Concept 25 of 42 · Circuit QED and Decoherence

Quasiparticles and dielectric loss

Microscopic defects and nonequilibrium excitations limit superconducting coherence.

01 / Mathematical framework

$$\frac1{Q_i}=\sum_j p_j\tan\delta_j+\frac1{Q_{\mathrm{other}}}$$

Physical intuition

Microscopic defects and nonequilibrium excitations limit superconducting coherence. This is the starting point for understanding how superconducting circuit parameters determine quantum energy levels and dynamics.

Deep dive: how the mechanism works

Electric-field participation in lossy interfaces and quasiparticle tunneling introduce dissipation.

How the quantum phenomenon appears

Surface treatment and geometry changes can improve resonator Q. To establish the predicted quantum behavior, connect the circuit Hamiltonian to a measured transition, coherence experiment, gate benchmark or calibrated readout signal.

Assumptions and limitations

Participation models depend on geometry, frequency and saturation of defects.

Research and simulation exercise

Identify the tunable parameters, predict the observable from the stated model, and sweep one parameter while holding the others fixed. Compare against a simpler reference model, quantify loss and uncertainty, and state which observations would falsify the hypothesis.

Connect the ideas

Echo and dynamical decoupling Thermalization and dilution refrigeration

See the chapter guide and further reading.

← Previous Next concept →

ΦQuasiparticles and dielectric loss