High-power readout of a transmon qubit using a nonlinear coupling

TL;DR

This paper demonstrates high-fidelity, high-power readout of a transmon qubit using a nonlinear coupling approach, significantly improving signal-to-noise ratio while maintaining quantum non-demolition properties.

Contribution

It introduces a nonlinear coupling scheme for transmon qubits, enabling high-power readout with minimal loss of QND characteristics, building on previous cross-Kerr coupling work.

Findings

Achieved 99.21% readout fidelity with 89 photons

Maintained 96.7% QND at high photon numbers

Theoretical critical photon number estimated at 377

Abstract

The field of superconducting qubits is constantly evolving with new circuit designs. However, when it comes to qubit readout, the use of simple transverse linear coupling remains overwhelmingly prevalent. This standard readout scheme has significant drawbacks: in addition to the Purcell effect, it suffers from a limitation on the maximal number of photons in the readout mode, which restricts the signal-to-noise ratio (SNR) and the Quantum Non-Demolition (QND) nature of the readout. Here, we explore the high-power regime by engineering a nonlinear coupling between a transmon qubit and its readout mode. Our approach builds upon previous work by Dassonneville et al. [Physical Review X 10, 011045 (2020)], on qubit readout with a non-perturbative cross-Kerr coupling in a transmon molecule. We demonstrate a readout fidelity of 99.21% with 89 photons utilizing a parametric amplifier. At this elevated photon number, the QND nature remains high at 96.7%. Even with up to 300 photons, the QNDness is only reduced by a few percent. This is qualitatively explained by deriving a critical number of photons associated with the nonlinear coupling, yielding a theoretical value of $\bar{n}_r^\text{crit} = 377$ photons for our sample's parameters. These results highlight the promising performance of the transmon molecule in the high-power regime, establishing it as a compelling platform for high-fidelity qubit readout.