Gradiometric quarton for nonlinear coupling of superconducting qubits and resonators

Nonlinear couplings between superconducting qubits and resonators such as the cross-Kerr interaction are used for important operations including qubit readout and gates. Most nonlinear coupling schemes including the long-established dispersive shift have limited cross-Kerr strength and non-ideal interactions such as residual self-Kerr. We previously proposed quarton couplers as promising nonlinear couplers between qubits and resonators that can facilitate cross-Kerr magnitudes of the order of gigahertz. Here, we present a device containing two transmon qubits coupled by a gradiometric quarton coupler. Through flux tuning the gradiometric quarton and transmon SQUID, we explore the potential of purely nonlinear coupling between the qubits and cancellation of qubit self-Kerr to linearize it into a resonator. We present experimental results on a device designed to operate in a parameter space that has large cross-Kerr couplings and large detuning to suppress other unwanted interactions. Large cross-Kerr between qubit and resonator is expected to enable applications including faster high fidelity qubit readout and gates.

*Author list: Yufeng Ye, Alec Yen, Gregory Cunningham, Jeremy Kline, Max Tan, Alicia Zang, Kaidong Peng, Kevin O’Brien