Qubit ZooTwo qubits placed at different antinodes of the same transmission line resonator can be entangled even when both are detuned from the cavity. The mechanism is virtual photon exchange: qubit 1 emits a virtual photon into the resonator mode, and qubit 2 absorbs it, producing an effective coupling:
No real photons populate the cavity during this process — the resonator mediates the interaction without being excited. This “quantum bus” architecture enables two-qubit gates between qubits separated by centimeter distances on a chip, far exceeding direct capacitive coupling range.
The bus coupling is always-on in the simplest version, which complicates idle operations. Later architectures introduced tunable couplers to switch on and off — see a-tunable-coupling-scheme-for-implementing-high-fidelity.
Source: blais-2004-circuit-qed Related: circuit-qed, transmon, a-tunable-coupling-scheme-for-implementing-high-fidelity