new and more sophisticated studies of fundamental aspects of quantum phenomena.” Artist’s impression, inspired by the work of the artist Maurits Cornelis Escher, of the continuous morphing between particle- and wavelike behavior of light achieved in the Bristol experiment. This image relates to the paper by Peruzzo et al. Courtesy of Nicolas Brunner and Jamie Simmonds. Florian Kaiser and colleagues at CNRS (National Center for Scientific Research) at the University of Nice performed Wheeler's experiment using pairs of entangled photons. One photon is tested in an interferometer and is detected, while the other photon lets the researchers determine whether wave, particle or some sort of in-between behavior has occurred. Their experimental setup lets the photon morph continuously from wave to particle. (doi: 10.1126/science.1226755) For more information, visit: www.bristol.ac.uk or http://unice.fr/