Composite pulse sequences for robust universal control of singlet-triplet qubits*

We consider composite pulse sequences for the exchange interaction in singlet-triplet qubits, in the presence of a finite magnetic field gradient (producing a term in the Hamiltonian similar in magnitude to the exchange interaction). We find pulse sequences achieving arbitrary rotations on the Bloch sphere, for which there is no first-order term in the error arising from fluctuations of the magnetic field gradient. We quantify the range of experimental parameters where our composite sequences can outperform naive uncorrected sequences.