Millan F. Welman | Searcharxiv

arXiv2026

Initialization with a Fock State Cavity Mode in Real-Time Nuclear--Electronic Orbital Polariton Dynamics

Molecular polaritons have drawn great interest in recent years as a possible avenue for providing optical control over chemical dynamics. A central challenge in the field is to identify physical phenomena that require a quantum rather than a classical treatment of electrodynamics. In this work, we use our recently developed mean-field quantum (mfq) and full-quantum (fq) real-time nuclear--electronic orbital (RT-NEO) time-dependent density functional theory methods to simulate polaritonic dynamics for a molecule under vibrational strong coupling when a quantized cavity mode is initialized in a Fock state rather than a coherent state. Our previous work showed that a coherent state initial condition for the cavity mode leads to polariton formation for both the mfq-RT-NEO and fq-RT-NEO methods. Herein, we show that the mfq-RT-NEO method, which does not allow light--matter entanglement, does not predict polariton formation for a Fock state initial condition. Similar to the mfq-RT-NEO method, the fq-RT-NEO method does not predict oscillations of the cavity mode coordinate and molecular dipole operator expectation values for a Fock state initial condition. However, the fq-RT-NEO method does predict oscillations of the expectation values of even powers of these operators as well as light--matter entanglement, implicating polariton formation with a Fock state initial condition. All these observations can be explained with model systems. These results suggest that using a quantized cavity mode initial condition that does not have a direct analogy to an initial condition in classical electrodynamics can lead to physical phenomena that can only be described by a quantum treatment of the cavity mode.

physics.chem-ph↗

arXiv2025

Light-Matter Entanglement in Real-Time Nuclear-Electronic Orbital Polariton Dynamics

Molecular polaritons are hybrid light-matter states that enable the exploration of potential cavity-modified chemistry. The development of dynamical, first-principles approaches for simulating molecular polaritons is important for understanding their origins and properties. Herein, we present a hierarchy of first-principles methods to simulate the real-time dynamics of molecular polaritons in the strong coupling regime. These methods are based on real-time time-dependent density functional theory (RT-TDDFT) and the corresponding real-time nuclear-electronic orbital (RT-NEO) approach, in which specified nuclei are treated quantum mechanically on the same level as the electrons. The hierarchy spans semiclassical, mean-field-quantum, and full-quantum approaches to simulate polariton dynamics under both electronic strong coupling and vibrational strong coupling. In the semiclassical approaches, the cavity mode is treated classically, whereas in the full-quantum approaches, the cavity mode is treated quantum mechanically with propagation of a joint molecule-mode density matrix. The semiclassical and full-quantum approaches produce virtually identical Rabi splittings and polariton peak locations for the systems studied. However, the full-quantum approaches allow exploration of molecule-mode quantum entanglement in the real-time dynamics. Although the degree of light-matter entanglement is relatively small in the systems considered, the oscillations of the von Neumann entropy reveal an entanglement Rabi splitting that differs from the Rabi splitting computed from the time-dependent dipole moment. These results suggest that a classical treatment of the cavity mode may provide an excellent description of polariton dynamics for macroscopic observables such as the Rabi splitting, but novel physics may be detectable by considering molecule-mode entanglement.

physics.chem-ph↗