Fast superconducting qubit control with sub-harmonic drives*

Increasing the fidelity of single-qubit gates requires a combination of faster pulses and increased qubit coherence.   However, with traditional resonant qubit driving via a capacitively coupled port, these two objectives are mutually contradictory, as higher qubit quality factor requires a weaker coupling, necessitating longer pulses for the same applied power.  Increasing drive power, on the other hand, can heat the qubit's environment and degrade coherence. In this talk, we will introduce a new parametric scheme for performing single-qubit gates by pumping at approximately 1/3 the qubit's resonant frequency.  The large separation in qubit and drive frequencies enables us to use filtering to perform rapid gates while protecting the qubit's coherence at the same time.  In addition, the Rabi rate of this process is proportional to driving voltage cubed, allowing rapidly increased gate speed with only modest increases in applied power. We will present our recent results, especially our gate calibration and tune up methods, which currently allow pi-pulses as short as tens of nanoseconds with gate error rate as low as 0.5% limited by qubit coherence.