High-fidelity parametric entangling gates at AC flux sweet spots

Realizing high-fidelity two-qubit gates is one of the main challenges in building quantum processors. A major limitation is often decoherence, in particular dephasing due to the ubiquitous presence of flux noise in tunable qubits. Under static magnetic flux biases, the dephasing time is greatly enhanced when the tunable qubit is parked at flux insensitive operating points, commonly referred to as "sweet spots". We show that under flux bias modulation around such DC flux sweet spots, even though the qubit continuously explores regions sensitive to flux noise, there exists a modulation amplitude where the qubit is insensitive to low-frequency flux fluctuations like 1/f noise - the AC flux sweet spot. We show how this sweet spot is preserved in presence of instrumental white noise by lowpass filtering the flux line. We present how the AC flux sweet spot allows to reach state-of-the-art fidelities in parametrically-activated entangling gates. arXiv:1807.01310.