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# 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.

## Decoherence Times

**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|QuTiP]] to simulate open system evolution.