Quarton couplers for ultrafast superconducting qubit readout: Part II, Experiment*

Fast, high-fidelity, quantum nondemolition (QND) qubit readout is an essential tool for quantum information processing. In superconducting qubits, state-of-the-art readout is enabled by a dispersive cross-Kerr coupling between a qubit and its readout resonator. Although dispersive readout can be high-fidelity and QND, it is limited to ~50ns readout time by the ~10 MHz dispersive cross-Kerr. We previously proposed (Ye et al., PRL 2021) using quarton couplers to reach ultrastrong (~1 GHz) cross-Kerr between superconducting qubits and resonators. Here, we present a new superconducting qubit readout scheme exploiting the large qubit-resonator cross-Kerr enabled by quarton couplers. In part 1/2, we present the theoretical work on new quarton coupling physics and its realistic application to ultrafast readout. Full stochastic master equation simulations indicate that ultrafast readout times <5 ns with high (~99%) readout fidelity and QND-ness can be achieved. In part 2/2, we present early experimental work demonstrating the key properties of quarton coupling schemes including large cross-Kerr coupling and self-Kerr cancellation to linearize qubits into resonators suitable for readout.