Ultrafast superconducting qubit readout and gates with a quarton coupler*

Faster entangling operations in the form of readout and two-qubit gates for superconducting qubits are essential for achieving the next milestones such as scalable quantum error correction. Entangling gate speed is directly proportional to the interaction energy, thus for CZ gates and operations involving photons including readout and bosonic code control, gate times are limited by the typically perturbatively weak nonlinear cross-Kerr or ZZ coupling energy. Here, we propose and present experimental progress towards ultrafast (order 1 nanosecond) entangling gates by using a new tunable quarton coupler capable of ultrastrong (order 1 GHz) cross-Kerr coupling. The quarton coupler is compatible with existing transmon architectures and can facilitate resonator-resonator, resonator-qubit, or qubit-qubit cross-Kerr coupling for a wide range of applications including ultrafast transmon qubit readout, CZ gates, bosonic code control, and single microwave photon detection.