UC Berkeley

Superconducting quantum circuits are a leading technology in the quest toward building a quantum computer, which promises to outperform conventional, or “classical” computers, in solving a variety of tasks. To be useful for computation, a quantum processing unit (QPU) architecture must be scalable and have low error rates. Microwave activated entangling interactions, while often being slower than interactions that use flux-tunable components, are a scalable approach to realizing entanglement in that they are compatible with a minimally complex QPU design: single-junction transmon qubits with fixed qubit-qubit coupling. A prominent microwave-activated gate in this architecture, the cross resonance (CR) gate, is experimentally investigated, including mitigation of leakage errors, and budgeting of coherent and incoherent errors. Importantly, the fidelity of the CR gate is limited in general by the static cross-Kerr, or ZZ, interaction between the qubits. A novel cross-Kerr entangling interaction that commutes with the static cross-Kerr interaction is presented, based on simultaneous off-resonant drives. This cross-Kerr interaction is tunable, enabling the complete cancellation or enhancement of qubit-qubit coupling, a first for this fixed-frequency, fixed-coupling architecture. We show the tunability of the sign of the interaction, and use it to implement a two qubit controlled-phase (CZ) gate. This gate is observed to have lower coherent error than the CR gate. Designing a QPU with this as the native entangling inter- action could enable faster microwave-activated gates, improving overall performance while maintaining minimal hardware complexity.