Squeezed gravitons from superradiant axion fields around rotating black holes

TL;DR

This work develops a semiclassical framework in which massive axion-like fields surrounding rotating Kerr black holes form clouds that drive superradiant amplification and produce multimode squeezed gravitons. By separating GR and gravitational Chern–Simons contributions, it shows that GR-induced squeezing dominates and can yield a potentially observable graviton yield for long-lived axion clouds, while CS terms imprint distinct polarization-entanglement structures but are EFT-suppressed. The analysis employs a Takagi–Autonne decomposition to map the theoretical squeezing kernel onto independent detector-relevant modes, providing a concrete link between microscopic axion dynamics and measurable interferometer signals. The results point to a viable astrophysical quantum gravity source and outline observational signatures, while noting important caveats such as curvature effects near the horizon and the need for detailed detector-frame modeling.

Abstract

We propose, in (3+1)-dimensional spacetimes, a novel astrophysical source of squeezed graviton states, due to superradiant axionic clouds surrounding rotating (Kerr-type) black holes (BH). The microscopic origin of these axions is diverse, ranging from the Kalb-Ramond (model-independent) axions and compactification axions in string theory, to \cm contorted geometries exemplified by a totally antisymmetric component of torsion in Einstein-Cartan theory. The axion fields couple to chiral gauge and gravitational Chern-Simons (CS) anomaly terms in the effective gravitational actions. In the presence of a Kerr BH background, such axions lead, upon acquiring a mass, to superradiance and the production of pairs of entangled gravitons in a squeezed state. The specific microscopic origin of the axions is not important, provided they are massive. This multimode squeezed-graviton state is examined through a Takagi-like decomposition, used in quantum optics. In the effective action it is shown that squeezing effects associated with conventional general relativity (GR) dominate, by many orders of magnitude, the corresponding effects due to the CS gravitational anomaly terms. For a sufficiently long lifetime of the axionic cloud of the BH, we find that significant squeezing (quantified through the average number of gravitons with respect to the appropriate vacuum) can be produced from the GR effects. It is also demonstrated explicitly that the structure of the entangled states (when the latter are expressed in a left-right polarization basis) depends highly on whether the GR or the anomalous CS effects produce the entanglement.

Figures (7)

• Figure 1: The superradiant axionic cloud around a rotating BH. The non-linear axion-graviton interactions are responsible for the production of pairs of entangled gravitons; the processes involved is the annihilation of two axions into two gravitons (in analogy to SFWM), and the axion decay into two gravitons (in analogy to SPDC). Picture taken from Dorlis:2025zzz.
• Figure 2: Polarization correlations from the GR induced interaction for LL and RR polarization pairs. The asymmetry is the result of the $2p$-state of the axionic cloud. The plots have been made for $a_{\mu}=0.1$, with $\vec{k}+\vec{k}^\prime$ lying on the y-axis, and $k=k^\prime=\mu_b$. Figure taken from Dorlis:2025zzz.
• Figure 3: Polarization correlations from the GR induced interaction for LR and RL polarization pairs. The plots have been made for $a_{\mu}=0.1$, with $\vec{k}+\vec{k}^\prime$ lying on the y-axis, and $k=k^\prime=\mu_b$. Figure taken from Dorlis:2025zzz.
• Figure 4: Polarization correlations from the GR induced interaction in the case that each graviton is emitted at different momenta ($k \neq k^\prime$) obeying though $k+k^\prime= 2\mu_b$. The plots have been made for $a_{\mu}=0.1$, with $\vec{k}+\vec{k}^\prime$ lying on the y-axis. One can observe that the case of $k=k^\prime=\mu_b$ is the dominant one. Left panel: $k=\frac{3}{2}\mu_b$ and $k^\prime = \frac{1}{2}\mu_b$. Right panel: $k=1.9\mu_b$ and $k^\prime = 0.1\mu_b$.
• Figure 5: Polarization correlations from the gravitational CS anomaly induced interaction. Only pairs of opposite polarizations are produced; Maximal entanglement between the Left and Right polarizations. The plots have been made for $a_{\mu}=0.1$, with $\vec{k}+\vec{k}^\prime$ lying on the y-axis, and $k=k^\prime=\mu_b/2$. Figure taken from Dorlis:2025zzz.