VacuumRabiSplitting

Vacuum Rabi splitting is a phenomenon in cavity quantum electrodynamics where a quantum emitter, such as a two-level atom or a quantum dot, interacts with a single mode of an electromagnetic cavity. Even when the cavity mode is in its vacuum state, the interaction with the emitter leads to a coherent exchange of energy and a characteristic splitting of the system’s spectral line. The effect is a hallmark of the quantum nature of light–matter coupling and demonstrates the formation of new eigenstates due to the interaction.

The theoretical description uses the Jaynes-Cummings model, with a Hamiltonian that couples the cavity field mode to the emitter with strength g. When the cavity resonance frequency matches the emitter transition frequency (resonance), the energy levels become dressed by the interaction. For one quantum of excitation (n = 1), the eigenstates split into two levels separated by 2g. In general, the eigenvalues are shifted by ±g√n, leading to the characteristic doublet structure in spectra such as transmission or emission from the cavity.

Experimentally, vacuum Rabi splitting requires the strong coupling regime, where g exceeds the decay rates of the cavity (κ) and the emitter (γ). This has been observed across platforms including atoms in optical cavities, quantum dots in solid-state microcavities, and superconducting qubits in circuit QED. Observations typically appear as two peaks in the spectrum, centered around the resonant frequencies, separated by 2g.

The concept underpins studies of quantum nonlinear optics at the single-photon level and informs approaches to quantum information processing and networking, where controlled light–matter interactions are essential.