Quantum induced superradiance

Superradiance, the phenomenon enabling energy extraction through radiation amplification, is not universal to all black holes. We show that semiclassical backreaction can induce superradiance, even when absent at the classical level. Specifically, we compute the quasinormal modes of a massless scalar field probing a family of rotating "quantum" black holes in three-dimensional anti-de Sitter space, accounting for all orders of backreaction due to quantum conformal matter. A subset of these modes is identified as superradiant, leading to the formation of quantum black hole "bombs." Further, we find all thermally unstable black holes exhibit modal linear instabilities, though the converse does not hold generally. Our findings thus suggest a semiclassical version of the Hollands-Wald conjecture on black hole stability. This motivates us to propose a stability criterion for quantum black holes.