Quantum Nondemolition Dispersive Readout of a Superconducting Artificial Atom Using Large Photon Numbers

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 n in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing n is generally observed to also monotonously increase these transition rates. Here we present a fluxonium artificial atom in which, despite the fact that the measured transition rates show nonmonotonous fluctuations within a factor of 6, for photon numbers up to n ≈ 200, the signal-to-noise ratio continuously improves with increasing n. Even without the use of a parametric amplifier, at n = 74, we achieve fidelities of 99% and 93% for feedback-assisted ground and excited state preparations, respectively. At higher n, leakage outside the qubit computational space can no longer be neglected and it limits the fidelity of quantum state preparation.

Minimizing the discrimination time for quantum states of an artificial atom

“…By performing a continuous measurement and benefiting from the remarkable insensitivity to n of a granular Aluminum (grAl) fluxonium [36] artificial atom, recently reported in Ref. [37], we demonstrate a significant decrease of the QND state discrimination time with increasing n, up to n = 110. To handle the correspondingly large signal power we use a high dynamic range dimer Josephson junction array parametric amplifier (DJJAA) [38].…”

“…This happens thanks to the grAl readout resonator's intrinsic nonlinearity [49] K grAl /2π = −2.4 kHz and the inherited state-dependent nonlinearities [54] K |g /2π = −2.6 Hz, K |e /2π = 2 kHz, and K |f /2π = −2 kHz at n = 50 (see Ref. [37]). Notice that for the |e state the Kerr coefficients almost cancel out, as evidenced by the reduced squeezing of the |e pointer state in Fig.…”

“…The incident readout signal reflects from a resonator coupled to a fluxonium artificial atom, both implemented using granular aluminum, which is the same device discussed in Ref. [37]. The reflected signal is amplified by a dimer Josephson junction array amplifier (DJJAA) [38] sufficient (projective measurement) or insufficient (weak measurement) to determine the qubit state.…”

“…The monitored quantum system is a fluxonium artificial atom [45], dispersively coupled via a shared inductance to a readout resonator (see Table I for the list of system parameters). The inductors required for both the fluxonium and readout antenna are implemented using grAl [36], and the fluxonium junction is implemented in the shape of a SQUID [37,46]. The sample chip was placed in a rectangular waveguide, following the concept in Ref.…”

“…(b) Simplified schematic of the experimental setup. The incident readout signal reflects from a resonator coupled to a fluxonium artificial atom, both implemented using granular aluminum, which is the same device discussed in Ref [37]…”