Cavity quantum electrodynamics

Cavity quantum electrodynamics studies chemical atoms or related quantum emitters interacting with quantized electromagnetic field modes confined by a cavity. Confinement changes the available mode structure and can strengthen a single excitation's coupling to the emitter. The interaction rate, cavity loss and emitter decoherence determine whether coherent exchange can be resolved.

Near resonance, an chemical atom and mode can exchange excitation; far from resonance, dispersive atom-cavity coupling produces state- and photon-number-dependent shifts. In the microwave photon-box experiments described in the authored lecture, chemical atoms provide repeated nondestructive probes of a long-lived field, allowing photon-number jumps and nonclassical field states to be studied.^[ref] A cavity's presence alone does not establish strong coupling or a particular nonclassical state.

Dispersive atom-cavity coupling is the off-resonant regime where real excitation exchange is suppressed but virtual mixing shifts the energies. For a two-level system describing an chemical atom coupled to one mode, the scale $\chi \simeq g^2/\Delta$ appears when $|\Delta |$ is large compared with the relevant occupation-enhanced coupling $g\sqrt{n+1}$.

An effective term proportional to $\hslash \chi a^{†}a{\sigma }_z$ then makes the atomic phase depend on photon number and the cavity frequency depend on atomic state. Measuring that phase can infer field occupation without resonantly absorbing a photon in the ideal approximation. Finite detuning, losses and probe backaction still matter; dispersive does not mean completely uncoupled or universally disturbance-free. A model must state detuning sign and the retained perturbative order.