We investigate the quantum optical behavior of quantum emitters coupled to a Weyl-like photonic environment. For the single emitter case, we will report on the formation of a novel photon-atom bound state featuring a tunable power-law localization of the excitation around the emitter. When two emitters are considered we show that the emergent bound state can mediate tunable and robust long-range coherent interactions.
We investigate the quantum optical behavior of quantum emitters coupled to a Weyl-like photonic environment. For the single emitter case, we will report on the formation of a novel photon-atom bound state featuring a tunable power-law localization of the excitation around the emitter. When two emitters are considered we show that the emergent bound state can mediate tunable and robust long-range coherent interactions.
Weyl photons appear when two three-dimensional photonic bands with linear dispersion are degenerate at a single momentum point, labeled as Weyl point. These points have remarkable properties such as being robust topological monopoles of Berry curvature as well as an associated vanishing density of states. Here, we study the quantum optical consequences of such topological Weyl photons by characterizing the individual and collective dynamics of quantum emitters close to resonance with these points. Using an exact non-perturbative treatment, we predict the development of non-exponential decay dynamics due to the emergence of localized photonic states around the emitters. We find that these bound states, whose wavefunction displays power-law spatial decay, can mediate coherent and topological long-range interactions when many emitters exchange energy through Weyl photons. Furthermore, by exploiting the topological protection of Weyl points, we provide a recipe to tune the range of the mediated interactions while keeping their power-law nature, something not possible in any other photonic platform. Thus, Weyl photons enable coherent, tunable, long-range interactions between emitters, and therefore can become a very valuable platform in the context of quantum simulation.
Weyl photons appear when two three-dimensional photonic bands with linear dispersion are degenerate at a single momentum point, labeled as Weyl point. These points have remarkable properties such as being robust topological monopoles of Berry curvature as well as an associated vanishing density of states. In this work, we report on a systematic theoretical study of the quantum optical consequences of such Weyl photons. First, we analyze the dynamics of a single quantum emitter coupled to a Weyl photonic bath as a function of its detuning with respect to the Weyl point and study the corrresponding emission patterns, using both perturbative and exact treatments. Our calculations show an asymmetric dynamical behavior when the emitter is detuned away from the Weyl frequency, as well as different regimes of highly collimated emission, which ultimately translate in a variety of directional collective decays. Besides, we find that the incorporation of staggered mass and hopping terms in the bath Hamiltonian both enriches the observed phenomenology and increases the tunability of the interaction. Finally, we analyze the competition between the coherent and dissipative components of the dynamics for the case of two emitters and derive the conditions under which an effective interacting spin model description is valid.
The interaction between quantum two-level systems is typically short-range in free space and most photonic environments. Here we show that Weyl photonic crystals can create significantly extended long-range interaction between distant quantum systems because of their diminishing momentum isosurfaces with equal frequencies around isolated Weyl points. The extended range of interaction is robust and does not rely on specific location or orientation of the transition dipoles. A general relation between the interaction range and properties of the isosurface is described, which provides a recipe to identify other photonic environments for that enable long-range interaction. This work lays the foundation to use Weyl photonic crystals as a platform to mediate quantum behavior.
We investigate the interactions between two-level emitters mediated by time-dependent, one-dimensional, structured photonic baths, focusing on Floquet topological lattices. Building on the framework of periodically driven photonic lattices, we demonstrate and characterize the emergence of tunable-range emitter interactions mediated by bound states absent in static photonic lattices. In particular, we show that one can obtain different spatial interaction dependencies not only with respect to the static bath scenarios, but also in qualitatively different regimes due to the time-dependent nature of the bath, for example, when the emitters have different frequencies. This work sheds light on the interplay between nonequilibrium photonics and quantum optics and can serve as a basis for analyzing Floquet photonic lattices in higher dimensions.
We discuss the properties of atom-photon bound states in waveguide QED systems consisting of single or multiple atoms coupled strongly to a finite-bandwidth photonic channel. Such bound states are formed by an atom and a localized photonic excitation and represent the continuum analog of the familiar dressed states in single-mode cavity QED. Here we present a detailed analysis of the linear and nonlinear spectral features associated with single- and multiphoton dressed states and show how the formation of bound states affects the waveguide-mediated dipole-dipole interactions between separated atoms. Our results provide both a qualitative and quantitative description of the essential strong-coupling processes in waveguide QED systems, which are currently being developed in the optical and microwave regimes.
Flat bands (FBs) are energy bands with zero group velocity, which in electronic systems were shown to favor strongly correlated phenomena. Indeed, a FB can be spanned with a basis of strictly localized states, the so called compact localized states (CLSs), which are yet generally non-orthogonal. Here, we study emergent dipole-dipole interactions between emitters dispersively coupled to the photonic analogue of a FB, a setup within reach in state-of the-art experimental platforms. We show that the strength of such photon-mediated interactions decays exponentially with distance with a characteristic localization length which, unlike typical behaviours with standard bands, saturates to a finite value as the emitter's energy approaches the FB. Remarkably, we find that the localization length grows with the overlap between CLSs according to an analytically-derived universal scaling law valid for a large class of FBs both in 1D and 2D. Using giant atoms (non-local atom-field coupling) allows to tailor interaction potentials having the same shape of a CLS or a superposition of a few of these.Flat Bands are dispersionless energy bands, feature causing a quenching of kinetic energy, making such systems extremely sensitive to small perturbations and non-linearities. Here, we examine the case in which the non-linearity is introduced through the coupling of two-level emitters (almost) resonant to the Flat Band. We find several features depending heavily on the orthogonality of the Flat Band basis, such as the emergence of compact bound states. We show that these compact states can be used to induce effective spin-Hamiltonians with exact finite-ranged interactions. Furthermore, in the non-orthogonal case we are able to observe the emergence of an exotic new class of bound states which, although resulting from the coupling in the bandgap of dispersive energy bands, can be completely studied through the Flat Band projector only. The localization length of these states is frequency-independent and is related to the overlap between states in the Flat Band basis. Finally, we also show that it is possible to engineer any shape of the bound state by controlling the coupling of a giant atom to the system.Flat bands (FBs) are band structures featuring zero-group velocity along the whole band, which in electronic systems have been shown to be source of strongly correlated phenomena. Here, we study the emergent photon-mediated interactions when emitter are tuned to the photonic analogue of FBs, a setup becoming within reach of state-of-art platforms In particular, we find that, unexpectedly, dispersive interactions between atoms are mediated by exponentially localized atom-photon bound states (BSs), whose range remains finite even for vanishing detuning from the FB. This represents a notable difference with respect to the cavity-QED case in which the presence of non-dispersive cavity modes does not lead to this kind of behaviour. We link this BS to a set of compact localized states (CLSs) spanning the FB eigenspace, which are generally non-orthogonal, and work out analytically the BS localization length as a function of the CLS non-orthogonality parameter for a large class of 1D and 2D lattices.We also show that photon-mediated interactions can be tailored in an exact way if giant atoms are coupled to the system in the vicinity of a FB.