Methods to achieve near-millisecond energy relaxation and dephasing times for a superconducting transmon qubit

  1. Mikko Tuokkola,
  2. Yoshiki Sunada,
  3. Heidi Kivijärvi,
  4. Leif Grönberg,
  5. Jukka-Pekka Kaikkonen,
  6. Visa Vesterinen,
  7. Joonas Govenius,
  8. and Mikko Möttönen
Superconducting qubits are one of the most promising physical systems for implementing a quantum computer. However, executing quantum algorithms of practical computational advantage
requires further improvements in the fidelities of qubit operations, which are currently limited by the energy relaxation and dephasing times of the qubits. Here, we report our measurement results of a high-coherence transmon qubit with energy relaxation and echo dephasing times surpassing those in the existing literature. We measure a qubit frequency of 2.890 GHz, an energy relaxation time with a median of 502 us and a maximum of (765 +/- 82.6) us, and an echo dephasing time with a median of 541 us and a maximum of (1057 +/- 138) us. We report details of our design, fabrication process, and measurement setup to facilitate the reproduction and wide adoption of high-coherence transmon qubits in the academia and industry.

Many-excitation removal of a transmon qubit using a single-junction quantum-circuit refrigerator and a two-tone microwave drive

  1. Wallace Teixeira,
  2. Timm Mörstedt,
  3. Arto Viitanen,
  4. Heidi Kivijärvi,
  5. András Gunyhó,
  6. Maaria Tiiri,
  7. Suman Kundu,
  8. Aashish Sah,
  9. Vasilii Vadimov,
  10. and Mikko Möttönen
Achieving fast and precise initialization of qubits is a critical requirement for the successful operation of quantum computers. The combination of engineered environments with all-microwave
techniques has recently emerged as a promising approach for the reset of superconducting quantum devices. In this work, we experimentally demonstrate the utilization of a single-junction quantum-circuit refrigerator (QCR) for an expeditious removal of several excitations from a transmon qubit. The QCR is indirectly coupled to the transmon through a resonator in the dispersive regime, constituting a carefully engineered environmental spectrum for the transmon. Using single-shot readout, we observe excitation stabilization times down to roughly 500 ns, a 20-fold speedup with QCR and a simultaneous two-tone drive addressing the e-f and f0-g1 transitions of the system. Our results are obtained at a 48-mK fridge temperature and without postselection, fully capturing the advantage of the protocol for the short-time dynamics and the drive-induced detrimental asymptotic behavior in the presence of relatively hot other baths of the transmon. We validate our results with a detailed Liouvillian model truncated up to the three-excitation subspace, from which we estimate the performance of the protocol in optimized scenarios, such as cold transmon baths and fine-tuned driving frequencies. These results pave the way for optimized reset of quantum-electric devices using engineered environments and for dissipation-engineered state preparation.