Researchers at the University of Rostock and Tampere University have experimentally demonstrated that studies in quantum photonics can be scaled up to encompass a broader spectrum of quantum mechanics than previously believed. The experiment made use of parity-time (PT) symmetry, a relatively new frame of reference that extends the traditional Hermitian quantum mechanics framework to increase the number of systems that can be described using quantum theory. The researchers demonstrated two-photon quantum interference in a PT-symmetric system. The experimental measurements were carried out at the University of Rostock in Germany. Professor Marco Ornigotti, formerly at the University of Rostock and now at Tampere University, highlights the contributions of his former colleague, physicist Alexander Szameit, to the study. “He had a major role in this research,” Ornigotti said. The researchers based their experimental setup on the Hong-Ou-Mandel (HOM) effect. Two photons were combined inside a beam splitter. To monitor their quantum interference, photodetectors were placed at each of the two output ports of the beam splitter. Although photons start out traveling in different directions when entering a beamsplitter, two photons will always leave the beamsplitter together out of the same output port. This phenomenon is called photon bunching and is at the heart of the HOM effect, the researchers said. “We used a very fundamental quantum property of light, namely two-photon interference, to show for the first time that the very fact that light is evolving in a PT-symmetric environment, rather than a traditional Hermitian one, makes the photon bunching happen earlier than expected.” When a photon arrives at the detector, it produces a click. If both detectors click at the same time, this would mean that the photons exited the system from different output ports and that they will be distinguishable from each other. “On the other hand, if one detector clicks and the other one does not, we can be sure that the photons are indistinguishable,” Ornigotti said. “In other words, they are bunched together. In this case the quantum properties of the two photons will be manifestly present, and [this is] the case we were looking at.” This phenomenon can only be observed when the two photons enter the beamsplitter at exactly the same time. The researchers tested HOM interference using a PT-symmetric beamsplitter consisting of two waveguides: one that experienced losses, and one that did not. They found that when the photons entered a lossy environment, they bunched together at a smaller scale than when the same experiment was performed in a Hermitian integrated beamsplitter. “We used a very fundamental quantum property of light, namely two-photon interference, to show for the first time that the very fact that light is evolving in a PT-symmetric environment, rather than a traditional Hermitian one, makes the photon bunching happen earlier than expected,” Ornigotti said. While the experimental setup must be carefully designed to match the requirements of PT-symmetry, this discovery could one day be used in quantum computers. Existing quantum computers are sensitive to all external interference and must be isolated and kept at temperatures close to absolute zero, making them prohibitively expensive. “The development of a light-powered quantum computer would be a major breakthrough akin to the arrival of electronics and the invention of conventional computers a few decades ago,” Ornigotti said. “Such a quantum computer would be capable of performing at room temperature and could be easily adapted to mass production.” The research was published in Nature Photonics (https://doi.org/10.1038/s41566-019-0517-0).