Circuit QED adapts ideas from cavity quantum electrodynamics to superconducting circuits. A qubit is coupled to a microwave resonator, and the resonator response shifts depending on the qubit state.
Readout Picture
In the dispersive regime, the qubit and resonator do not exchange energy resonantly. Instead, the qubit state changes the effective resonator frequency. Measuring the phase and amplitude of a microwave probe gives information about whether the qubit was in |0> or |1>.
What Good Readout Needs
- High contrast between the resonator response for
|0>and|1>. - Low added noise from amplifiers and measurement electronics.
- Minimal measurement-induced transitions.
- Fast enough acquisition before T1 decay changes the state.