Quasinormal Mode Spectroscopy via Horizon-Brightened Quantum Optics

Abstract: We develop a quantum optical framework for probing black hole quasinormal modes (QNMs) using two-level atoms in the spirit of the horizon-brightened acceleration radiation (HBAR) program. Starting from the QNM contribution to the Wightman function of a scalar field on a static, spherically symmetric black hole background, we derive the response function of a two-level Unruh--DeWitt detector following simple trajectories (static at fixed radius, with comments on radial free fall). The QNM sector imprints a set of Lorentzian resonances in the detector spectrum at the redshifted real parts of the QNM frequencies, with widths determined by the imaginary parts. We then treat a single dominant QNM as an effective non-Hermitian cavity mode coupled to an ensemble of driven two-level atoms, and derive a master equation of Dicke laser type. The resulting lasing threshold condition depends explicitly on the QNM damping rate, providing a direct quantum optical interpretation of the imaginary part of the QNM frequency. Specializing to the Schwarzschild geometry, we express the resonant frequencies, linewidths, and threshold in terms of photon-sphere data in the eikonal limit. We discuss several extensions and propose our framework as a unifying language connecting black hole ringdown, near-horizon conformal quantum mechanics, and quantum optics, thereby enriching the emerging program of black hole spectroscopy in the gravitational-wave era.