A Tunable, Modeless, and Hybridization-free Cross-Kerr Coupler for a Miniaturized Quantum Processor Architecture

Superconducting quantum processors typically use capacitive charge-based coupling between circuit elements. However, design goals such as maintaining large anharmonicity and suppressing dielectric loss often impose a capacitance budget which can constrain achievable coupling rates and qubit packing density. In addition, tunable interactions relying on dynamical change in mode hybridization accompany nonadiabatic transitions, inducing leakage errors and limiting gate speed.

In this talk, we propose an architecture that can address these constraints by using a SQUID (superconducting quantum interference device) coupler [1, 2] with small Josephson energies. The SQUID coupler provides cross-Kerr coupling that does not rely on hybridization, controlled by external fluxes. We show that the SQUID coupler realizes a fast and high-fidelity CZ gate between transmons with minimal adiabaticity overhead, and the operation is robust against junction asymmetry for high-frequency qubits. Despite the large SQUID loops due to the choice of grounded transmon design that eliminates the sloshing mode [2], the small Josephson energy of the coupler effectively suppresses dephasing from flux noise. Using junction-based coupling schemes and merged-element transmons, we propose a scalable tiling strategy for a fully miniaturized quantum processor.

[1] Brooks et al., Phys. Rev. A 87, 052306, 2013.

[2] Kounalakis et al., npj Quantum Inf 4, 38, 2018.