L. Grönberg

Initial experimental results on a superconducting-qubit reset based on photon-assisted quasiparticle tunneling

  1. V. A. Sevriuk,
  2. W. Liu,
  3. J. Rönkkö,
  4. H. Hsu,
  5. F. Marxer,
  6. T. F. Mörstedt,
  7. M. Partanen,
  8. J. Räbinä,
  9. M. Venkatesh,
  10. J. Hotari,
  11. L. Grönberg,
  12. J. Heinsoo,
  13. T. Li,
  14. J. Tuorila,
  15. K.W. Chan,
  16. J. Hassel,
  17. K. Y. Tan,
  18. and M. Möttönen
We present here our recent results on qubit reset scheme based on a quantum-circuit refrigerator (QCR). In particular, we use the photon-assisted quasiparticle tunneling through a superconductor–insulator–normal-metal–insulator–superconductor
junction to controllably decrease the energy relaxation time of the qubit during the QCR operation. In our experiment, we use a transmon qubit with dispersive readout. The QCR is capacitively coupled to the qubit through its normal-metal island. We employ rapid, square-shaped QCR control voltage pulses with durations in the range of 2–350 ns and a variety of amplitudes to optimize the reset time and fidelity. Consequently, we reach a qubit ground-state probability of roughly 97% with 80-ns pulses starting from the first excited state. The qubit state probability is extracted from averaged readout signal, where the calibration is based of the Rabi oscillations, thus not distinguishing the residual thermal population of the qubit.
arxiv pdf

Qubit-compatible substrates with superconducting through-silicon vias

  1. K. Grigoras,
  2. N. Yurttagül,
  3. J.-P. Kaikkonen,
  4. E. T. Mannila,
  5. P. Eskelinen,
  6. D. P. Lozano,
  7. H.-X. Li,
  8. M. Rommel,
  9. D. Shiri,
  10. N. Tiencken,
  11. S. Simbierowicz,
  12. A. Ronzani,
  13. J. Hätinen,
  14. D. Datta,
  15. V. Vesterinen,
  16. L. Grönberg,
  17. J. Biznárová,
  18. A. Fadavi Roudsari,
  19. S. Kosen,
  20. A. Osman,
  21. J. Hassel,
  22. J. Bylander,
  23. and J. Govenius
We fabricate and characterize superconducting through-silicon vias and electrodes suitable for superconducting quantum processors. We measure internal quality factors of a million for
test resonators excited at single-photon levels, when vias are used to stitch ground planes on the front and back sides of the wafer. This resonator performance is on par with the state of the art for silicon-based planar solutions, despite the presence of vias. Via stitching of ground planes is an important enabling technology for increasing the physical size of quantum processor chips, and is a first step toward more complex quantum devices with three-dimensional integration.
arxiv pdf

A superconductor free of quasiparticles for seconds

  1. E. T. Mannila,
  2. P. Samuelsson,
  3. S. Simbierowicz,
  4. J.T. Peltonen,
  5. V. Vesterinen,
  6. L. Grönberg,
  7. J. Hassel,
  8. V. F. Maisi,
  9. and J P Pekola
Superconducting devices, based on the Cooper pairing of electrons, are of outstanding importance in existing and emergent technologies, ranging from radiation detectors to quantum computers.
Their performance is limited by spurious broken Cooper pairs also known as quasiparticle excitations. In state-of-the-art devices, the time-averaged number of quasiparticles can be on the order of one. However, realizing a superconductor with no excitations remains an outstanding challenge. Here, we experimentally demonstrate a superconductor completely free of quasiparticles up to seconds. The quasiparticle number on a mesoscopic superconductor is monitored in real time by measuring the charge tunneling to a normal metal contact. Quiet excitation-free periods are interrupted by random-in-time events, where one or several Cooper pairs break, followed by a burst of charge tunneling within a millisecond. Our results vindicate the opportunity to operate devices without quasiparticles with potentially improved performance. In addition, our present experiment probes the origins of nonequilibrium quasiparticles in it; 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 sources.
arxiv pdf