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High-fidelity single- and two-qubit gates are essential building blocks for afault-tolerant quantum computer. While there has been much progress insuppressing single-qubit gate errors in superconducting qubit systems,two-qubit gates still suffer from error rates that are orders of magnitudehigher. One limiting factor is the residual ZZ-interaction, which originatesfrom a coupling between computational states and higher-energy states. Whilethis interaction is usually viewed as a nuisance, here we experimentallydemonstrate that it can be exploited to produce a universal set of fast single-and two-qubit entangling gates in a coupled transmon qubit system. To implementarbitrary single-qubit rotations, we design a new protocol called the two-axisgate that is based on a three-part composite pulse. It rotates a single qubitindependently of the state of the other qubit despite the strong ZZ-coupling.We achieve single-qubit gate fidelities as high as 99.1% from randomizedbenchmarking measurements. We then demonstrate both a CZ gate and a CNOT gate.Because the system has a strong ZZ-interaction, a CZ gate can be achieved byletting the system freely evolve for a gate time $t_g=53.8$ ns. To design theCNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHTprotocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6%and 97.8% for the CNOT and CZ gates respectively from quantum progresstomography.

A universal quantum gate set for transmon qubits with strong ZZ interactions