arXiv subjects

Johannes Feist

Publications and source records attributed to Johannes Feist.

At least 19 recordsLinked to original sources

arXiv2026

Single-molecule strong optomechanical regimes in SERS via hybrid plasmonic cavities

We identify enhanced strong coherent Surface-Enhanced Raman Scattering (SERS) interactions facilitated by hybridized metallo-dielectric cavities with a resonant fluorescent molecule. By illuminating a detuned electronic transition near saturation via a narrowband hybrid plasmonic mode, we observe enhanced splittings in the Stokes and anti-Stokes spectra, revealing strong optomechanical coupling and nonlinear vibrational modifications at intensities far below irreversible SERS damage thresholds. We compute the second-order photon correlations that allows identifying the nonclassical character associated to these strong optomechanical interactions and SERS configurations that enables them to optically characterize the anharmonic character of the vibrational modes.

quant-ph↗

arXiv2026

Spin injection of exciton-polaritons with halide perovskites at room temperature

Exciton-polaritons, hybrid photon-exciton quasiparticles, constitute a useful platform for the study of light-matter interaction and nonlinear photonic applications. In this work, we realize a monolithic Tamm-plasmon microcavity embedding a thin film of two-dimensional halide perovskites with a tunable polymer spacer that controls the exciton-photon detuning. Angle-resolved optical spectroscopy at room temperature reveals the lower polariton branch dispersions in the linear regime for several detunings. Under circularly polarized, quasi-resonant laser excitation, the spin injection of excitons and their relaxation towards the lower polariton branch demonstrates its preservation. The observed spin-polarized emission is consistent with the fast decay of polaritons: spin relaxation mechanisms become inefficient during this short time. Our results provide promising insights into the spin control of polaritonic devices, including chiral lasers and switches.

cond-mat.mes-hall↗

arXiv2026

High-harmonic generation driven by temporal-mode quantum states of light

We develop a theoretical framework for high-harmonic generation (HHG) driven by quantum states of light based on a temporal-mode expansion of the electromagnetic field. This approach extends previous single plane-wave mode treatments to realistic pulse configurations and arbitrary multi-mode states of light, resolving conceptual inconsistencies arising from non-normalizable infinite plane waves and establishing consistency between analytical and numerical methods. We derive a correction factor that quantifies deviations from the diagonal approximation (in which the yield becomes a statistical average over classical-field simulations) both for the response of a single atom and in the many-atom regime. Our results confirms that the HHG spectrum for atoms driven by any quantum state of light in free space is accurately described by averaging semi-classical calculations over the Husimi distribution, with no observable genuine quantum effects in the spectrum. We also demonstrate that in the many-atom regime, the mean-field coherent-state approximation underlying this treatment does not preserve probabilities, although unitarity is restored by in the diagonal approximation. The absence of genuine quantum effects in the HHG yield is attributed to the large photon numbers ($\sim 10^{11}$) required to reach HHG intensities in free space, which render quantum fluctuations negligible. We discuss nanophotonic environments with ultrasmall mode volumes as potential platforms where few-photon strong-field processes could exhibit genuine quantum signatures.

quant-ph↗

arXiv2026

Engineering recoil heating in coherent-scattering levitated optomechanics

Recoil heating from photon scattering is a fundamental source of decoherence in optical trapping, severely limiting the preparation of nonclassical motional states. In cavity setups in the coherent scattering configuration, a predictive theory of recoil heating rate is missing, as usual perturbative approaches fail in the presence of sharp optical resonances. Hence, current works assume that the recoil heating rate is approximately equal to its free-space value. Here we show that this is not the case, as the electromagnetic environment can strongly modify recoil heating rate by the Purcell effect. Specifically, we predict that this rate can be significantly suppressed in state-of-the-art microcavities, for both center-of-mass and librational motion. To establish these results, we develop a general theoretical framework based on macroscopic quantum electrodynamics and on the few-mode quantization approach developed in nanophotonics. Our method applies to particles trapped in the presence of arbitrary electromagnetic structures, thus providing a route to engineering motional decoherence in levitated optomechanics by photonic structure design.

quant-ph↗

arXiv2026

Close encounters between periodic light and periodic arrays of quantum emitters

We introduce crystal polaritons, hybrid excitations formed when the collective excitations of a periodic quantum-emitter array strongly couple to the resonant Bloch modes of a metasurface. This realizes a cavity-QED platform in which periodic light and periodic matter are treated on the same footing, allowing strong collective light-matter coupling in an extended, lossy, and dispersive nanophotonic structure. To describe this regime, we develop a reciprocal-space few-mode quantization based on macroscopic quantum electrodynamics, which maps the metasurface resonances seen by the emitter array onto a cavity-QED Hamiltonian at each in-plane momentum. We show that both plasmonic surface-lattice resonances and dielectric bound states in the continuum can enter the strong-coupling regime with a single emitter per unit cell. As a consequence of the resonant nonlinearities of the resulting crystal polaritons, the platform enables quantum light generation with efficiencies orders of magnitude higher than those achieved in conventional nonlinear metasurfaces.

quant-ph↗

arXiv2026

Breakdown of the isotropic asymptotic approximation in two-colour photoionisation

The Wigner delay is defined as the energy derivative of the scattering phase of a particle in a given potential, unveiling the time taken (or gained) due to the interaction. The characterisation of this delay plays a central role in attosecond science, where the time resolution allows to gain information on the time interval required for a photoelectron to be emitted into the continuum after the absorption of a single photon. Attosecond interferometric techniques, based on two-colour (extreme ultraviolet and near-infrared) photoionisation schemes, cannot provide a direct measurement of the Wigner delay, because the low-frequency photon contributes with an additional delay, which is imprinted on the outgoing photoelectron. The isolation of the Wigner delay is usually achieved by appealing to the asymptotic approximation, which assumes that the two-photon delay is separable into a Wigner and a near-infrared-induced phase and provides a universal analytical expression for the latter. In this study, we introduce a self-referencing approach based on the implementation of non-consecutive extreme ultraviolet harmonics, in order to test the validity of the asymptotic approximation. We demonstrate its breakdown by observing a deviation of a few tens of milliradians (corresponding to a few attoseconds) between its predictions and the experimentally measured phases of the sideband oscillations generated in our scheme, in agreement with full-dimensional simulations.

physics.atom-ph↗

arXiv2025

Casimir-Lifshitz Theory for Cavity Modification of Ground-State Energy

A theory for ground-state modifications of matter embedded in a Fabry-Perot cavity and whose excitations are described as harmonic oscillators is presented. Based on Lifshitz's theory for vacuum energy and employing a Lorentz model for the material permittivity, a nonperturbative macroscopic QED model that accounts for the infinite number of cavity modes with a continuum of their wavevectors was built. Differences from the commonly used single-mode Hopfield Hamiltonian are revealed. The nonresonant role of polaritons in the ground-state energy shift is also demonstrated, showing that the cavity effect is mainly caused by static screening occurring at very low frequencies. The theory allows for a straightforward incorporation of losses and temperature effects.

quant-ph↗

arXiv2025

Can Intense Quantum Light Beat Classical Uncertainty Relations?

Uncertainty relations are fundamental to quantum mechanics, encoding limits on the simultaneous measurement of conjugate observables. Violations of joint uncertainty bounds can certify entanglement -- a resource critical for quantum information protocols and increasingly relevant in strong-field physics. Here, we investigate the pairwise time-delay and frequency-bandwidth uncertainties for arbitrary multimode quantum states of light, deriving a general lower bound for their joint product. We find that the nonclassical correction scales inversely with the average photon number, a behavior rooted in the so-called ``monogamy of entanglement''. These results clarify the intensity scaling of quantum advantages in nonclassical light states and highlight the interplay between entanglement and photon statistics.

quant-ph↗

arXiv2025

Incorporating QM/MM molecular dynamics into the few-mode quantization approach for light-matter interactions in nanophotonic structures

In the context of light-matter interactions between organic chromophores and confined photons of (plasmonic) nano-resonators, we introduce a general framework that couples ab initio QM/MM molecular dynamics with few-mode field quantization to simulate light-matter interactions of molecular emitters at the nanoscale. Arbitrary, lossy, and spatially inhomogeneous photonic environments are represented by a minimal set of interacting modes fitted to their spectral density, while geometry-dependent molecular properties are computed on the fly. Applications to few-molecule strong coupling show that strong coupling persists when molecular degrees of freedom and disorder are included for the chosen system consisting of a nanoparticle dimer coupled to multiple emitters. At the same time, symmetry-protected degeneracies of two-level models are lifted. The framework further reveals how spatial field inhomogeneity and molecular disorder shape cavity-mediated energy transfer, illustrated for an HBQ-Methylene Blue donor-acceptor combination in a five-emitter system.

physics.optics↗

arXiv2025

Entangled Polariton States in the Visible and Mid-Infrared Spectral Ranges

Entanglement generation in polariton systems is fundamentally constrained by high losses and decoherence, which typically outweigh polariton nonlinearities. Here, we propose a conceptually different approach that uses optomechanical interactions, rather than polariton-polariton interactions, to generate entangled polaritons. Our double-resonant scheme relies on strong exciton-phonon coupling, found in both inorganic and molecular semiconductors, enabling room-temperature generation of spectrally disparate photon pairs. The quantum coherent and delocalized nature of polariton states inside optical cavities ensures efficient single-mode outcoupling and allows for unconditional quantum state preparation - not relying on any post-selection or projective measurements. When conditioned on exciton-polariton emission, single phonon-polariton states can be prepared that subsequently yield bright, heralded single-photon emission in the mid-IR/THz. We introduce a double-resonant optomechanical platform that enables scalable, room-temperature quantum polaritonics without relying on conventional excitonic nonlinearities.

quant-ph↗

arXiv2025

Interference-induced cavity resonances and imaginary Rabi splitting

Polaritons are usually described within single-mode cavity QED models. However, nanophotonic environments typically involve several modes that spectrally overlap and interfere, giving rise to sharp dip features such as Fano profiles in the electromagnetic spectral density. Here, we identify these features as interference-induced resonances, effective electromagnetic modes with complex, non-Hermitian couplings to quantum emitters. We show that these modes hybridize with emitters to form polaritons even when the system parameters do not satisfy the single-mode strong-coupling criterion. Moreover, the resulting polaritons differ in their decay rates, a phenomenon we term imaginary Rabi splitting. Extending the analysis to ensembles, we find that coupling to interference-induced resonances produces long-lived polaritons that can outlast excitonic dark states. Numerical simulations of a realistic hybrid metallodielectric platform confirm these predictions and demonstrate their robustness against disorder and loss. Our results reveal a new polaritonic regime beyond the single-mode description, offering new opportunities for controlling light-matter interactions in complex electromagnetic environments.

physics.optics↗

arXiv2025

There is no ultrastrong coupling with photons

Theoretical accounts of ultrastrongly coupled light-matter systems commonly assume that it arises from the interaction of an emitter with propagating photon modes supported by a structure, understanding photons as the excitations of the transverse electromagnetic field. This description discards the Coulomb interaction between the emitter and structure charges. Here, we show with a general argument based on electromagnetic constraints that the emitter-photon coupling strength is fundamentally limited. Accordingly, we conclude that the ultrastrong coupling regime cannot be reached with photons. Instead, it must originate from the Coulomb interactions between charges. A further corollary is that the so-called polarization self-energy term does not need to be included. We illustrate our claims by solving an analytical model of the paradigmatic case of an emitter next to a metallic nanosphere. These findings shed light on the fundamental processes underlying ultrastrong coupling, clarify the role of the polarization self-energy term and compel a reevaluation of previous literature.

quant-ph↗

arXiv2025

Squeezing and quantum control of antiferromagnetic magnon pseudospin

Antiferromagnets have been shown to harbor strong magnon squeezing in equilibrium, making them a potential resource for quantum correlations and entanglement. Recent experiments have also found them to host coherently coupled magnonic excitations forming a magnon pseudospin, in analogy to electronic spin. Here, we delineate the quantum properties of antiferromagnetic magnon pseudospin by accounting for spin non-conserving interactions and going beyond the rotating wave approximation. Employing concrete examples of nickel oxide and hematite, we find strong squeezing of the magnon pseudospin highlighting its important role in determining the eigenmode quantum properties. Via ground state quantum fluctuations engineering, this pseudospin squeezing enables an enhancement and control of coupling between the magnonic modes and other excitations. Finally, we evaluate the quantum superpositions that comprise a squeezed pseudospin ground state and delineate a qubit spectroscopy protocol to detect them. Our results are applicable to any system of coupled bosons and thus introduce quantum fluctuations engineering of a general bosonic pseudospin.

cond-mat.mtrl-sci↗

arXiv2024

Enhanced photoisomerization with hybrid metallodielectric cavities based on mode interference

The ability to control chemical reactions by coupling organic molecules to confined light in a cavity has recently attracted much attention. While most previous studies have focused on single-mode photonic or plasmonic cavities, here we investigate the effect of hybrid metallodielectric cavities on photoisomerization reactions. Hybrid cavities, which support both photonic and plasmonic modes, offer unique opportunities that arise from the interplay between these two distinct types of modes. Specifically, we demonstrate that interference in the spectral density due to a narrow photonic mode and a broad plasmonic mode that are coupled to each other enables hybrid cavities to provide an energy-selective Purcell effect. This effect enhances electronic relaxation only to the desired molecular geometry, providing the ability to increase the yield of photoisomerization reactions. As a test case, we study the asymmetric proton transfer reaction in the electronic excited state of 3-aminoacrolein. Our results, which are robust for a range of realistic cavity parameters, highlight the advantages of hybrid cavities in cavity-induced photochemical processes.

physics.chem-ph↗

arXiv2024

Spatially resolved photon statistics of general nanophotonic systems

While experimental measurements of photon correlations have become routine in laboratories, theoretical access to these quantities for the light generated in complex nanophotonic devices remains a major challenge. Current methods are limited to specific simplified cases and lack generality. Here we present a novel method that provides access to photon statistics resolved in space and frequency in arbitrary electromagnetic environments. Within the macroscopic QED framework, we develop a practical tool to compute electric field correlations for complex quantum systems by including lossy two-level systems that act as field detectors within the system. To make the implementation feasible, we use a recently developed multi-emitter few-mode quantization method to correctly account for fully retarded light propagation to the detectors. We demonstrate the effectiveness and robustness of the proposed technique by studying the photon correlations of one and two emitters in close proximity to a plasmonic nanoparticle. The simulations show that even in these relatively simple configurations, the light statistics exhibit a strong angular dependence. These results highlight the importance of going beyond conventional quantum-optical approaches to fully capture the analyzed physical effects and enable the study of the quantum light generation in realistic nanophotonic devices.

quant-ph↗

arXiv2024

General theory of cavity-mediated interactions between low-energy matter excitations

The manipulation of low-energy matter properties such as superconductivity, ferromagnetism and ferroelectricity via cavity quantum electrodynamics engineering has been suggested as a way to enhance these many-body collective phenomena. In this work, we investigate the effective interactions between low-energy matter excitations induced by the off-resonant coupling with cavity electromagnetic modes. We extend previous work by going beyond the dipole approximation accounting for the full polarization and magnetization densities of matter. We further include the often neglected diamagnetic interaction and, for the cavity, we consider general linear absorbing media with possibly non-local and non-reciprocal response. We demonstrate that, even in this general scenario, the effective cavity-induced interactions between the matter degrees of freedom are of electrostatic and magnetostatic nature. This confirms the necessity of a multimode description for cavity engineering of matter systems where the low-energy assumption holds. Our findings provide a theoretical framework for studying the influence of general optical environments on extended low-energy matter excitations.

quant-ph↗

arXiv2024

Photon Antibunching in Single-Molecule Vibrational Sum-Frequency Generation

Sum-frequency generation (SFG) allows for coherent upconversion of an electromagnetic signal and has applications in mid-infrared vibrational spectroscopy of molecules. Recent experimental and theoretical studies have shown that plasmonic nanocavities, with their deep sub-wavelength mode volumes, may allow to obtain vibrational SFG signals from a single molecule. In this article, we compute the degree of second order coherence ($g^{(2)}(0)$) of the upconverted mid-infrared field under realistic parameters and accounting for the anharmonic potential that characterizes vibrational modes of individual molecules. On the one hand, we delineate the regime in which the device should operate in order to preserve the second-order coherence of the mid-infrared source, as required in quantum applications. On the other hand, we show that an anharmonic molecular potential can lead to antibunching of the upconverted photons under coherent, Poisson-distributed mid-infrared and visible drives. Our results therefore open a path toward a new kind of bright and tunable source of indistinguishable single photons by leveraging ``vibrational blockade'' in a resonantly and parametrically driven molecule, without the need for strong light-matter coupling.

physics.optics↗

arXiv2024

Memory loss is contagious in open quantum systems

Memoryless (Markovian) system-bath interactions are of fundamental interest in physics. While typically, the absence of memory originates from the characteristics of the bath, here we demonstrate that it can result from the system becoming lossy due to the Markovian interaction with a second bath. This uncovers an interesting interplay between independent baths and suggests that Markovianity is ``contagious'', i.e., it can be transferred from one bath to another through the system with which they both interact. We introduce a Bloch-Redfield-inspired approach that accounts for this distinct origin of Markovianity and uniquely combines non-Hermitian Hamiltonian formalism with master equations. This method significantly improves the description of the interaction between a lossy system (associated with a Lindblad master equation) and a non-Markovian bath, reducing the computational demands of complex system-bath setups across various fields.

quant-ph↗