Engineering non-perturbative cross-Kerr coupling for Transmon qubit readout

Abstract

Fast, high-fidelity qubit readout is essential because it enables critical mid-circuit feedback operations—such as qubit initialization, entanglement generation, teleportation, and particularly quantum error correction—that directly determine the overall performance and scalability of quantum algorithms. For superconducting qubits, the standard dispersive readout architecture leverages the qubit-state-dependent dispersive shift of the resonance frequency of a resonator coupled to the qubit to infer the qubit state. However, even at moderate measurement powers, unwanted multiphoton transitions between the qubit and resonator emerge, which impair both the speed and accuracy of the readout. As a result, dispersive techniques often fall short of matching the rapid, high-fidelity operation of single- and two-qubit gates.

In this thesis, we aim to demonstrate the recent proposal 'Junction readout' [\href{https://arxiv.org/abs/2501.09010}{Chapple \& Benhayoune-Khadraoui et. al.}] as a promising alternative approach for realizing fast, high-fidelity qubit readout. By connecting the qubit and resonator through a Josephson junction, this proposal exploits a strong, non-perturbative cross-Kerr interaction that naturally separates the measurement tones for the qubit ground or excited states. By further adding a parallel capacitive coupling, the transverse coupling terms can be balanced to suppress Purcell-induced qubit decay without resorting to bulky Purcell filters. We theoretically analyzed the Hamiltonian, numerically designed the physical parameters, experimentally engineered and characterized the device, and demonstrated the readout performance. The measured flux-tunable features are as predicted. We achieved 100 ns and 99.46\% readout fidelity in the bifurcation regime using high-loss readout resonator without extra Purcell filter and Josephson amplifier. This work illustrates the potential of junction-based readout architecture as a flexible and reliable solution for realizing fast, high-fidelity readout in superconducting qubits. It is a promising alternative with many variants for superconducting qubit readout to simplify current hardware set-up and compatible for scale-up architectures.