Axion clouds around black holes could reveal gravitons directly

Rotating black holes can feed energy into surrounding axion-like-particle clouds through superradiance, and this paper shows that process can simultaneously squeeze gravitational radiation into coherent, correlated multi-mode states containing up to ten million gravitons. Those squeezed states carry specific polarization correlations and quantum-noise patterns distinct from classical gravitational waves. Future interferometers sensitive enough to spot these signatures would provide direct evidence that gravity is quantized; a null result would constrain how long such axion clouds survive.