Jason Walter

Single Flux Quantum Circuit Operation at Millikelvin Temperatures

  1. Jason Walter,
  2. Adam C. Weis,
  3. Kan-Ting Tsai,
  4. Meng-Ju Yu,
  5. Naveen Katam,
  6. Alex F. Kirichenko,
  7. Oleg A. Mukhanov,
  8. Shu Jen Han,
  9. and Igor V. Vernik
As quantum computing processors increase in size, there is growing interest in developing cryogenic electronics to overcome significant challenges to system scaling. Single flux-quantum
(SFQ) circuits offer a promising alternative to remote, bulky, and power-hungry room temperature electronics. To meet the need for digital qubit control, readout, and co-processing, SFQ circuits must be adapted to operate at millikelvin temperatures near quantum processors. SEEQC’s SFQuClass digital quantum management approach proximally places energy-efficient SFQ (ERSFQ) circuits and qubits in a multi-chip module. This enables extremely low power dissipation, compatible with a typical dilution cryostat’s limited cooling power, while maintaining high processing speed and low error rates. We report on systematic testing from 4 K to 10 mK of a comprehensive set of ERSFQ cells, as well as more complex circuits such as programmable counters and demultiplexers used in digital qubit control. We compare the operating margins and error rates of these circuits and find that, at millikelvin, bias margins decrease and the center of the margins (i.e., the optimal bias current value) increases by ~15%, compared to 4.2 K. The margins can be restored by thermal annealing by reducing Josephson junction (JJ) critical current Ic. To provide guidance for how circuit parameters vary from 4.2 K to millikelvin, relevant analog process control monitors (PCMs) were tested in the temperature range of interest. The measured JJ critical current (of the PCM JJ arrays) increases by ~15% when decreasing temperature from 4.2 K to millikelvin, in good agreement with both theory and the empirically measured change in the center of bias margins for the tested digital circuits.
arxiv pdf

Quantum Computer Controlled by Superconducting Digital Electronics at Millikelvin Temperature

  1. Jacob Bernhardt,
  2. Caleb Jordan,
  3. Joseph Rahamim,
  4. Alex Kirchenko,
  5. Karthik Bharadwaj,
  6. Louis Fry-Bouriaux,
  7. Katie Porsch,
  8. Aaron Somoroff,
  9. Kan-Ting Tsai,
  10. Jason Walter,
  11. Adam Weis,
  12. Meng-Ju Yu,
  13. Mario Renzullo,
  14. Daniel Yohannes,
  15. Igor Vernik,
  16. Oleg Mukhanov,
  17. and Shu Jen Han
Current superconducting quantum computing platforms face significant scaling challenges, as individual signal lines are required for control of each qubit. This wiring overhead is a
result of the low level of integration between control electronics at room temperature and qubits operating at millikelvin temperatures, which raise serious doubts among technologists about whether utility-scale quantum computers can be built. A promising alternative is to utilize cryogenic, superconducting digital control electronics that coexist with qubits. Here, we report the first multi-qubit system integrating this technology. The system utilizes digital demultiplexing, breaking the linear scaling of control lines to number of qubits. We also demonstrate single-qubit fidelities above 99%, and up to 99.9%. This work is a critical step forward in realizing highly scalable chip-based quantum computers.
arxiv pdf

Flip-Chip Packaging of Fluxonium Qubits

  1. Aaron Somoroff,
  2. Patrick Truitt,
  3. Adam Weis,
  4. Jacob Bernhardt,
  5. Daniel Yohannes,
  6. Jason Walter,
  7. Konstantin Kalashnikov,
  8. Raymond A. Mencia,
  9. Igor V. Vernik,
  10. Oleg Mukhanov,
  11. Maxim G. Vavilov,
  12. and Vladimir E. Manucharyan
The strong anharmonicity and high coherence times inherent to fluxonium superconducting circuits are beneficial for implementing quantum information processors. In addition to requiring
high-quality physical qubits, a quantum processor needs to be assembled in a manner that reduces crosstalk and decoherence. In this letter, we report work on fluxonium qubits packaged in a flip-chip architecture. Here, the fluxonium qubits are embedded in a multi-chip module (MCM), where a classical control and readout chip is bump-bonded to the quantum chip. The modular approach allows for improved connectivity between qubits and control/readout elements, and separate fabrication processes. We demonstrate that this configuration does not degrade the fluxonium qubit performance, and identify the main decoherence mechanisms to improve on the reported results.
arxiv pdf