Fast high-fidelity entangling gates in Si double quantum dots*

High-fidelity two-qubit gates remain a challenge for spin qubits in quantum dots, partly due to the exchange coupling’s sensitivity to charge noise and relatively longer gate times. Here we show that, by using simple smooth pulses to control the amplitude of the oscillating magnetic field, entangling gates locally equivalent to control-NOT and control-Z gates can be implemented in times as low as 26 ns and with fidelities up to 99.99%. Moreover, we provide the particular single-qubit gates necessary to make the two-qubit gates exactly equal to the CNOT and CZ gates.