How Much Can Gravitons Be Squeezed?

TL;DR

This paper proposes an astrophysical mechanism involving black holes and axion-like particles to generate multimode squeezed graviton states, potentially enabling direct detection of quantum gravitational effects.

Contribution

It introduces a novel method to produce detectable squeezed graviton states via superradiant axion clouds around black holes, offering a new avenue for quantum gravity tests.

Findings

Squeezed graviton states with up to 10^7 correlated quanta can be generated.

Distinctive polarization and quantum-noise signatures are predicted for future detectors.

Observation can confirm quantum nature of gravity or constrain axion-cloud properties.

Abstract

Quantum Gravity remains elusive, largely because its observable effects are suppressed by powers of the Planck scale. Direct detection of single gravitons is widely believed to be impossible. Here we propose a concrete astrophysical mechanism that may overcome this suppression. We show that superradiant axion-like-particle clouds surrounding rotating black holes can generate multimode squeezed states of gravitons containing up to $10^6$ - $10^7$ correlated quanta. Such states exhibit distinctive polarization correlations and quantum-noise signatures that could be detectable in future gravitational-wave interferometers. Observation of these signatures would constitute direct evidence for the quantum nature of gravitational radiation. Conversely, their absence can place constraints on axion-cloud lifetimes. Our approach also provides a test of General Relativity as an effective field theory.