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.

Based on the theory of macroscopic quantum electrodynamics, we generalize the expression of theCasimir force for nonreciprocal media. The essential ingredient of this result is the Green’s tensor betweentwo nonreciprocal semi-infinite slabs, including a reflexion matrix with four coefficients that mixes opticalpolarizations. This Green’s tensor does not obey Lorentz’s reciprocity and thus violates time-reversal symmetry.The general result for the Casimir force is analyzed in the retarded and nonretarded limits, concentrating on theinfluences arising from reflections with or without change of polarization. In a second step, we apply our generalresult to a photonic topological insulator whose nonreciprocity stems from an anisotropic permittivity tensor,namely InSb. We show that there is a regime for the distance between the slabs where the magnitude of theCasimir force is tunable by an external magnetic field. Furthermore, the strength of this tuning depends on theorientation of the magnetic field with respect to the slab surfaces.

We present a semi-analytical model of the resonance phenomena occurring in a hybrid system made of a 1D array of periodic subwavelength slits deposited on an insulator/graphene layer. We show that the spectral response of this hybrid system can be fully explained by a simple semi-analytical model based on weak and strong couplings between two elementary sub-systems.

Light-matter interactions in conventional nanophotonic structures typically lack directionality. For example, differently from microwave antenna systems, most optical emitters (e.g., excited atoms/molecules and simple nanoantennas) exhibit quasi-isotropic dipolar radiation patterns with low directivity. Furthermore, surface waves supported by conventional material substrates do not usually have a preferential direction of propagation, and their wavefront tends to spread as it propagates along the surface, unless the surface or the excitation is properly engineered and structured. In this article, we theoretically demonstrate the possibility of realizing unidirectional and diffractionless surface plasmon polariton modes on a nonreciprocal platform, namely, a gyrotropic magnetized plasma. Based on a rigorous Green’s function approach, we provide a comprehensive and systematic analysis of all the available physical mechanisms that may bestow the system with directionality, both in the sense of one-way excitation of surface waves and in the sense of directive diffractionless propagation along the surface. The considered mechanisms include (i) the effect of strong and weak forms of nonreciprocity, (ii) the elliptic-like or hyperbolic-like topology of the modal dispersion surfaces, and (iii) the source polarization state, with the associated possibility of chiral surface-wave excitation governed by angular-momentum matching. We find that three-dimensional gyrotropic plasmonic platforms support a previously unnoticed wave-propagation regime that exhibit several of these physical mechanisms simultaneously, allowing us to theoretically demonstrate unidirectional surface plasmon polariton modes that propagate as a single ultranarrow diffractionless beam. We also assess the impact of dissipation and nonlocal effects. Our theoretical findings may enable a new generation of plasmonic structures and devices with highly directional response.

Radiative heat transfer (RHT) between two metallic nanoparticles clusters in both near field and far field areexplored using many-body radiative heat transfer theory implemented with the coupled electric and magneticdipole (CEMD) approach, which effectively takes into account the contribution of magnetic polarization ofmetallic nanoparticles on heat exchange. The effects of magnetic polarization, many-body interaction (MBI),fractal dimension, and relative orientation of the clusters on RHT were analyzed. The results show thatthe contribution of magnetically polarized eddy-current Joule dissipation dominates the RHT between Agnanoparticle clusters. If the electric polarization (EP approach) only is considered, the heat conductance will beunderestimated as compared with the CEMD approach in both near field and far field regime. The effect of MBIon the RHT between Ag nanoparticle clusters is insignificant at room temperature, which is quite different fromthe SiC nanoparticle clusters. For the latter, MBI tends to suppress RHT significantly. The relative orientationhas remarkable effect on radiative heat flux for clusters with lacy structure when the separation distance is in thenear field. While for the separation distance in far field, both the relative orientation and the fractal dimensionhas a weak influence on radiative heat flux. This work will help the understanding of thermal transport in denseparticulate system.

The transient Casimir-Polder force on a two-level atom introduced into a three-dimensional, inhomogeneous, generally nonreciprocal environment is evaluated using non-Markovian Weisskopf-Wigner theory in the strong- and weak-coupling regimes. Ground-state and excited atoms are shown to decouple into two separate initial-value problems, and both the short-time and long-time atomic population and force are evaluated. The results are compared with various Markov approximations of the Weisskopf-Wigner theory and with previous Markov results from the Heisenberg picture.

The study of open quantum systems often relies on approximate master equations derived under the assumptions of weak coupling to the environment. However when the system is made of several interacting subsystems such a derivation is in many cases very hard. An alternative method, employed especially in the modeling of transport in mesoscopic systems, consists in using local master equations (LMEs) containing Lindblad operators acting locally only on the corresponding subsystem. It has been shown that this approach however generates inconsistencies with the laws of thermodynamics. In this paper we demonstrate that using a microscopic model of LMEs based on repeated collisions all thermodynamic inconsistencies can be resolved by correctly taking into account the breaking of global detailed balance related to the work cost of maintaining the collisions. We provide examples based on a chain of quantum harmonic oscillators whose ends are connected to thermal reservoirs at different temperatures. We prove that this system behaves precisely as a quantum heat engine or refrigerator, with properties that are fully consistent with basic thermodynamics.