  We propose a set of universal gate operations for the singlet-triplet qubit
realized by two electron spins in a double quantum dot, in the presence of a
fixed inhomogeneous magnetic field. All gate operations are achieved by
switching the potential offset between the two dots with an electrical bias,
and do not require time-dependent control of the tunnel coupling between the
dots. We analyze the two-electron dynamics and calculate the effective qubit
rotation angle as a function of the applied electric bias. We present explicit
gate sequences for single-qubit rotations about two orthogonal axes, and a CNOT
gate sequence, completing the universal gate set.
