Measurement-Induced Dephasing
Photons in a readout resonator carry qubit information; the same information leak dephases the qubit whenever the resonator population fluctuates.
Information and Back-Action
Dispersive readout works because the readout resonator's frequency depends on the qubit state. But this coupling runs both ways. Any photon in the resonator experiences a phase shift that depends on the qubit, which means the environment, through the resonator field, is continuously learning about the qubit. Learning about a qubit's Z state necessarily dephases its X-Y coherence. This is measurement-induced dephasing, and it is the price the qubit pays for being measurable.
The Two Cases
During deliberate readout, this dephasing is expected and irrelevant, since the qubit's phase is being collapsed anyway. The problem is dephasing when no measurement is intended. Residual or thermal photons in the readout resonator, present even when idle, fluctuate randomly, and each fluctuation shifts the qubit frequency through the dispersive coupling. The random frequency shifts dephase the qubit exactly as if a weak measurement were running.
- Thermal photons in the resonator cause idle dephasing proportional to their number and to the dispersive shift.
- A larger dispersive shift speeds intentional readout but worsens photon-induced dephasing.
- Photon-number fluctuations, not just the average, drive the effect.
Managing the Trade
Designers face a tension: a large dispersive shift gives fast, high-fidelity readout but makes the qubit more sensitive to stray photons. The remedies attack the photons rather than the coupling. The readout resonator is thermalized aggressively with well-attenuated input lines so its thermal photon population is minimized. Improved filtering and shielding cut the photon influx. Some schemes actively empty the resonator after readout to prevent lingering photons from dephasing the next operation.
Why It Belongs in the Budget
Measurement-induced dephasing links readout design directly to coherence, so it cannot be treated as a separate concern. Choosing the dispersive shift, the resonator linewidth, the input attenuation, and the operating temperature is a joint optimization of readout speed against idle coherence, and it is a standard line item in a well-constructed qubit error budget.