Binary gravitational waves as probes of quantum graviton states

TL;DR

This work proposes a graviton quantum-test based on Hanbury Brown–Twiss interferometry of gravitational waves from binaries, linking nonclassical graviton statistics to inflation-imprinted states. It treats GW emission as a coherent displacement of a primordial squeezed vacuum, yielding a coherent–squeezed state that can exhibit sub-Poissonian number statistics under suitable conditions. The authors derive a sub-Poissonian criterion, relate it to present-day frequencies, and estimate a GW150914-like scenario where a sub-Hertz signal could reveal quantum features of gravitons, potentially informing inflationary physics and quantum gravity. If detected, these signatures would provide a direct glimpse into the quantum nature of gravity and the early Universe, while acknowledging that decoherence and alternative inflationary dynamics could modify the expected signals.

Abstract

It is well known that the most reliable way to reveal the quantum nature of light is through photon number statistics, since photons exhibiting sub-Poissonian statistics unambiguously demonstrate their quantum behavior. In this paper, we show that gravitons emitted by binary systems can, in principle, exhibit analogous sub-Poissonian statistics. The key idea is that the vacuum state of gravitons may not be the standard Minkowski vacuum but rather a nonclassical state imprinted with the physics of the early Universe, such as inflation. Accordingly, gravitational waves from binary systems provide a means to probe the graviton states generated in the early Universe. As a concrete example, we show that squeezed graviton states originating from inflation may be detected through the observation of gravitational waves from binary systems.