Single-shot readout and state preparation of a fluxonium qubit without the use of a parametric amplifier

High-fidelity qubit readout is an essential requirement for the implementation of quantum algorithms. A commonly used readout technique involves the dispersive interaction between a qubit and a resonator, which encodes the qubit state into the phase and amplitude of the microwave readout tone. In theory, the state discrimination can be substantially improved if the resonator is populated with high photon numbers [1]. In practice however, the optimal photon number, at which the best readout fidelity is obtained, is usually in the range of a few photons [2, 3], and parametric amplifiers with near quantum-limited noise are needed to overcome the noise of even the best commercial high electron mobility transistor amplifiers.
Using a fluxonium qubit with a granular aluminum superinductance [4], we demonstrate single shot qubit measurements without a parametric amplifier, at a readout power corresponding to hundreds of circulating photons. The dispersive shift decreases with photon number, and the qubit decay rate increases, resulting in a tradeoff between applied power and readout fidelity.

[1] A. Blais, et al. PRA 69, 062320, (2004)
[2] T. Walter, et al. PRA 75, 054020, (2017)
[3] U. Vool, et al. PRL. 113, 247001 (2014)
[4] L. Grünhaupt & M. Spiecker, et al. Nature Materials (2019)