Combining CSFQ and transmon qubits to suppress unwanted ZZ interaction

Building a fault-tolerant quantum computer requires not only highly coherent qubits but also tailored interactions between qubits to implement high-fidelity two-qubit entangling gates. One limiting factor to gate errors is crosstalk in the device corresponding to unwanted terms in the system Hamiltonian. Thus, mitigating crosstalk errors, whether classical or quantum mechanical, is critically important for achieving high-fidelity entangling gates in multi-qubit circuits. This is a particular concern in a superconducting qubit architecture with fixed-frequency transmons coupled to nearest neighbors via a static exchange term J. In this architecture, the two-qubit gate is enabled by activating the cross-resonance (CR) effect, whose strength is proportional to J. However, this J also produces an always-on ZZ coupling term. Such a ZZ interaction is an ever-present source of error since it leads to unwanted entanglement between pairs. This unwanted ZZ interactions can be suppressed by combining qubits with opposite anharmonicity. In this talk, we present the first such hybrid two-qubit device with a capacitively shunted flux qubit (CSFQ) and a transmon. Also, we show experimental measurements and theoretical modeling of two-qubit gate error for gates based on the cross resonance interaction, and demonstrate the elimination of the static ZZ interaction.

In collaboration with: B.L.T. Plourde (Syracuse University, USA); Xuexin Xu, Mohammad H. Ansari (Forschungszentrum Jülich, Germany); Markus Brink, David C. McKay, Jared B. Hertzberg (IBM Quantum, USA)