Flux Noise
Flux noise is the random fluctuation of magnetic flux through a SQUID loop, and it is the dominant dephasing source for flux-tunable qubits away from their sweet spots.
The Observation
Superconducting loops show a small, slowly varying random magnetic flux threading them, even when the external field is held fixed. The fluctuations have a power spectrum that rises toward low frequency roughly as one over frequency, so-called 1/f noise, with a strikingly universal amplitude of about a few micro-flux-quanta across many devices, sizes, and materials. This universality has made flux noise a long-standing puzzle.
How It Dephases Qubits
A flux-tunable qubit's frequency depends on the flux through its SQUID. Any flux fluctuation therefore shifts the qubit frequency, and a randomly wandering frequency causes the qubit's phase to diffuse, which is dephasing. The sensitivity is the slope of frequency versus flux. At a sweet spot this slope vanishes and the qubit is first-order insensitive to flux noise; away from the sweet spot the qubit dephases faster the steeper the slope.
- Spectrum rises toward low frequency, roughly one over frequency.
- Amplitude is remarkably similar across devices, a few micro-flux-quanta.
- Dephasing rate scales with the qubit's flux sensitivity.
The Suspected Origin
The leading explanation attributes flux noise to unpaired electron spins on the surfaces and interfaces of the superconducting metal and substrate. Large numbers of these surface spins flip randomly, and their fluctuating magnetic moments produce the observed flux noise. Evidence includes the dependence of the noise on surface treatment and its behavior with temperature and magnetic field.
Mitigation
Because it is largely low-frequency, flux noise can be partly refocused by dynamical decoupling sequences such as spin echo, which cancel slow drifts. Operating at sweet spots removes first-order sensitivity. Reducing loop area lowers the collected flux, and surface treatments that remove or passivate the responsible spins reduce the noise at its source. For tunable-qubit architectures, quiet flux lines and stable bias sources are essential to keep the added dephasing within the error budget.