Although quantum phenomena are being explored for applications in computing, encryption, and sensing, the physics behind quantum phenomena is not yet fully understood. New work by a research team at the University of Science and Technology of China (USTC), to be presented at the Frontiers in Optics + Laser Science conference in September, could help improve techniques for probing quantum mechanics. In the new study, the researchers demonstrate ways to close loopholes in experiments conducted to test Bell’s inequality, an approach to testing quantum mechanics devised in the 1960s by physicist John Bell. The USTC study contributes evidence that quantum mechanics governs the interactions between two photons. Researchers created entangled photon pairs and distributed the two photons of each pair to two measurement stations in opposite directions. At each measurement station, a telescope received the photons from the selected cosmic radiation source, which is at least 11 light-years from Earth. The cosmic photon detection signals generate random bits for measurement-setting choices for the loophole-free Bell test. In this experiment, the researchers closed detection and locality loopholes, and pushed the time constraint to rule out local hidden variable models to 11 years before the experiment. Courtesy of Ming-Han Li, USTC, Shanghai. The researchers demonstrated quantum interactions between two particles spaced more than 590 ft apart. At the same time, the researchers eliminated the possibility that shared events during the past 11 years could have affected these interactions. The experimental setup consisted of one device that periodically sent out pairs of entangled photons and two stations that measured the photons. The first measurement station was 305 ft from the photon pair source and the second station was 295 ft away in the opposite direction. The entangled photons traveled through a single-mode optical fiber to the measurement stations, where their polarization state was measured with a Pockels cell. The photons were detected by superconducting nanowire single-photon detectors. The experiment was designed to examine and overcome three ideas: 1) the idea that loss and noise make detection unreliable (the detection loophole); 2) the idea that any communication that affects measurement choices about two particles that are located far apart but are connected via quantum entanglement makes the measurement cheatable (the locality loophole); and 3) the idea that a measurement-setting choice that is not truly free and random leads to a result that can be controlled by a hidden cause in the common past (the freedom-of-choice loophole). The researchers demonstrated that their setup achieved a sufficiently low level of loss and noise by comparing measurements made at the start and at the end of the photon’s journey. This addressed the concept of a detection loophole. To address the locality loophole, the researchers built space-like separation between the events of measurement-setting choice into the experimental setup. To address the freedom-of-choice loophole, the team based its measurement-setting choices on cosmic photon behavior from 11 years earlier to instill a high level of confidence that nothing in the photons’ shared past — at least for the past 11 years — had created a hidden variable affecting the outcome. By combining theoretically calculated predictions with experimental results, the researchers were able to demonstrate quantum interactions between the entangled photon pairs with a high degree of confidence and fidelity. Their experiment could be considered robust evidence that quantum effects, rather than hidden variables, are behind the photons’ behavior. “We realized a loophole-free Bell test with the measurement settings determined by remote cosmic photons,” said USTC researcher Ming-Han Li. “Thus we verified the completeness of quantum mechanics with high-confidence probability.” The research, “Test of Local Realism into the Past without Detection and Locality Loopholes” by Ming-Han Li et al., will be presented Sept. 16, 2019, at the Frontiers in Optics + Laser Science (FiO + LS) conference in Washington, D.C.