IGFAE Colloquia | Squeezed Gravitons as a Potential Probe of Quantum Gravity

Despite decades of theoretical work on quantum gravity, the quantum nature of the gravitational interaction, if any, remains experimentally untested. One major challenge is the difficulty of detecting individual gravitons, the hypothetical quantum particles of the gravitational field.

Freeman Dyson argued that such a detection might be fundamentally impossible, as a detector sensitive enough to register a single graviton could collapse into a black hole. However, this does not necessarily rule out the possibility of detecting quantum effects of gravity through collective phenomena. In this talk, I will explore how such effects might appear in gravitational-wave detectors as non-classical fluctuations whose statistical properties reflect the quantum state of gravitons. I will first introduce quantum squeezing, a concept from quantum optics, and explain how it can arise in effective quantum field theories of gravity. I will then discuss an astrophysical mechanism in which rotating black holes surrounded by clouds of axion-like particles can produce large numbers of correlated, squeezed gravitons—potentially up to 106–107. Under suitable conditions, these gravitons could generate gravitational-wave signals with non-classical properties, such as sub-Poissonian statistics and entanglement between polarization states. These features may also reveal information about how the gravitons were produced. By connecting quantum optics, particle physics, black-hole physics, and gravitational-wave astronomy, this scenario suggests a possible new way to investigate the quantum nature of gravity through collective effects, potentially overcoming some of the challenges highlighted by Dyson\\\\\\\'s argument.