Atoms Meet Photons
When a single atom is placed inside a high-finesse optical cavity, the atom-photon interaction is enhanced by the cavity's ability to recirculate photons thousands of times. If the coupling rate g exceeds both the cavity decay rate κ and atomic spontaneous emission rate γ, the system enters the strong coupling regime — a cornerstone of quantum optics recognized by the 2012 Nobel Prize awarded to Serge Haroche.
The Jaynes-Cummings Ladder
The Jaynes-Cummings model, the simplest fully quantum model of light-matter interaction, predicts an anharmonic energy ladder. Each rung with n photons splits into two levels separated by 2g√(n+1). This √(n+1) scaling — the hallmark of quantum field theory — means the splitting grows with photon number but not linearly, distinguishing it from any classical model.
Vacuum Rabi Oscillations
Even when no photons are present, an excited atom in a cavity oscillates between emitting and reabsorbing a photon at frequency 2g. These vacuum Rabi oscillations demonstrate that the vacuum itself has structure. Adding photons increases the oscillation frequency as √(n+1), and with many photons, collapses and revivals appear as the different frequency components dephase and rephase.
From Cavities to Circuits
The same physics governs superconducting qubits coupled to microwave resonators in circuit QED — the foundation of modern quantum computing platforms from IBM and Google. The cooperativity C = g²/(κγ) determines gate fidelity, readout contrast, and entanglement generation rates. Understanding cavity QED is essential for designing the next generation of quantum processors.