Volodymyr Sivak

Optimizing quantum gates towards the scale of logical qubits

  1. Paul V. Klimov,
  2. Andreas Bengtsson,
  3. Chris Quintana,
  4. Alexandre Bourassa,
  5. Sabrina Hong,
  6. Andrew Dunsworth,
  7. Kevin J. Satzinger,
  8. William P. Livingston,
  9. Volodymyr Sivak,
  10. Murphy Y. Niu,
  11. Trond I. Andersen,
  12. Yaxing Zhang,
  13. Desmond Chik,
  14. Zijun Chen,
  15. Charles Neill,
  16. Catherine Erickson,
  17. Alejandro Grajales Dau,
  18. Anthony Megrant,
  19. Pedram Roushan,
  20. Alexander N. Korotkov,
  21. Julian Kelly,
  22. Vadim Smelyanskiy,
  23. Yu Chen,
  24. and Hartmut Neven
A foundational assumption of quantum error correction theory is that quantum gates can be scaled to large processors without exceeding the error-threshold for fault tolerance. Two major
challenges that could become fundamental roadblocks are manufacturing high performance quantum hardware and engineering a control system that can reach its performance limits. The control challenge of scaling quantum gates from small to large processors without degrading performance often maps to non-convex, high-constraint, and time-dependent control optimization over an exponentially expanding configuration space. Here we report on a control optimization strategy that can scalably overcome the complexity of such problems. We demonstrate it by choreographing the frequency trajectories of 68 frequency-tunable superconducting qubits to execute single- and two-qubit gates while mitigating computational errors. When combined with a comprehensive model of physical errors across our processor, the strategy suppresses physical error rates by ∼3.7× compared with the case of no optimization. Furthermore, it is projected to achieve a similar performance advantage on a distance-23 surface code logical qubit with 1057 physical qubits. Our control optimization strategy solves a generic scaling challenge in a way that can be adapted to other quantum algorithms, operations, and computing architectures.
arxiv pdf

Fast Universal Control of an Oscillator with Weak Dispersive Coupling to a Qubit

  1. Alec Eickbusch,
  2. Volodymyr Sivak,
  3. Andy Z. Ding,
  4. Salvatore S. Elder,
  5. Shantanu R. Jha,
  6. Jayameenakshi Venkatraman,
  7. Baptiste Royer,
  8. S. M. Girvin,
  9. Robert J. Schoelkopf,
  10. and Michel H. Devoret
Efficient quantum control of an oscillator is necessary for many bosonic applications including error-corrected computation, quantum-enhanced sensing, robust quantum communication,
and quantum simulation. For these applications, oscillator control is often realized through off-resonant hybridization to a qubit with dispersive shift χ where typical operation times of 2π/χ are routinely assumed. Here, we challenge this assumption by introducing and demonstrating a novel control method with typical operation times over an order of magnitude faster than 2π/χ. Using large auxiliary displacements of the oscillator to enhance gate speed, we introduce a universal gate set with built-in dynamical decoupling consisting of fast conditional displacements and qubit rotations. We demonstrate the method using a superconducting cavity weakly coupled to a transmon qubit in a regime where previously known methods would fail. Our demonstrations include preparation of a single-photon state 30 times faster than 2π/χ with 98±1(%) fidelity and preparation of squeezed vacuum with a squeezing level of 11.1 dB, the largest intracavity squeezing reported in the microwave regime. Finally, we demonstrate fast measurement-free preparation of logical states for the binomial and Gottesman-Kitaev-Preskill (GKP) code, and we identify possible fidelity limiting mechanisms including oscillator dephasing.
arxiv pdf