Sergey Novikov

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

Anneal-path correction in flux qubits

  1. Mostafa Khezri,
  2. Jeffrey A. Grover,
  3. James I. Basham,
  4. Steven M. Disseler,
  5. Huo Chen,
  6. Sergey Novikov,
  7. Kenneth M. Zick,
  8. and Daniel A. Lidar
Quantum annealers require accurate control and optimized operation schemes to reduce noise levels, in order to eventually demonstrate a computational advantage over classical algorithms.
We study a high coherence four-junction capacitively shunted flux qubit (CSFQ), using dispersive measurements to extract system parameters and model the device. We confirm the multi-level structure of the circuit model of our CSFQ by annealing it through small spectral gaps and observing quantum signatures of energy level crossings. Josephson junction asymmetry inherent to the device causes a deleterious nonlinear cross-talk when annealing the qubit. We implement a nonlinear annealing path to correct the asymmetry in-situ, resulting in a 50% improvement in the qubit performance. Our results demonstrate a low-level quantum control scheme which enhances the success probability of a quantum annealer.
arxiv pdf

Exploring More-Coherent Quantum Annealing

  1. Sergey Novikov,
  2. Robert Hinkey,
  3. Steven Disseler,
  4. James I. Basham,
  5. Tameem Albash,
  6. Andrew Risinger,
  7. David Ferguson,
  8. Daniel A. Lidar,
  9. and Kenneth M. Zick
In the quest to reboot computing, quantum annealing (QA) is an interesting candidate for a new capability. While it has not demonstrated an advantage over classical computing on a real-world
application, many important regions of the QA design space have yet to be explored. In IARPA’s Quantum Enhanced Optimization (QEO) program, we have opened some new lines of inquiry to get to the heart of QA, and are designing testbed superconducting circuits and conducting key experiments. In this paper, we discuss recent experimental progress related to one of the key design dimensions: qubit coherence. Using MIT Lincoln Laboratory’s qubit fabrication process and extending recent progress in flux qubits, we are implementing and measuring QA-capable flux qubits. Achieving high coherence in a QA context presents significant new engineering challenges. We report on techniques and preliminary measurement results addressing two of the challenges: crosstalk calibration and qubit readout. This groundwork enables exploration of other promising features and provides a path to understanding the physics and the viability of quantum annealing as a computing resource.
arxiv pdf