  The dephasing time of spin-orbit qubits is limited by the coupling with
electrical and charge noise. However, there may exist "dephasing sweet spots"
where the qubit decouples (to first order) from the noise so that the dephasing
time reaches a maximum. Here we discuss the nature of the dephasing sweet spots
of a spin-orbit qubit electrically coupled to some fluctuator. We characterize
the Zeeman energy $E_\mathrm{Z}$ of this qubit by the tensor $G$ such that
$E_\mathrm{Z}=\mu_B\sqrt{\vec{B}^\mathrm{T}G\vec{B}}$ (with $\mu_B$ the Bohr
magneton and $\vec{B}$ the magnetic field), and its response to the fluctuator
by the derivative $G^\prime$ of $G$ with respect to the fluctuating field. The
geometrical nature of the sweet spots on the unit sphere describing the
magnetic field orientation depends on the sign of the eigenvalues of
$G^\prime$. We show that sweet spots usually draw lines on this sphere. We then
discuss how to characterize the electrical susceptibility of a spin-orbit qubit
with test modulations on the gates. We apply these considerations to a Ge/GeSi
spin qubit heterostructure, and discuss the prospects for the engineering of
sweet spots.
