Intracavity Photon Statistics from Correlated Molecular Electronic Structure

Abstract

Quantum optics characterizes light through photon correlations, but ab initio cavity quantum electrodynamics (QED) has only calculated raw field moments at present. 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 well defined even for molecular ions. The photon subsystem of QED Hartree-Fock is exactly coherent, so the invariant statistics measure electron-photon correlation. The field fluctuations are super-Poissonian, and their degree of second-order coherence is set by the cavity frequency rather than by the coupling strength, falling from 18.5 to 4.9 for H2 as the cavity is detuned from 5 to 20 eV. A parity selection rule makes the sign of the third cumulant a probe of the orientation of the molecular charge asymmetry along the cavity polarization.