Novel approaches to high fidelity qubit state measurement in circuit quantum electrodynamics

Nicholas T. Bronn, Easwar Magesan, Nicholas A. Masluk +3 more

Physical implementations of qubits can be extremely sensitive to environmental coupling, which can result in decoherence. While efforts are made for protection, coupling to the environment is necessary to measure and manipulate the state of the qubit. As such, the goal of having long qubit energy relaxation times is in competition with that of achieving high-fidelity qubit control and measurement. Here, we propose a method that integrates filtering techniques for preserving superconducting qubit lifetimes together with the dispersive coupling of the qubit to a microwave resonator for control and measurement. The result is a compact circuit that protects qubits from spontaneous loss to the environment, while also retaining the ability to perform fast, high-fidelity readout. Importantly, we show the device operates in a regime that is attainable with current experimental parameters and provide a specific example for superconducting qubits in circuit quantum electrodynamics.

S. Kundu, Sumeru Hazra, Tanay Roy +4 more

Eran Ginossar, Lev S. Bishop, S. M. Girvin

This chapter analyzes the high excitation nonlinear response of the Jaynes–Cummings model in quantum optics when the qubit and cavity are strongly coupled. It studies the response in the parameter ranges appropriate for transmon qubits in the circuit quantum electrodynamics architecture. It focuses on the situation where the system behaves essentially as a nonlinear quantum oscillator and explores quantum and semi-classical dynamics. One of the central motivations is that under strong excitation tones, the nonlinear response can lead to qubit quantum state discrimination and the chapter then presents results for the cases when the qubit and cavity are on resonance or far off-resonance (dispersive).

Oliver Viehmann

M. D. Reed, L. DiCarlo, Blake Johnson +4 more

We demonstrate a qubit readout scheme that exploits the Jaynes-Cummings nonlinearity of a superconducting cavity coupled to transmon qubits. We find that, in the strongly driven dispersive regime of this system, there is the unexpected onset of a high-transmission "bright" state at a critical power which depends sensitively on the initial qubit state. A simple and robust measurement protocol exploiting this effect achieves a single-shot fidelity of 87% using a conventional sample design and experimental setup, and at least 61% fidelity to joint correlations of three qubits.

Will Shanks, Devin Underwood, James J. Raftery +3 more