Tabletop experiment helps reconcile fundamental physics

Tabletop experiment helps reconcile fundamental physics Gaby Clark Scientific Editor Andrew Zinin Lead Editor Assistant Professor Haocun Yu is something of a scientific diplomat. In a recent Physical Review Letters publication, she and her colleagues show how a tabletop experiment can bring together two bedrock physics theories that have never been fully reconciled. Subatomic gravity? More than a century ago, Albert Einstein gave us the theory of general relativity, describing gravity in relation to space and time on a large scale. Within a decade, physicists were developing a deeper knowledge of quantum mechanics, the laws that govern the subatomic world, including atoms, photons and other microscopic systems. "Quantum mechanics and general relativity are two of the most successful theories in physics, but they describe nature in very different ways," Yu explained. "To understand nature at its deepest level, we need experiments that probe where these two frameworks overlap. Studying gravitational effects in genuinely quantum systems can help reveal whether the two theories remain fully compatible in that regime, and it may point the way toward new physics." She said the challenge in those studies is that, compared with other physical effects, gravity is "extraordinarily weak" at the scale of single quantum particles. This gives it an extremely small signature that's difficult to gauge. To overcome that obstacle, she and her colleagues built a highly stable 50-kilometer (31-mile) optical interferometer using compact fiber coils and tested it with single photons. The entire apparatus can fit on a tabletop. An optical interferometer splits a beam of light, then recombines the resulting beams to create an interference pattern that makes precision measurements. Most lab-based interferometers lack the sensitivity required to pick up the elusive signal of gravity in a quantum system. In this experiment, the team's unique design successfully detected a gravitationally induced phase signal small enough to reach the regime needed for laboratory-scale measurements of gravitational redshift, a prediction of general relativity, with quantum light. "Experiments involving both general relativity and quantum mechanics are necessarily at the cutting edge of precision measurement," Yu explained. "Experiments simply did not have the stability, size and phase sensitivity needed to reach that regime. Our work helps bring these tests closer to experimental reach." Quantum phenomena on a human scale Yu is a Ph.D. graduate of the Massachusetts Institute of Technology (MIT), and the PRL research comes from her work as a Marie Curie Postdoctoral Fellow at the University of Vienna. She joined the University of Tennessee, Knoxville, physics faculty in January 2026 and is drawing on her experience to create new research opportunities. "I am building a quantum optics and sensing research program with broad applications, where advancing and developing new quantum tools for fundamental science is a central part of that vision," she said. Yu is actively looking for students and postdocs to join her research group and contribute to work she said has long fascinated her. "What especially drew me in is that fragile quantum effects can be harnessed as practical tools for precision measurement," she said. "In return, these enhanced experiments can reveal mesoscopic and macroscopic quantum behavior in measurement devices—bringing quantum phenomena closer to human scales and advancing quantum science itself." Publication details Haocun Yu et al, 50-km Fiber Interferometer for Testing Gravitational Signatures in Quantum Interference, Physical Review Letters (2026). DOI: 10.1103/yvlk-t1j9 Journal information: Physical Review Letters Provided by University of Tennessee at Knoxville