Noise and Decoherence

Noise causes decoherence—the loss of quantum information. Main noise sources in superconducting qubits:

• Charge noise: fluctuations in gate voltage or offset charge ($\delta n_g$). Causes dephasing. Transmons reduce charge noise sensitivity via the $E_J \gg E_C$ regime.
• Flux noise: fluctuations in applied magnetic flux. Affects qubit frequency (flux-tunable systems) and couplings.
• 1/f noise: low-frequency noise (spectra $\propto 1/f$) causes slow frequency drifts.
• Photon shot noise: from the measurement apparatus, causes measurement back-action.

T1 (energy relaxation): $|1\rangle \to |0\rangle$ via photon emission, timescale microseconds to milliseconds

T2 (phase decoherence): loss of superposition coherence, limited by charge and flux noise

T2*: measured dephasing time (includes inhomogeneous broadening)

Typical values (state-of-the-art transmons):

• T1 ~ 50–100 μs
• T2 ~ 10–50 μs
• Gate time ~ 10–100 ns (so ~1000 gates per T1)

scqubits models noise analytically or numerically to predict decoherence effects. For detailed dynamics, use QuTiP to simulate open system evolution.