A unique setup involving an integrated photonic quantum chip in a quantum beamsplitter is helping to answer one of the most fundamental questions in physics: Is light made of waves or particles? Debates on the particle-wave theories of light have raged since the earliest days of science, with notable advocates on both sides of the issue. But the debate changed dramatically in 1905, when Albert Einstein showed that it was possible to explain the photoelectric effect using the idea that light is made of particles called photons. This discovery affected physics, greatly contributing to the development of quantum mechanics. Findings by University of Bristol physicists and quantum theorists comprise one of two research projects to measure this particle-wave duality. Both papers appeared in the same issue of Science (doi: 10.1126/science.1226719, doi: 10.1126/science.1226755). The quantum photonic chip used to test wave-particle duality in the University of Bristol experiment. Single photons are sent into the circuit using optical fibers and are detected at the output using extremely sensitive detectors. Both the Bristol and the Nice tests were based on a classic experiment first proposed by theoretical physicist John Wheeler in the 1980s. His experiment theorized that the act of observing a photon is what ultimately determines whether it will behave as a particle or a wave. The Bristol team, which included Dr. Alberto Peruzzo, Peter Shadbolt and professor Jeremy O’Brien, devised a novel apparatus that can measure both particle- and wavelike behavior simultaneously. They used a quantum beamsplitter to entangle a single second photon with a single first photon. A measurement on the second photon can decide what kind of measurement is done on the first, while in the process allowing them to explore the continuous passage of light from wavelike behavior to particlelike action. “Previously, many important experiments studying the fundamental behavior of quantum mechanics have been realized using bulk optical setups the size of a dining table,” Peruzzo told Photonics Spectra. “The use of integrated photonics allows the most important part of our experiment to be implemented on a chip, which is only 70 x 3 mm in size – a paradigm shift in quantum optics.” The integrated quantum photonic device they used, which was pioneered at Bristol, is a silica-on-silicon technology that enables low-loss waveguides at the near-infrared wavelength. Several small heaters placed on top of the waveguides were reconfigured using a computer-controlled system that enabled automated measurements throughout the setup. “A clear technological result of our work is the design and demonstration of an integrated controlled-Hadamard gate that will be used in future quantum technologies,” Peruzzo said. “On the fundamental side, we introduced entanglement-enabled measurements to tests of wave-particle duality, which will allow fundamentally new tests of Bohr’s principle of complementarity.” The team also found out just how contentious quantum phenomena remain. “We now have firsthand experience – in the form of emails, online comments and other correspondence from around the world – of the extent to which this question remains controversial, and the fact that the interpretation of the quantum nature of light is by no means a closed question in physics,” he said. The next step, he said, is to “perform more accurate, more sophisticated experiments to test the principle of complementarity, with the goal of being able to make stronger and stronger statements about the wave-particle duality of light.” They also want to continue to test the fundamentals of quantum mechanics using their integrated photonics approach.