Cavity QED: Vacuum Rabi Oscillations

Coupling

Coupling strength g 6.0 MHz

Losses

Cavity decay κ 1.0 MHz

Atomic decay γ 0.5 MHz

Atom excitation

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Photon number

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Cooperativity C

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Regime

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How it works

The Jaynes-Cummings model describes a two-level atom coupled to a single cavity mode. One quantum of excitation coherently swaps between "atom excited, cavity empty" and "atom ground, one photon in the cavity" at the vacuum Rabi frequency — as long as coupling beats the losses.

Ω_R = 2g                         (vacuum Rabi frequency, on resonance)
C = g² / (κ·γ)                   (cooperativity — strong coupling needs C ≫ 1)
strong coupling: g ≫ κ, γ  ⇒  visible oscillations
weak coupling:   g ≪ κ, γ  ⇒  excitation just leaks away (Purcell regime)

Coupling strength g — how strongly the atom's dipole talks to the cavity field; the whole effect scales with it.
Cavity decay κ — how fast photons leak out of the cavity mirrors.
Atomic decay γ — how fast the atom spontaneously emits into free space instead of the cavity mode.
Excite atom — injects one quantum of excitation and starts the exchange.

Real-world relevance: this exact strong-coupling regime is the working principle behind cavity-QED quantum computers and single-photon sources — Serge Haroche and David Wineland shared the 2012 Nobel Prize for experiments built on it.

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