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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.

In the 2007 cavity quantum electrodynamics experiment of Christine Guerlin and collaborators, including Sébastien Gleyzes, Michel Brune, Jean-Michel Raimond and Serge Haroche, an injected coherent state occupied a 51.1 GHz microwave cavity with niobium-coated mirrors exhibiting superconductivity. Velocity-selected rubidium chemical atoms were prepared as circular Rydberg atoms in levels with principal quantum numbers 50 and 51. Repeated atomic readouts progressively narrowed the inferred photon number to a Fock state; later records revealed jumps. The cavity damping time was $T_c=0.130\mathrm{s}$. Empty-cavity Ramsey interferometry scans fixed the analysis-phase origin; weak injected fields refined the response calibration. Finite fringe contrast required calibrated likelihoods; photon losses could interrupt convergence.^[ref]

The functional sequence is preparation, first Ramsey interferometry zone $R_1$, dispersive cavity $C$, second zone $R_2$, and state-selective detection. The first $\pi /2$ microwave pulse prepares an atomic quantum superposition; dispersive atom-cavity coupling adds a photon number-dependent relative quantum phase while suppressing energy exchange. The second pulse converts that quantum phase into atomic populations; state-selective field ionization reads $e$ or $g$. Successive chemical atoms carry the meter records away while the same field remains available.^[ref]

This arrangement makes quantum nondemolition measurement useful for following a field repeatedly and preparing a Fock state by conditioning. The photon number is inferred from a sequence of physical records; it is not a directly visible label attached to a photon. Each atomic probe makes a weak, partial measurement in quantum mechanics of photon number, and the accumulated records can approach a resolved count. A single binary meter need not identify one among many possible Fock states, as perfect discrimination of pure states requires orthogonality.