Gabriele Orlando | Searcharxiv

arXiv2026

Quantum Vacuum Radiation Near a Critical Point

Equilibrium quantum phase transitions profoundly reshape the ground state of light-matter systems; yet, the resulting quantum correlations, such as squeezing and entanglement, remain experimentally inaccessible since they involve virtual ground state excitations. We investigate how a nonadiabatic modulation of a Hamiltonian parameter can convert these virtual excitations into real photons, enabling quantum vacuum radiation. We show that proximity to the critical point strongly enhances the emitted photon flux and the non-classical nature of the emitted radiation, even when thermal fluctuations are expected to dominate. In addition, higher-order processes become relevant even for small modulation amplitudes, and we develop a framework that systematically incorporates them. Our results reveal that criticality can act as an efficient amplifier of vacuum fluctuations, offering new routes to probe and exploit quantum critical ground states.

quant-ph↗

arXiv2024

Superradiant Quantum Phase Transition in Open Systems: System-Bath Interaction at the Critical Point

The occurrence of a second-order quantum phase transition in the Dicke model is a well-established feature. On the contrary, a comprehensive understanding of the corresponding open system, particularly in the proximity of the critical point, remains elusive. When approaching the critical point, the system inevitably enters first the system-bath ultrastrong coupling regime and finally the deepstrong coupling regime, causing the failure of usual approximations adopted to describe open quantum systems. We study the interaction of the Dicke model with bosonic bath fields in the absence of additional approximations, which usually relies on the weakness of the system-bath coupling. We find that the critical point is not affected by the interaction with the environment. Moreover, the interaction with the environment is not able to affect the system ground-state condensates in the superradiant phase, whereas the bath fields are infected by the system and acquire macroscopic occupations. The obtained reflection spectra display lineshapes which become increasingly asymmetric, both in the normal and superradiant phases, when approaching the critical point.

quant-ph↗