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arXiv:1909.02866 · Physical Review Research

Waiting time distributions in a two-level fluctuator coupled to a superconducting charge detector

Máté Jenei, Elina Potanina, Ruichen Zhao, Kuan Yen Tan, Alessandro Rossi, Tuomo Tanttu, K. W. Chan, Vasilii Sevriuk, Mikko Möttönen, Andrew S. Dzurak

Published Dec 10, 2019• 13 citations
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Abstract

The authors propose and realize experimentally a method based on distributions of waiting times to determine the optimal working regime of a charge detector. This proposal includes the finite-bandwidth of the superconducting charge sensor and directly extract the short timescales of charge transitions in a strongly-coupled two-level fluctuator.

DOI
10.1103/physrevresearch.1.033163

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The authors propose and realize experimentally a method based on distributions of waiting times to determine the optimal working regime of a charge detector. This proposal includes the finite-bandwidth of the superconducting charge sensor and directly extract the short timescales of charge transitions in a strongly-coupled two-level fluctuator.
We analyze charge fluctuations in a parasitic state strongly coupled to a superconducting Josephson-junction-based charge detector. The charge dynamics of the state resembles that of electron transport in a quantum dot with two charge states, and hence we refer to it as a two-level fluctuator. By constructing the distribution of waiting times from the measured detector signal and comparing it with a waiting time theory, we extract the electron in- and out-tunneling rates for the two-level fluctuator, which are severely asymmetric.
Extensive studies of dielectric loss due to two level fluctuators (TLFs) in superconducting resonators have provided routes for low loss resonators. The research is motivated not only by the use of resonators as detectors and in quantum information processing, but more generally due to TLFs being a source of noise and decoherence in all quantum devices. In this work a frequency locked loop was used to measure frequency fluctuations at timescales in excess of 104 seconds, thereby accurately probing the TLF induced low- frequency noise of the resonator. Our measurement method lead to very high statistical confidence even for very long timescales, and here we can therefore present results explicitly identifying power dependent flicker frequency noise (S = 1/fawhere a=1) persisting down to the mHz level.
Spectroscopic analysis of superconducting qubits often shows clear signatures of avoided level crossings, indicating the presence of microscopic two-level fluctuators (TLFs) coupling to the qubit. Within ''direct coherent control'' method it was possible for the first time to investigate coherence times and temperatures properties of single TLFs. Measurements of relaxation time shows unusual behaviour of temperature dependence. Within Bloch Redfield formalism we analyse the influence of higher qubit levels on the resonance shape of relaxation time.
Schemes aimed at transferring individual electrons in semiconductor devices and detecting possible transfer errors have increasing importance for metrological applications. We study the coupling of a superconducting Josephson-junction-based charge detector to an electron island defined by field-effect in silicon. The flexibility of our device allows one to tune the coupling using the detector as an additional gate electrode. We study the reliability of the electron sensor in different device configurations and the suitability of various operation modes for error detection in electron pumping experiments. As a result, we obtain a charge detection bandwidth of 5.87 kHz with unity signal to noise ratio at 300 mK bath temperature.
Low-frequency charge noise is a dominant source of decoherence in many solid-state qubit platforms. Conventionally, the noise is modeled as a bath of two-level fluctuators that activate or tunnel between minima in a potential energy landscape. Here, the authors describe measurements of charge noise in a superconducting qubit that suggest an unexpected dependence of the noise on the device scale. The measurements cannot be explained by the two-level fluctuate model. Rather, they are consistent with charge drift.
In time-domain measurements of a Poisson two-level system, the observed transition rates are always smaller than those of the actual system, a general consequence of a finite measurement bandwidth in an experiment. This underestimation of the rates is significant even when the measurement and detection apparatus are 10 times faster than the process under study. We derive here a quantitative form for this correction, using a straightforward state transition model that includes the detection apparatus, and provide a method for determining a system's actual transition rates from bandwidth-limited measurements. We support our results with computer simulations and experimental data from time-domain measurements of quasiparticle tunneling in a single-Cooper-pair transistor.