High Density Fabrication Process for Single Flux Quantum Circuits

  1. D. Yohannes,
  2. M. Renzullo,
  3. J. Vivalda,
  4. A. C. Jacobs,
  5. M. Yu,
  6. J. Walter,
  7. A. F. Kirichenko,
  8. I. V. Vernik,
  9. and O. A. Mukhanov
We implemented, optimized and fully tested over multiple runs a superconducting Josephson junction fabrication process tailored for the integrated digital circuits that are used for
control and readout of superconducting qubits operating at millikelvin temperatures. This process was optimized for highly energy efficient single flux quantum (ERSFQ) circuits with the critical currents reduced by factor of ~10 as compared to those operated at 4.2 K. Specifically, it implemented Josephson junctions with 10 uA unit critical current fabricated with a 10 uA/um2 critical current density. In order to circumvent the substantial size increase of the SFQ circuit inductors, we employed a NbN high kinetic inductance layer (HKIL) with a 8.5 pH/sq sheet inductance. Similarly, to maintain the small size of junction resistive shunts, we used a non-superconducting PdAu alloy with a 4.0 ohm/sq sheet resistance. For integration with quantum circuits in a multi-chip module, 5 and 10 um height bump processes were also optimized. To keep the fabrication process in check, we developed and thoroughly tested a comprehensive Process Control Monitor chip set.

Quantum–Classical Interface Based on Single Flux Quantum Digital Logic

  1. R. McDermott,
  2. M.G. Vavilov,
  3. B. L. T. Plourde,
  4. F.K. Wilhelm,
  5. P. J. Liebermann,
  6. O. A. Mukhanov,
  7. and T. A. Ohki
We describe an approach to the integrated control and measurement of a large-scale superconducting multiqubit circuit using a proximal coprocessor based on the Single Flux Quantum (SFQ)
digital logic family. Coherent control is realized by irradiating the qubits directly with classical bitstreams derived from optimal control theory. Qubit measurement is performed by a Josephson photon counter, which provides access to the classical result of projective quantum measurement at the millikelvin stage. We analyze the power budget and physical footprint of the SFQ coprocessor and discuss challenges and opportunities associated with this approach.