  A common approach to realize conditional-phase (CZ) gates in transmon qubits
relies on flux control of the qubit frequency to make computational states
interact with non-computational ones using a fast-adiabatic trajectory to
minimize leakage. We develop a bipolar flux-pulsing method with two key
advantages over the traditional unipolar variant. First, the action of the
bipolar pulse is robust to long-timescale linear-dynamical distortions in the
flux-control line, facilitating tuneup and ensuring atomic repeatability.
Second, the flux symmetry of the transmon Hamiltonian makes the conditional
phase and the single-qubit phase of the pulsed qubit first-order insensitive to
low-frequency flux noise, increasing fidelity. By harnessing destructive
interference to minimize leakage, the bipolar pulse can approach the speed
limit set by the exchange coupling. We demonstrate a repeatable, high-fidelity
($99.1\%$), low-leakage ($0.1\%$), and fast ($40~\mathrm{ns}$) CZ gate in a
circuit QED quantum processor. Detailed numerical simulations with excellent
match to experiment show that leakage is dominated by remaining short-timescale
distortions and fidelity is limited by high-frequency flux noise.
