Showing 1–6 of 6 results for author: Karle, V

Collective rotational cat states of molecules in microwave cavities

Authors: Volker Karle, Florian Kluibenschedl, Mikhail Lemeshko, Vasil Rokaj

Abstract: 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 Wig… ▽ More 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. △ Less

Submitted 24 June, 2026; originally announced June 2026.

Hybrid light-matter boundaries of graphene in a chiral cavity

Authors: Volker Karle, Oriana K. Diessel, Vasil Rokaj, Ceren B. Dağ

Abstract: 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… ▽ More 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. △ Less

Submitted 13 November, 2025; v1 submitted 15 October, 2025; originally announced October 2025.

Anomalous multi-gap topological phases in periodically driven quantum rotors

Authors: Volker Karle, Mikhail Lemeshko, Adrien Bouhon, Robert-Jan Slager, F. Nur Ünal

Abstract: We demonstrate that periodically driven quantum rotors provide a promising and broadly applicable platform to implement multi-gap topological phases, where groups of bands can acquire topological invariants due to non-Abelian braiding of band degeneracies. By adiabatically varying the periodic kicks to the rotor we find nodal-line braiding, which causes sign flips of topological charges of band no… ▽ More We demonstrate that periodically driven quantum rotors provide a promising and broadly applicable platform to implement multi-gap topological phases, where groups of bands can acquire topological invariants due to non-Abelian braiding of band degeneracies. By adiabatically varying the periodic kicks to the rotor we find nodal-line braiding, which causes sign flips of topological charges of band nodes and can prevent them from annihilating, indicated by non-zero values of the %non-Abelian patch Euler class. In particular, we report on the emergence of an anomalous Dirac string phase arising in the strongly driven regime, a truly out-of-equilibrium phase of the quantum rotor. This phase emanates from braiding processes involving all (quasienergy) gaps and manifests itself with edge states at zero angular momentum. Our results reveal direct applications in state-of-the-art experiments of quantum rotors, such as linear molecules driven by periodic far-off-resonant laser pulses or artificial quantum rotors in optical lattices, whose extensive versatility offers precise modification and observation of novel non-Abelian topological properties. △ Less

Submitted 29 August, 2024; originally announced August 2024.

Journal ref: Phys. Rev. A 113, 012216 (2026)

Area-law entangled eigenstates from nullspaces of local Hamiltonians

Authors: Volker Karle, Maksym Serbyn, Alexios A. Michailidis

Abstract: Eigenstate thermalization in quantum many-body systems implies that eigenstates at high energy are similar to random vectors. Identifying systems where at least some eigenstates are non-thermal is an outstanding question. In this work we show that interacting quantum models that have a nullspace -- a degenerate subspace of eigenstates at zero energy (zero modes), which corresponds to infinite temp… ▽ More Eigenstate thermalization in quantum many-body systems implies that eigenstates at high energy are similar to random vectors. Identifying systems where at least some eigenstates are non-thermal is an outstanding question. In this work we show that interacting quantum models that have a nullspace -- a degenerate subspace of eigenstates at zero energy (zero modes), which corresponds to infinite temperature, provide a route to non-thermal eigenstates. We analytically show the existence of a zero mode which can be represented as a matrix product state for a certain class of local Hamiltonians. In the more general case we use a subspace disentangling algorithm to generate an orthogonal basis of zero modes characterized by increasing entanglement entropy. We show evidence for an area-law entanglement scaling of the least entangled zero mode in the broad parameter regime, leading to a conjecture that all local Hamiltonians with the nullspace feature zero modes with area-law entanglement scaling, and as such, break the strong thermalization hypothesis. Finally, we find zero-modes in constrained models and propose setup for observing their experimental signatures. △ Less

Submitted 20 September, 2021; v1 submitted 26 February, 2021; originally announced February 2021.

Journal ref: Phys. Rev. Lett. 127, 060602 (2021)

Coupled superfluidity of binary Bose mixtures in two dimensions

Authors: Volker Karle, Nicolo Defenu, Tilman Enss

Abstract: We consider a two-component Bose gas in two dimensions at low temperature with short-range repulsive interaction. In the coexistence phase where both components are superfluid, inter-species interactions induce a nondissipative drag between the two superfluid flows (Andreev-Bashkin effect). We show that this behavior leads to a modification of the usual Berezinskii-Kosterlitz-Thouless (BKT) transi… ▽ More We consider a two-component Bose gas in two dimensions at low temperature with short-range repulsive interaction. In the coexistence phase where both components are superfluid, inter-species interactions induce a nondissipative drag between the two superfluid flows (Andreev-Bashkin effect). We show that this behavior leads to a modification of the usual Berezinskii-Kosterlitz-Thouless (BKT) transition in two dimensions. We extend the renormalization of the superfluid densities at finite temperature using the renormalization group approach and find that the vortices of one component have a large influence on the superfluid properties of the other, mediated by the nondissipative drag. The extended BKT flow equations indicate that the occurrence of the vortex unbinding transition in one of the components can induce the breakdown of superfluidity also in the other, leading to a locking phenomenon for the critical temperatures of the two gases. △ Less

Submitted 9 July, 2019; v1 submitted 15 March, 2019; originally announced March 2019.

Comments: 12 Pages, 7 Figures

Journal ref: Phys. Rev. A 99, 063627 (2019)

Classical dynamical localization

Authors: Italo Guarneri, Giulio Casati, Volker Karle

Abstract: We consider classical models of the kicked rotor type, with piecewise linear kicking potentials designed so that momentum changes only by multiples of a given constant. Their dynamics display quasi-localization of momentum, or quadratic growth of energy, depending on the arithmetic nature of the constant. Such purely classical features mimic paradigmatic features of the {\it quantum} kicked rotor,… ▽ More We consider classical models of the kicked rotor type, with piecewise linear kicking potentials designed so that momentum changes only by multiples of a given constant. Their dynamics display quasi-localization of momentum, or quadratic growth of energy, depending on the arithmetic nature of the constant. Such purely classical features mimic paradigmatic features of the {\it quantum} kicked rotor, notably dynamical localization in momentum, or quantum resonances. We present a heuristic explanation, based on a classical phase space generalization of a well known argument, that maps the quantum kicked rotor on a tight-binding model with disorder. Such results suggest reconsideration of generally accepted views, that dynamical localization and quantum resonances are a pure result of quantum coherence. △ Less

Submitted 23 October, 2014; originally announced October 2014.

Journal ref: Phys. Rev. Lett. 113, 174101 (2014)