Résumé
We investigate the quantum optical torque on an atom interacting with an inhomogeneous electromagneticenvironment described by the most general linear constitutive relations. The atom is modeled as a two-levelsystem prepared in an arbitrary initial energy state. Using the Heisenberg equation of motion (HEM) and underthe Markov approximation, we show that the optical torque has a resonant and nonresonant part, associated,respectively, with a spontaneous-emission process and Casimir-type interactions with the quantum vacuum,which can both be written explicitly in terms of the system Green function. Our formulation is valid forany three-dimensional inhomogeneous, dissipative, dispersive, nonreciprocal, and bianisotropic structure. Weapply this general theory to a scenario in which the atom interacts with a material characterized by strongnonreciprocity and modal unidirectionality. In this case, the main decay channel of the atom energy is representedby the unidirectional surface waves launched at the nonreciprocal material-vacuum interface. To provide relevantphysical insight into the role of these unidirectional surface waves in the emergence of nontrivial optical torque,we derive closed-form expressions for the induced torque under the quasistatic approximation. Finally, weinvestigate the equilibrium states of the atom polarization, along which the atom spontaneously tends to aligndue to the action of the torque. Our theoretical predictions may be experimentally tested with cold Rydbergatoms and superconducting qubits near a nonreciprocal material. We believe that our general theory may findbroad application in the context of nanomechanical and biomechanical systems.