Coherence describes how long a qubit keeps quantum information before noise erases it. For superconducting qubits, coherence is shaped by materials, circuit design, temperature, electromagnetic environment, and control quality.
Key Timescales
T1: energy relaxation time, or how long|1>tends to survive before decaying to|0>.T2: dephasing time, or how long phase information remains predictable.T2 echo: a refocused dephasing measurement that reduces slow noise effects.
Common Noise Sources
- Dielectric loss in materials and interfaces.
- Flux noise from magnetic defects or environmental pickup.
- Charge noise, especially in more charge-sensitive designs.
- Photon shot noise in readout resonators.
- Leakage or heating from imperfect filtering and shielding.
Why It Matters
Gate operations must finish much faster than decoherence. Error correction also depends on physical error rates being low enough that encoding helps rather than hurts.