Polariton Autler-Townes effect: Classical versus Quantum Features in Polaritonic Spectra

Understanding whether a polaritonic phenomenon is inherently quantum or classical is crucial for developing accurate theoretical models, optimizing experimental designs, and resolving conflicting findings. In this work, we explore this question within the context of molecular rotational-vibrational spectra. We examine the role of treating the cavity mode either classically or quantum mechanically when coupled to a quantum molecular system and report an intriguing new feature of the polaritonic spectrum that requires a quantum treatment of light. This feature, which we call the polaritonic Autler-Townes (PAT) effect, represents a new additional splitting beyond the usual resonant polariton splitting.

We investigate the PAT effect for HCl dimers coupled to a cavity, calculated fully ab initio, and corroborate our findings with a tunable three-level model system. Additionally, we explore the thermodynamic limit as the number of molecules approaches infinity. Our results show that the PAT effect persists in the many-molecule case, and its description requires a quantum treatment of light due to light-matter entanglement. Interestingly, the PAT splitting, in addition to the main resonant polariton splitting, follows the same rule: a linear increase in splitting with coupling strength. This offers a unique way to tune a quantum feature using a classical one, providing new insights for both theoretical and experimental approaches to polaritonic systems.