  Spin-orbit coupling is ubiquitous in quantum dot quantum computing
architectures, and makes spin qubits susceptible to charge noise. We derive a
Hamiltonian describing the effect of spin-orbit and noise on a single-spin
qubit in a quantum dot. Relaxation is due to noise coupling different orbital
levels and is dominated by screened whole charge defects near the dot.
Dephasing stems from noise causing relative fluctuations between orbital
levels, and is driven by screened whole charge defects and unscreened dipole
defects in the substrate. Dephasing times are vastly different between common
materials such as Si and GaAs. They can be enhanced by increasing gate fields,
choosing materials with weak spin-orbit such as Si, making dots narrower, or
using accumulation dots.
