The crossover from quantum to semiclassical behavior in the seminal Rabi model of light-matter interaction still, surprisingly, lacks a complete and rigorous understanding. A formalism for deriving the semiclassical model directly from the quantum Hamiltonian is developed here. Working in a displaced Fock-state basis $|\alpha ,n⟩$, the semiclassical limit is obtained by taking $|\alpha |\to \infty$ and the coupling to zero. This resolves the discrepancy between coherent-state dynamics and semiclassical Rabi oscillations in both standard and ultrastrong coupling and driving regimes. Furthermore, it provides a framework for studying the quantum-to-semiclassical transition, with potential applications in quantum technologies.

• Received 30 May 2022
• Accepted 19 September 2022
• Corrected 27 March 2024

DOI:https://doi.org/10.1103/PhysRevLett.129.183603