arXiv:0912.1956 · Physica Scripta

Spectral measurement of the thermal excitation of a superconducting qubit

Agustin Palacios-Laloy, F. Mallet, François Nguyen, Florian R. Ong, Patrice Bertet, D. Vion, D Esteve

Published Dec 1, 2009• 12 citations

Abstract

We report the measurement of the fluctuations of a transmon qubit through the noise spectrum of the microwave signal that measures its state. The amplitude of the Lorentzian noise power spectrum allows us to determine the average qubit excitation, in agreement with the estimated thermal radiation reaching the sample. Its width yields the qubit energy relaxation rate that decreases with temperature, contrary to the predictions for a two-level system solely coupled to thermal radiation. This indicates the existence of another non-radiative energy relaxation channel for the qubit.

DOI: 10.1088/0031-8949/2009/t137/014015

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arXiv:0912.1956 · Physica Scripta

Spectral measurement of the thermal excitation of a superconducting qubit

Agustin Palacios-Laloy, F. Mallet, François Nguyen, Florian R. Ong, Patrice Bertet, D. Vion, D Esteve

Published Dec 1, 2009• 12 citations

arXiv HTML PDF

Abstract

DOI: 10.1088/0031-8949/2009/t137/014015

Related papers

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\overline {T_1}

T 1 ¯ = 49 μs and

\overline {T_2^ \ast }

T 2 * ¯ = 95 μs; however, both of these quantities fluctuate, explaining the need for frequent re-calibration in qubit setups. Our main finding is that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems (TLS). Through statistical analysis, we determine sub-millihertz switching rates of these TLS and observe the coherent coupling between an individual TLS and a transmon qubit. Finally, we find evidence that the qubit’s frequency stability produces a 0.8 ms limit on the pure dephasing which we also observe. These findings raise the need for performing qubit metrology to examine the reproducibility of qubit parameters, where these fluctuations could affect qubit gate fidelity.

T_1

extraction of the first and second excited states reveals characteristic correlations in the decay rates of adjacent transitions. From these correlations we identify one or more dominant TLSs and reconstruct their frequency drift over time. Remarkably, we find that TLSs detuned by

\gtrsim 100\,\mathrm{MHz}

from the qubit transition can still significantly influence relaxation. The proposed method provides a powerful tool for TLS spectroscopy without the need to tune the transmon frequency, either via a flux-tunable inductor or AC-Stark shifts.

T_1

) exhibit high robustness against thermal stress, characterized by frequency deviations typically confined within 0.5\% and non-degraded coherence baselines. In stark contrast, the environmental variables, specifically the background magnetic flux offsets and the microscopic landscape of two-level system (TLS) defects, undergo a significant stochastic reconfiguration after each cycle. By employing frequency-dependent relaxation spectroscopy and a quantitative metric, the

T_1

Spectral Topography Fidelity, we demonstrate that thermal cycling acts as a ``hard reset'' for the local defect environment. This process introduces a level of spectral randomization equivalent to thousands of hours of continuous low-temperature evolution. These findings confirm that while the fabrication quality is preserved, the specific noise realization is statistically distinct for each thermal cycle, necessitating automated recalibration strategies for large-scale quantum systems.