arXiv subjects

Vasil Rokaj

Publications and source records attributed to Vasil Rokaj.

At least 19 recordsLinked to original sources

arXiv2026

Robustness of quantized Hall resistivity under cavity coupling at zero temperature

Recent experiments have shown that strong light-matter coupling in electromagnetic cavities can modify transport properties of quantum Hall systems through the formation of Landau polaritons, prompting questions about the robustness of topological protection. While earlier theory demonstrated that the Hall conductivity can be modified at finite temperature and finite polariton lifetime (or finite broadening), experiments primarily probe the resistivity tensor. Our phenomenological model reveals an asymmetry between conductivity and resistivity in quantum Hall systems under strong light-matter interaction, showing that at zero temperature the Hall resistivity remains completely immune to cavity-induced modifications arising from polariton broadening, independent of the light-matter coupling strength. These results provide a deeper explanation for the absence of renormalization in the von Klitzing constant in experiments probing the even QH plateaus through the Hall resistivity at low temperature, and clarify the distinct roles of dissipation and strong light-matter coupling in hybrid light-matter systems.

cond-mat.mes-hall↗

arXiv2026

Collective rotational cat states of molecules in microwave cavities

We show theoretically that an ensemble of polar molecules coupled to a microwave cavity supports hybrid rotational-photonic cat states. The cavity couples to a symmetric rotor in the bright manifold of $N$ molecules with $\sqrt{N}$-enhancement. In the dispersive limit of the collective strong coupling regime, virtual multilevel transitions induce an effective Kerr nonlinearity, as confirmed by Wigner tomography and a Schrieffer-Wolff analysis, leading to parity-locked cat structure in the cavity sectors. Collective molecular rotations thus provide a new route to hybrid light-matter cat states.

quant-ph↗

arXiv2025

Multimode equilibrium approximations in light-matter systems from weak to strong coupling

In this work, we detail different approaches to treat multi-mode photonic environments within non-relativistic quantum electrodynamics in the long-wavelength approximation efficiently. Specifically we show that for equilibrium properties of coupled light-matter systems, we can approximately capture the effects of multi-mode photonic environments on matter systems by either only keeping the polarization part of the electric field in the length-gauge formulation or by a few effective modes. We present a comprehensive set of approximation methods designed to accurately capture equilibrium phenomena in quantum light-matter systems across a range of complex photonic environments, from weak to strong coupling. These methods are applied to atomic and molecular models as well as to a two-dimensional quantum ring, demonstrating the versatility of our approach and laying the groundwork for first-principles simulations of real materials in cavity quantum electrodynamics.

quant-ph↗

arXiv2025

Hybrid light-matter boundaries of graphene in a chiral cavity

Recent advances in chiral cavities that can couple coherently to two-dimensional materials have opened a powerful route to reshape electronic topology without an external drive. Here we establish the bulk-boundary correspondence for graphene embedded in a circularly polarized cavity. By combining exact diagonalization (ED) of zigzag ribbons, a semi-analytic T-matrix for half-infinite lattices, and analytical insights from a Dirac-Jaynes-Cummings model, we show that (i) every light-matter interaction-induced gap hosts pairs of unidirectional light-matter edge currents depending on the Chern number of the band while some of them are even bright; (ii) these chiral states persist throughout the entire photon ladder; and (iii) their dispersion, localization length and photon distribution exhibit a universal scaling controlled by the light-matter interaction. Time-evolution simulations further demonstrate that a dark electronic edge excitation can be converted into a bright and unidirectionally propagating current that remains coherent over long time scales. Our results predict an experimental signature of the hybrid band topology and a blueprint for tunable chiral channels in next generation quantum optical solid-state devices.

cond-mat.mes-hall↗

arXiv2025

Realization of a Chiral Photonic-Crystal Cavity with Broken Time-Reversal Symmetry

Light-matter interactions in chiral cavities offer a compelling route to manipulate material properties by breaking fundamental symmetries such as time-reversal symmetry. However, only a limited number of chiral cavity implementations exhibiting broken time-reversal symmetry have been demonstrated to date. These typically rely on either the application of strong magnetic fields, circularly polarized Floquet driving, or the hybridization of cavity modes with matter excitations in the ultrastrong coupling regime. Here, we present a one-dimensional terahertz photonic-crystal cavity that exhibits broken time-reversal symmetry. The cavity consists of a high-resistivity silicon wafer sandwiched between lightly n-doped InSb wafers. By exploiting the nonreciprocal response of a terahertz magnetoplasma and the exceptionally low effective mass of electrons in InSb, we demonstrate a circularly polarized cavity mode at 0.67 THz under a modest magnetic field of 0.3 T, with a quality factor exceeding 50. Temperature-, magnetic field-, and polarization-dependent measurements, supported by simulations, confirm the realization of a chiral cavity with broken time-reversal symmetry. This platform offers a robust and accessible approach for exploring chiral light--matter interactions and vacuum dressed quantum condensed matter in the terahertz regime.

physics.optics↗

arXiv2025

Cavity-Mediated Collective Resonant Suppression of Local Molecular Vibrations

Recent advances in polaritonic chemistry suggest that chemical reactions can be controlled via collective vibrational strong coupling (VSC) in a cavity. In this fully analytical work, we demonstrate that the collective vibrations of a molecular ensemble under VSC execute a beating with a period inversely proportional to the collective vacuum Rabi splitting. Significantly, this collective beating is imprinted on the local dynamics and resonantly suppresses individual molecular vibrations when a fraction of molecules are vibrationally excited, as in activated complexes formed in chemical reactions. This emergent beating occurs on significantly longer time scales than the individual molecular vibration or the cavity field oscillation period, peaking at the cavity-molecule resonance, consistent with polaritonic chemistry experiments. The cavity mediates an energy exchange between excited and ground-state molecules, affecting the dynamics of the entire ensemble. These findings suggest that the dynamics in polaritonic chemical reactions may not be in full equilibrium. In the ultra-strong coupling regime, we find that the local vibrations are modified by the cavity even at short time scales. Notably, these dynamical effects also extend to isotropic molecular ensembles in our model. Our analytical model offers insights into how collective VSC can dampen local molecular vibrations at resonance, potentially altering chemical reactivity.

quant-ph↗

arXiv2025

Non-perturbative Mass Renormalization Effects in Non-relativistic Quantum Electrodynamics

In this work we investigate the effects that multi-mode photonic environments, e.g., optical cavities, have on the properties of quantum matter. We highlight the importance of the non-perturbative mass renormalization procedure for ab initio quantum electrodynamics simulations and how it connects to common approximations used in polaritonic chemistry and cavity materials engineering. We focus on one-dimensional systems which can be solved exactly for large number of photon modes. First, we apply mass renormalization to free particles. The value of the renormalized mass depends on the details of the photonic environment and on the number of particles. We then show how the multi-mode photon field influences various ground- and excited-state properties of atomic and molecular systems. For instance, we observe the enhancement of particle confinement in the binding potential for the atomic system, and the modification of the potential energy surfaces of the molecular dimer due to photon-mediated long-range interactions. We also highlight how these changes compare to the common free-space mass-renormalization approximation employed in electronic structure theory and quantum chemistry. Since such phenomena are enhanced under strong light-matter coupling in a cavity environment they will become relevant for the emerging fields of polaritonic chemistry and cavity materials engineering.

quant-ph↗

arXiv2024

Terahertz chiral photonic-crystal cavities for Dirac gap engineering in graphene

Strong coupling between matter and vacuum electromagnetic fields in a cavity can induce novel quantum phases in thermal equilibrium via symmetry breaking. Particularly intriguing is the coupling with circularly polarized cavity fields, which can break time-reversal symmetry (TRS) and lead to topological bands. This has spurred significant interest in developing chiral cavities that feature broken TRS, especially in the terahertz (THz) frequency range, where various large-oscillator-strength resonances exist. Here, we present a design for high-quality-factor THz chiral photonic-crystal cavities (PCCs) that achieves broken TRS using a magnetoplasma in a lightly doped semiconductor. We incorporate ab initio density functional theory calculations into the derived microscopic model, allowing a realistic estimate of the vacuum-induced gap in graphene when coupled to our chiral cavity. Our calculations show an enhancement in the light-matter interaction due to Dirac nodes and predict an energy gap on the order of 1 meV. The THz chiral PCCs offer a promising platform for exploring cavity-dressed condensed matter with broken TRS.

physics.optics↗

arXiv2024

Dynamical generation and transfer of nonclassical states in strongly interacting light-matter systems in cavities

We propose leveraging strong and ultrastrong light-matter coupling to efficiently generate and exchange nonclassical light and quantum matter states. Two initial conditions are considered: (a) a displaced quadrature-squeezed matter state, and (b) a coherent state in a cavity. In both scenarios, polaritons mediate the dynamical generation and transfer of nonclassical states between light and matter. By monitoring the dynamics of both subsystems, we uncover the emergence of beatings in the collective matter oscillations. The beating period depends on the particle density through the vacuum Rabi splitting and peaks sharply under light-matter resonance conditions. For initial condition (a), nonclassicality is efficiently transferred from matter to photons under strong and ultrastrong coupling. However, for initial condition (b), nonclassical photonic states are generated only in the ultrastrong coupling regime due to the counter-rotating terms, highlighting the advantages of ultrastrong coupling. Furthermore, in the ultrastrong coupling regime, distinctive asymmetries relative to cavity detuning emerge in dynamical observables of both light and matter. The nonclassical photons can be extracted through a semi-transparent cavity mirror, while nonclassical matter states can be detected via time-resolved spectroscopy. This work highlights that hybrid polariton states can be utilized for dynamically generating nonclassical states, with potential applications in quantum state transfer.

quant-ph↗

arXiv2024

Multiphoton dressed Rydberg excitations in a microwave cavity with ultracold Rb atoms

We investigate magneto-optical trap loss spectroscopy of Rydberg excited $^{85}$Rb ($66\leq n \leq 68~S_{1/2}$) atoms, placed inside a tailored microwave cavity. The cavity frequency at 13.053 GHz is in resonance with the $67S_{1/2} \rightarrow 66P_{3/2}$ transition, inducing a ladder multiphoton microwave Rydberg absorption and emission. The observed spectra are modeled with an extended Jaynes-Cumming formalism that accounts for multiphoton absorption from and emission into the cavity, the loss from the trap due to Rydberg excitation, and cavity imperfection. We calculate the average photons in each spectral feature and find evidence for fractional photon emission into the cavity modes. The microwave cavity Rydberg spectroscopy in this work should inform the application and technology development of Rydberg based sensors and hybrid Rydberg atom-superconducting resonator quantum gates.

physics.atom-ph↗

arXiv2024

Weakened Topological Protection of the Quantum Hall Effect in a Cavity

We study the quantum Hall effect in a two-dimensional homogeneous electron gas coupled to a quantum cavity field. As initially pointed out by Kohn, Galilean invariance for a homogeneous quantum Hall system implies that the electronic center of mass (CM) decouples from the electron-electron interaction, and the energy of the CM mode, also known as Kohn mode, is equal to the single particle cyclotron transition. In this work, we point out that strong light-matter hybridization between the Kohn mode and the cavity photons gives rise to collective hybrid modes between the Landau levels and the photons. We provide the exact solution for the collective Landau polaritons and we demonstrate the weakening of topological protection at zero temperature due to the existence of the lower polariton mode which is softer than the Kohn mode. This provides an intrinsic mechanism for the recently observed topological breakdown of the quantum Hall effect in a cavity [Appugliese et al., Science 375, 1030-1034 (2022)]. Importantly, our theory predicts the cavity suppression of the thermal activation gap in the quantum Hall transport. Our work paves the way for future developments in the cavity control of quantum materials.

cond-mat.mes-hall↗

arXiv2023

Cavity Induced Topology in Graphene

Strongly coupling materials to cavity fields can affect their electronic properties altering the phases of matter. We study the monolayer graphene whose electrons are coupled to both left and right circularly polarized photons, and time-reversal symmetry is broken due to a phase shift between the two polarizations. We develop a many-body perturbative theory, and derive cavity mediated electronic interactions. This theory leads to a gap equation which predicts a sizable topological band gap at Dirac nodes in vacuum and when the cavity is prepared in an excited Fock state. Remarkably, band gaps also open in light-matter hybridization points away from the Dirac nodes giving rise to topological photo-electron bands with high Chern numbers. We reveal that the physical mechanism behind this phenomenon lies on the exchange of chiral photons with electronic matter at the hybridization points, and the number and polarization of exchanged photons determine the Chern number. This is a generic microscopic mechanism for the photo-electron band topology. Our theory shows that graphene-based materials, with no need of Floquet engineering and hence protected from the heating effects, host high Chern insulator phases when coupled to chiral cavity fields.

cond-mat.mes-hall↗

arXiv2023

Polaritonic ultracold reactions: cavity controlled molecular photoassociation

We introduce a prototypical model for cavity polaritonic control of ultracold photochemistry by considering the resonant vibrational strong coupling of a rubidium dimer to a terahertz cavity. We demonstrate that at avoided crossings between a vibrational excitation and the vacuum photon absorption, the resulting polaritonic states between the molecule and photons can efficiently control the molecular vibrational Franck-Condon (FC) factors. Due to the entanglement between light and matter, FC factor is transferred from one polaritonic branch to other, leading to a polariton with a substantially enhanced FC factor. Utilizing this polariton state for photoassociation results in the enhanced formation of ultracold molecules. This work suggests a path to controlling photoassociation with cavity vacuum fields, and lays the ground for the emerging subfield of polaritonic ultracold chemistry.

cond-mat.quant-gas↗

arXiv2023

Testing the Renormalization of the von Klitzing Constant by Cavity Vacuum Fields

In light of recent developments demonstrating the impact of cavity vacuum fields inducing the breakdown of topological protection in the integer quantum Hall effect, a compelling question arises: what effects might cavity vacuum fields have on fundamental constants in solid-state systems? In this work we present an experiment that assesses the possibility of the von Klitzing constant itself being modified. By employing a Wheatstone bridge, we precisely measure the difference between the quantized Hall resistance of a cavity-embedded Hall bar and the resistance standard, achieving an accuracy down to 1 part in 105 for the lowest Landau level. While our results do not suggest any deviation that could imply a modified Hall resistance, our work represents pioneering efforts in exploring the fundamental implications of vacuum fields in solid-state systems.

cond-mat.mes-hall↗

arXiv2022

Polaritonic Hofstadter Butterfly and Cavity-Control of the Quantized Hall Conductance

In a previous work [Phys. Rev. Lett. 123, 047202 (2019)] a translationally invariant framework called quantum-electrodynamical Bloch (QED-Bloch) theory was introduced for the description of periodic materials in homogeneous magnetic fields and strongly coupled to the quantized photon field in the optical limit. For such systems, we show that QED-Bloch theory predicts the existence of fractal polaritonic spectra as a function of the cavity coupling strength. In addition, for the energy spectrum as a function of the relative magnetic flux we find that a terahertz cavity can modify the standard Hofstadter butterfly. In the limit of no quantized photon field, QED-Bloch theory captures the well-known fractal spectrum of the Hofstadter butterfly and can be used for the description of 2D materials in strong magnetic fields, which are of great experimental interest. As a further application, we consider Landau levels under cavity confinement and show that the cavity alters the quantized Hall conductance and that the Hall plateaus are modified as $σ_{xy}=e^2ν/h(1+η^2)$ by the light-matter coupling $η$. Most of the aforementioned phenomena should be experimentally accessible and corresponding implications are discussed.

cond-mat.mes-hall↗

arXiv2022

New Class of Landau Levels and Hall Phases in a 2D Electron Gas Subject to an Inhomogeneous Magnetic Field: An Analytic Solution

An analytic closed form solution is derived for the bound states of electrons subject to a static, inhomogeneous ($1/r$-decaying) magnetic field, including the Zeeman interaction. The solution provides access to many-body properties of a two-dimensional, non-interacting, electron gas in the thermodynamic limit. Radially distorted Landau levels can be identified as well as magnetic field induced density and current oscillations close to the magnetic impurity. These radially localised oscillations depend strongly on the coupling of the spin to the magnetic field, which give rise to non-trivial spin currents. Moreover, the Zeeman interaction introduces a lowest flat band for $E_F=0^+$ assuming a spin $g_s$-factor of two. Surprisingly, in this case the charge and current densities can be computed analytically in the thermodynamic limit. Numerical calculations show that the total magnetic response of the electron gas remains diamagnetic (similar to Landau levels) independent of the Fermi energy. However, the contribution of certain, infinitely degenerate energy levels may become paramagnetic. Furthermore, numerical computations of the Hall conductivity reveal asymptotic properties of the electron gas, which are driven by the anisotropy of the vector potential instead of the magnetic field, i.e. become independent of spin. Eventually, the distorted Landau levels give rise to different Hall conductivity phases, which are characterized by sharp sign flips at specific Fermi energies. Overall, our work merges "impurity" with Landau-level physics, which provides novel physical insights, not only locally, but also in the asymptotic limit. This paves the way for a large number of future theoretical as well as experimental investigations.

cond-mat.mes-hall↗

arXiv2022

Condensed Matter Systems in Cavity Quantum Electrodynamics

Condensed matter physics and quantum electrodynamics (QED) have been long considered as distinct disciplines. This situation is changing by the progress in cavity QED materials. Motivated by these advances we aim to bridge these fields by merging fundamental concepts coming from both sides. In the first part of the thesis we present how non-relativistic QED can be constructed and we discuss the light-matter interaction in different gauges and that neglecting particular quadratic terms can lead to instabilities. In the second part, we revisit the Sommerfeld model of the free electron gas in cavity QED and provide the analytic solution for this paradigmatic system coupled to the cavity. We show that the cavity field modifies the optical conductivity of the electron gas and suppresses its Drude peak. Further, by constructing an effective field theory in the continuum of photon modes we show how the photon field leads to a many-body renormalization of the electron mass, which modifies the fermionic quasiparticle excitations of the Fermi liquid. In the last part, we show that translational symmetry for periodic materials in homogeneous magnetic fields can be restored by embedding the problem into QED. This leads to a generalization of Bloch's theory for electron-photon systems, that we named as QED-Bloch theory, which can be applied for the description of periodic materials in homogeneous magnetic fields and strongly coupled to the quantized cavity field. As a first application we consider Landau levels coupled to a cavity and we show that quasiparticle excitations between Landau levels and photons appear, called Landau polaritons. Further, for periodic materials in such setups, QED-Bloch theory predicts the emergence of novel fractal polaritonic energy spectra, which we name as fractal polaritons. The fractal polaritons are a polaritonic, QED analogue of the Hofstadter butterfly.

quant-ph↗

arXiv2021

The Free Electron Gas in Cavity Quantum Electrodynamics

Cavity modification of material properties and phenomena is a novel research field largely motivated by the advances in strong light-matter interactions. Despite this progress, exact solutions for extended systems strongly coupled to the photon field are not available, and both theory and experiments rely mainly on finite-system models. Therefore a paradigmatic example of an exactly solvable extended system in a cavity becomes highly desireable. To fill this gap we revisit Sommerfeld's theory of the free electron gas in cavity quantum electrodynamics (QED). We solve this system analytically in the long-wavelength limit for an arbitrary number of non-interacting electrons, and we demonstrate that the electron-photon ground state is a Fermi liquid which contains virtual photons. In contrast to models of finite systems, no ground state exists if the diamagentic $\textbf{A}^2$ term is omitted. Further, by performing linear response we show that the cavity field induces plasmon-polariton excitations and modifies the optical and the DC conductivity of the electron gas. Our exact solution allows us to consider the thermodynamic limit for both electrons and photons by constructing an effective quantum field theory. The continuum of modes leads to a many-body renormalization of the electron mass, which modifies the fermionic quasiparticle excitations of the Fermi liquid and the Wigner-Seitz radius of the interacting electron gas. Lastly, we show how the matter-modified photon field leads to a repulsive Casimir force and how the continuum of modes introduces dissipation into the light-matter system. Several of the presented findings should be experimentally accessible.

quant-ph↗