David G. Ferguson

Superconducting Flux Memory for Cryogenic Applications

  1. Tony X. Zhou,
  2. John McFarland,
  3. Aruna N. Ramanayaka,
  4. Brian Sears,
  5. Colin Stack,
  6. Aref Fouladi,
  7. Robert Smith,
  8. Sambarta Rakshit,
  9. Zachary A. Stegen,
  10. Keith D. Hillaire,
  11. Moe Khalil,
  12. Robert M. Young,
  13. David G. Ferguson,
  14. Anthony J. Przybysz,
  15. John X. Pryzbysz,
  16. Mark Covington,
  17. Gregory R Boyd,
  18. Jeremy Clark,
  19. and Aaron Pesetski
We report the development of flux memory for use with superconducting circuits. This technology stores persistent currents in superconducting loops on-chip to be used to provide flux
biasing for superconducting circuits, like qubits. We developed three types of flux memory and draw comparisons among them for circuit design. We demonstrate the utility of flux memory by using an in-situ flux detector and characterize each approach and further demonstrate that once flux is set in a memory cell, benchtop DC control sources can be powered off, leaving the on-chip flux bias in place. We propose that flux memory can be arranged in a two-dimensional configuration to multiplex control signals and reduce how line counts scale (N^2 devices -> 2N control lines), and our experimental results pave the path to the proposed scalability. We demonstrate the use of flux memory to flux bias a transmon qubit and show the tunability of the qubit state to a target frequency which remained stable on-chip for 20 hours.
arxiv pdf

Fast, Lifetime-Preserving Readout for High-Coherence Quantum Annealers

  1. Jeffrey A. Grover,
  2. James I. Basham,
  3. Alexander Marakov,
  4. Steven M. Disseler,
  5. Robert T. Hinkey,
  6. Moe Khalil,
  7. Zachary A. Stegen,
  8. Thomas Chamberlin,
  9. Wade DeGottardi,
  10. David J. Clarke,
  11. James R. Medford,
  12. Joel D. Strand,
  13. Micah J. A. Stoutimore,
  14. Sergey Novikov,
  15. David G. Ferguson,
  16. Daniel Lidar,
  17. Kenneth M. Zick,
  18. and Anthony J. Przybysz
We demonstrate, for the first time, that a quantum flux parametron (QFP) is capable of acting as both isolator and amplifier in the readout circuit of a capacitively shunted flux qubit
(CSFQ). By treating the QFP like a tunable coupler and biasing it such that the coupling is off, we show that T1 of the CSFQ is not impacted by Purcell loss from its low-Q readout resonator (Qe=760) despite being detuned by only 40 MHz. When annealed, the QFP amplifies the qubit’s persistent current signal such that it generates a flux qubit-state-dependent frequency shift of 85 MHz in the readout resonator, which is over 9 times its linewidth. The device is shown to read out a flux qubit in the persistent current basis with fidelities surpassing 98.6% with only 80 ns integration, and reaches fidelities of 99.6% when integrated for 1 μs. This combination of speed and isolation is critical to the readout of high-coherence quantum annealers.
arxiv pdf

Symmetries and collective excitations in large superconducting circuits

  1. David G. Ferguson,
  2. A. A. Houck,
  3. and Jens Koch
The intriguing appeal of circuits lies in their modularity and ease of fabrication. Based on a toolbox of simple building blocks, circuits present a powerful framework for achieving
new functionality by combining circuit elements into larger networks. It is an open question to what degree modularity also holds for quantum circuits — circuits made of superconducting material, in which electric voltages and currents are governed by the laws of quantum physics. If realizable, quantum coherence in larger circuit networks has great potential for advances in quantum information processing including topological protection from decoherence. Here, we present theory suitable for quantitative modeling of such large circuits and discuss its application to the fluxonium device. Our approach makes use of approximate symmetries exhibited by the circuit, and enables us to obtain new predictions for the energy spectrum of the fluxonium device which can be tested with current experimental technology.
arxiv pdf