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A superconductor free of quasiparticles for seconds

Abstract

Superconducting devices, based on the Cooper pairing of electrons, play an important role in existing and emergent technologies, ranging from radiation detectors1,2 to quantum computers3. Their performance is limited by spurious quasiparticle excitations formed from broken Cooper pairs4,5,6,7,8,9,10,11,12. Efforts to achieve ultra-low quasiparticle densities have reached time-averaged numbers of excitations on the order of one in state-of-the-art devices2,12,13,14,15. However, the dynamics of the quasiparticle population as well as the timescales for adding and removing individual excitations remain largely unexplored. Here, we experimentally demonstrate a superconductor completely free of quasiparticles for periods lasting up to seconds. We monitor the quasiparticle number on a mesoscopic superconductor in real time by measuring the charge tunnelling to a normal metal contact. Quiet, excitation-free periods are interrupted by random-in-time Cooper pair breaking events, followed by a burst of charge tunnelling within a millisecond. Our results demonstrate the possibility of operating devices without quasiparticles with potentially improved performance. In addition, our experiment probes the origins of nonequilibrium quasiparticles in our device. The decay of the Cooper pair breaking rate over several weeks following the initial cooldown rules out processes arising from cosmic or long-lived radioactive sources16,17,18,19.

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Fig. 1: Real-time monitoring of the number of quasiparticles on a superconducting island via charge tunnelling.
Fig. 2: Statistics of Cooper pair breaking events.
Fig. 3: Dynamics of quasiparticle relaxation within a burst.
Fig. 4: Decay of burst rate over time.

Data availability

Data for figures that support the manuscript are available at https://doi.org/10.5281/zenodo.5574415. All other data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

The authors thank O. Maillet for useful discussions, J. Ala-Heikkilä for support with shielding solutions and J. Lehtinen and M. Prunnila from VTT Technical Research Center of Finland Ltd., who were also involved in the JPA development. This work was performed as part of the Academy of Finland Centre of Excellence program (projects 312057, 312059 and 312294). We acknowledge the provision of facilities and technical support by Aalto University at OtaNano - Micronova Nanofabrication Centre and OtaNano - Low Temperature Laboratory. E.T.M. and J.P.P. acknowledge financial support from Microsoft. V.V. acknowledges financial support from the Academy of Finland through grant no. 321700. P.S. and V.F.M. acknowledge financial support from the Swedish National Science Foundation, and V.F.M. acknowledges financial support from the QuantERA project ‘2D hybrid materials as a platform for topological quantum computing’ and NanoLund.

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E.T.M., P.S., V.F.M. and J.P.P. conceived the experiment and model and interpreted the results. E.T.M. fabricated the sample and performed the experiment with support from J.T.P., and E.T.M. analysed the data. P.S. performed the theoretical modelling. S.S., V.V., L.G. and J.H. provided the Josephson parametric amplifier. E.T.M. and J.T.P. integrated the Josephson parametric amplifier into the set-up with assistance from S.S. and V.V. The manuscript was written by E.T.M. and P.S. with input from all coauthors.

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Correspondence to E. T. Mannila.

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Supplementary Notes 1–8 and Figs. S1–S13.

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Mannila, E.T., Samuelsson, P., Simbierowicz, S. et al. A superconductor free of quasiparticles for seconds. Nat. Phys. 18, 145–148 (2022). https://doi.org/10.1038/s41567-021-01433-7

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