Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2608.21071 · The Journal of Chemical Physics

Intracavity photon statistics from correlated molecular electronic structure

Kurt R. Brorsen

Published Oct 27, 2026• 0 citations
arXivHTMLPDF

Abstract

Quantum optics characterizes light through photon correlations, motivating their connection to correlated ab initio molecular electronic structure. In this study, intracavity photon statistics of cavity-coupled molecules are computed from QED coupled-cluster (QED-CCSD-22) densities and validated against exact diagonalization of the Pauli-Fierz Hamiltonian. A change of coordinate origin or dipole convention displaces the ground state coherently, so second-order and higher cumulants of the cavity field are independent of these conventions even for molecular ions. The photon subsystem of QED Hartree-Fock is exactly coherent, so the invariant statistics describe fluctuations beyond this electron-photon product reference. The field fluctuations are super-Poissonian, and their degree of second-order coherence depends primarily on cavity frequency in the tested weak-coupling regime, falling from 18.5 to 4.9 for H2 as the cavity is detuned from 5 to 20 eV. The third cumulant identifies non-Gaussian fluctuations and changes sign under reversal of the coupling direction.

DOI
10.1063/5.0357762

Related papers

Cavity QED System with Optical Lattice: Photon Statistics and Conditioned Homodyne Detection

Mambwe Mumba, Dyan L. Jones, Perry R. Rice

We investigate the quantum fluctuations of a single atom in a weakly driven cavity, with an intracavity optical lattice. The weak driving field is on resonance with the atoms and the cavity, and is the second-harmonic of the lattice beam. In this special case we can find eigenstates of the Hamiltonian in terms of Mathieu functions.We present analytic results for the second order intensity correlation function $g^{(2)}(τ)$ and the intensity-field correlation function $h_θ(τ)$, for both transmitted and fluorescent light for weak driving fields. We find that the coupling of the center of mass motion to the intracavity field mode can be deleterious to nonclassical effects in photon statistics; less so for the intensity-field correlations, and compare the use of trapped atoms in a cavity to atomic beams.

arXiv:1807.05703•Jul 16, 2018•0 citations

Vacuum-Field Squeezing by Molecules in Optical Cavities: An Exact Bound and Linear-Response Theory

Kurt R. Brorsen

The ground state of a molecule coupled to an optical cavity contains virtual photons, and squeezing of the photonic field has been found in exactly solvable models, perturbation theory, and numerical calculations. Here, two exact sum rules for the field quadratures show that, within the dipole approximation, the vector-potential fluctuations of a linearly polarized mode never exceed their vacuum value in the ground state of any molecule at any coupling strength. Consequently, the field is squeezed whenever the mode couples to a dipole-allowed transition from a nondegenerate ground state, and the cavity-induced ground-state energy shift cannot increase with the cavity frequency at a fixed coupling vector, which is an exact constraint on approximate methods. Moreover, the incoherent photon statistics, which exclude the coherent part of the field, are never sub-Poissonian. At weak coupling, an exact resolvent expression reduces the squeezing and the ratio of the anomalous to the normal photon moment to linear-response properties of the isolated molecule, with the ratio set by a mean excitation energy. Time-dependent density functional theory with CAM-B3LYP and PBE0 reproduces exact and QED coupled-cluster references for both quantities to within 5% at 45 benchmark points. A survey of 29 molecules gives mean excitation energies of 9.5-21 eV at a cavity frequency of 2 eV and shows that the most polarizable molecules squeeze the field the most.

arXiv:2610.26732•Oct 19, 2026•0 citations

Super-Poissonian Squeezed Light in the Ground State of Strongly Coupled Light-matter Systems

Cankut Tasci, Mohammad Hassan, Leon Orlov-Sullivan +2 more

Strong light-matter coupling enables hybrid states in which photonic and electronic degrees of freedom become correlated even in the ground state. While many-body effects in long-range dispersion interactions are known to reshape electronic properties under such conditions, their impact on quantum-optical observables remains largely unexplored. Here, we address this problem using quantum electrodynamical density-functional theory (QEDFT) combined with the recently developed photon-many-body dispersion (pMBD) functional, which can capture higher-order electron-photon correlations and multi-photon processes. We compute ground-state photonic observables including photon number fluctuations, second-order correlations, and quadrature variances, and find squeezing and super-Poissonian photon statistics emerging from light-matter interactions in the strong coupling regime. Our results demonstrate that capturing the full hierarchy of many-body, electron-photon and multi-photon correlations is essential for a consistent description of quantum-optical properties in strongly coupled molecular systems, establishing QEDFT as a first-principles framework for predicting nonclassical photonic features in the ground state of complex systems.

physics.chem-phCross-listedarXiv:2512.18242•Dec 23, 2025•0 citations

Beyond ideal cavities: quantifying the impact of cavity dissipation on light-matter strong coupling

Giovanna Bruno, Enrico Busani, Lorenzo Gialli +2 more

Cavity Quantum Electrodynamics (Cavity-QED) offers a powerful framework for controlling light-matter interactions, yet first-principles approaches commonly assume ideal, lossless cavity fields. Here, we introduce a minimal extension to the Pauli-Fierz Hamiltonian that accounts for cavity dissipation through the experimentally accessible cavity quality factor Q. The electromagnetic field is described as a damped harmonic oscillator within the very weak damping approximation. In this framework, cavity losses only enter the light-matter interaction and dipole self-energy terms, without introducing additional photonic degrees of freedom. A time-dependent unitary transformation yields a real-valued Hermitian Hamiltonian that can be incorporated into existing ab-initio QED methods without additional computational cost or changes in scaling. We combined the approach to the QED-CCSD method and investigated the effects of cavity dissipation on the optical spectrum of a p-nitroaniline molecule under strong light-matter coupling. We found that, for realistic quality factors, cavity dissipation can reduce the Rabi splitting by more than 20% compared to the ideal-cavity limit, even though the system remains in the strong-coupling regime. Convergence towards the ideal-cavity limit then requires substantially higher quality factors than those needed to establish strong coupling. These results demonstrate that cavity dissipation can significantly affect polaritonic energetics and highlight the importance of incorporating experimentally realistic cavity losses into quantitative ab-initio Cavity-QED simulations to accurately guide experimental protocols.

physics.chem-phCross-listedarXiv:2610.04534•Oct 6, 2026•0 citations

Coherence and statistics of a one-atom laser in a photonic crystal microcavity

Xiaoyong Guo, Shuchen Lü, Zhongzhou Ren

Through calculating the quantum degree of second-order coherence and photon number distribution numerically, we investigate the coherence and statistics of a one-atom laser where a coherently driven two-level atom is coupled to a photonic crystal microcavity. By going beyond the previous theoretical studies, we show that in the presence of non-secular terms the cavity field displays a variable degree of second-order coherence as a function of discontinuities in the photonic density of states. The cavity field is characterized by photon antibunching and sub-Poissonian statistics for large discontinuities. In the case of a vanishing photonic density of states on the lower Mollow sideband, we can obtain only weak antibunching. If the jump in the photonic density of states is small, the bunching and super-Poissonian statistics field is generated in the cavity. Consequently, the cavity field exhibits certain non-classical features when non-secular interaction is included into the dynamics of the system. The theoretical analysis and numerical calculations presented here coincide with recent experimental observations.

Journal of Physics B Atomic Molecular and Optical Physics•Oct 29, 2010•5 citations

Testing electron-photon exchange-correlation functional performance for many-electron systems under weak and strong light-matter coupling

Iman Ahmadabadi, I-Te Lu, Leonardo A. Cunha +3 more

We present results of a photon-free exchange-correlation functional within the local density approximation (pxcLDA) for quantum electrodynamics density functional theory (QEDFT) that efficiently describes the electron density of many-electron systems across weak to strong light-matter coupling. Building on previous work [I-Te. Lu et al., Phys. Rev. A 109, 052823 (2024)] that captured electron-photon correlations via an exchange-correlation functional derived from the nonrelativistic Pauli-Fierz Hamiltonian and tested on one-electron systems, we use a simple procedure to compute a renormalization factor describing electron-photon correlations and inhomogeneity in the weak-coupling regime by comparing it with quantum electrodynamics coupled-cluster, and previous QEDFT optimized effective potential methods. Across various atoms and molecules, pxcLDA reproduces cavity-modified densities in close agreement with these references. The renormalization factor approaches unity as the system size or collective coupling increases, reflecting an electron-photon exchange-dominated behavior and improved accuracy for larger systems. This approach now offers a practical route to applying QEDFT functionals based on electron density to realistic electron systems.

quant-pharXiv:2512.14655•Dec 17, 2025
Quantum optics characterizes light through photon correlations, motivating their connection to correlated ab initio molecular electronic structure. In this study, intracavity photon statistics of cavity-coupled molecules are computed from QED coupled-cluster (QED-CCSD-22) densities and validated against exact diagonalization of the Pauli-Fierz Hamiltonian. A change of coordinate origin or dipole convention displaces the ground state coherently, so second-order and higher cumulants of the cavity field are independent of these conventions even for molecular ions. The photon subsystem of QED Hartree-Fock is exactly coherent, so the invariant statistics describe fluctuations beyond this electron-photon product reference. The field fluctuations are super-Poissonian, and their degree of second-order coherence depends primarily on cavity frequency in the tested weak-coupling regime, falling from 18.5 to 4.9 for H2 as the cavity is detuned from 5 to 20 eV. The third cumulant identifies non-Gaussian fluctuations and changes sign under reversal of the coupling direction.
We investigate the quantum fluctuations of a single atom in a weakly driven cavity, with an intracavity optical lattice. The weak driving field is on resonance with the atoms and the cavity, and is the second-harmonic of the lattice beam. In this special case we can find eigenstates of the Hamiltonian in terms of Mathieu functions.We present analytic results for the second order intensity correlation function g(2)(τ)g^{(2)}(τ)g(2)(τ) and the intensity-field correlation function hθ(τ)h_θ(τ)hθ​(τ), for both transmitted and fluorescent light for weak driving fields. We find that the coupling of the center of mass motion to the intracavity field mode can be deleterious to nonclassical effects in photon statistics; less so for the intensity-field correlations, and compare the use of trapped atoms in a cavity to atomic beams.
The ground state of a molecule coupled to an optical cavity contains virtual photons, and squeezing of the photonic field has been found in exactly solvable models, perturbation theory, and numerical calculations. Here, two exact sum rules for the field quadratures show that, within the dipole approximation, the vector-potential fluctuations of a linearly polarized mode never exceed their vacuum value in the ground state of any molecule at any coupling strength. Consequently, the field is squeezed whenever the mode couples to a dipole-allowed transition from a nondegenerate ground state, and the cavity-induced ground-state energy shift cannot increase with the cavity frequency at a fixed coupling vector, which is an exact constraint on approximate methods. Moreover, the incoherent photon statistics, which exclude the coherent part of the field, are never sub-Poissonian. At weak coupling, an exact resolvent expression reduces the squeezing and the ratio of the anomalous to the normal photon moment to linear-response properties of the isolated molecule, with the ratio set by a mean excitation energy. Time-dependent density functional theory with CAM-B3LYP and PBE0 reproduces exact and QED coupled-cluster references for both quantities to within 5% at 45 benchmark points. A survey of 29 molecules gives mean excitation energies of 9.5-21 eV at a cavity frequency of 2 eV and shows that the most polarizable molecules squeeze the field the most.
Strong light-matter coupling enables hybrid states in which photonic and electronic degrees of freedom become correlated even in the ground state. While many-body effects in long-range dispersion interactions are known to reshape electronic properties under such conditions, their impact on quantum-optical observables remains largely unexplored. Here, we address this problem using quantum electrodynamical density-functional theory (QEDFT) combined with the recently developed photon-many-body dispersion (pMBD) functional, which can capture higher-order electron-photon correlations and multi-photon processes. We compute ground-state photonic observables including photon number fluctuations, second-order correlations, and quadrature variances, and find squeezing and super-Poissonian photon statistics emerging from light-matter interactions in the strong coupling regime. Our results demonstrate that capturing the full hierarchy of many-body, electron-photon and multi-photon correlations is essential for a consistent description of quantum-optical properties in strongly coupled molecular systems, establishing QEDFT as a first-principles framework for predicting nonclassical photonic features in the ground state of complex systems.
Cavity Quantum Electrodynamics (Cavity-QED) offers a powerful framework for controlling light-matter interactions, yet first-principles approaches commonly assume ideal, lossless cavity fields. Here, we introduce a minimal extension to the Pauli-Fierz Hamiltonian that accounts for cavity dissipation through the experimentally accessible cavity quality factor Q. The electromagnetic field is described as a damped harmonic oscillator within the very weak damping approximation. In this framework, cavity losses only enter the light-matter interaction and dipole self-energy terms, without introducing additional photonic degrees of freedom. A time-dependent unitary transformation yields a real-valued Hermitian Hamiltonian that can be incorporated into existing ab-initio QED methods without additional computational cost or changes in scaling. We combined the approach to the QED-CCSD method and investigated the effects of cavity dissipation on the optical spectrum of a p-nitroaniline molecule under strong light-matter coupling. We found that, for realistic quality factors, cavity dissipation can reduce the Rabi splitting by more than 20% compared to the ideal-cavity limit, even though the system remains in the strong-coupling regime. Convergence towards the ideal-cavity limit then requires substantially higher quality factors than those needed to establish strong coupling. These results demonstrate that cavity dissipation can significantly affect polaritonic energetics and highlight the importance of incorporating experimentally realistic cavity losses into quantitative ab-initio Cavity-QED simulations to accurately guide experimental protocols.
Through calculating the quantum degree of second-order coherence and photon number distribution numerically, we investigate the coherence and statistics of a one-atom laser where a coherently driven two-level atom is coupled to a photonic crystal microcavity. By going beyond the previous theoretical studies, we show that in the presence of non-secular terms the cavity field displays a variable degree of second-order coherence as a function of discontinuities in the photonic density of states. The cavity field is characterized by photon antibunching and sub-Poissonian statistics for large discontinuities. In the case of a vanishing photonic density of states on the lower Mollow sideband, we can obtain only weak antibunching. If the jump in the photonic density of states is small, the bunching and super-Poissonian statistics field is generated in the cavity. Consequently, the cavity field exhibits certain non-classical features when non-secular interaction is included into the dynamics of the system. The theoretical analysis and numerical calculations presented here coincide with recent experimental observations.
We present results of a photon-free exchange-correlation functional within the local density approximation (pxcLDA) for quantum electrodynamics density functional theory (QEDFT) that efficiently describes the electron density of many-electron systems across weak to strong light-matter coupling. Building on previous work [I-Te. Lu et al., Phys. Rev. A 109, 052823 (2024)] that captured electron-photon correlations via an exchange-correlation functional derived from the nonrelativistic Pauli-Fierz Hamiltonian and tested on one-electron systems, we use a simple procedure to compute a renormalization factor describing electron-photon correlations and inhomogeneity in the weak-coupling regime by comparing it with quantum electrodynamics coupled-cluster, and previous QEDFT optimized effective potential methods. Across various atoms and molecules, pxcLDA reproduces cavity-modified densities in close agreement with these references. The renormalization factor approaches unity as the system size or collective coupling increases, reflecting an electron-photon exchange-dominated behavior and improved accuracy for larger systems. This approach now offers a practical route to applying QEDFT functionals based on electron density to realistic electron systems.