Quantum non-demolition dispersive readout of a superconducting artificial atom using large photon numbers

TL;DR

This paper demonstrates a fluxonium artificial atom with stable transition rates at high photon numbers, enabling high-fidelity, rapid quantum state readout without increased transition errors, even at large photon numbers.

Contribution

It shows that fluxonium atoms maintain low transition rates at high photon numbers, allowing improved readout fidelity without additional error from increased power.

Findings

Transition rates remain flat up to $ar{n} oughly 200$

Fidelities of 99% and 93% achieved for state preparation

Signal-to-noise ratio improves with increasing photon number

Abstract

Reading out the state of superconducting artificial atoms typically relies on dispersive coupling to a readout resonator. For a given system noise temperature, increasing the circulating photon number $\bar{n}$ in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing $\bar{n}$ is generally observed to also increase these transition rates. Here we present a fluxonium artificial atom in which we measure an overall flat dependence of the transition rates between its first two states as a function of $\bar{n}$, up to $\bar{n}\approx200$. Despite the fact that we observe the expected decrease of the dispersive shift with increasing readout power, the signal-to-noise ratio continuously improves with increasing $\bar{n}$. Even without the use of a parametric amplifier, at $\bar{n}=74$, we measure fidelities of 99% and 93% for feedback-assisted ground and excited state preparation, respectively.