Daniel L. Campbell

Fast single-qubit gates for continuous dynamically decoupled systems

  1. Michael Senatore,
  2. Daniel L. Campbell,
  3. James A. Williams,
  4. and Matthew D. LaHaye
Environmental noise that couples longitudinally to a quantum system dephases that system and can limit its coherence lifetime. Performance using quantum superposition in clocks, information
processors, communication networks, and sensors depends on careful state and external field selection to lower sensitivity to longitudinal noise. In many cases time varying external control fields–such as the Hahn echo sequence originally developed for nuclear magnetic resonance applications–can passively correct for longitudinal errors. There also exist continuous versions of passive correction called continuous dynamical decoupling (CDD), or spin-locking depending on context. However, treating quantum systems under CDD as qubits has not been well explored. Here, we develop universal single-qubit gates that are „fast“ relative to perturbative Rabi gates and applicable to any CDD qubit architecture. We demonstrate single-qubit gates with fidelity =0.9947(1) on a frequency tunable CDD transmon superconducting circuit operated where it is strongly sensitive to longitudinal noise, thus establishing this technique as a potentially useful tool for operating qubits in applications requiring high fidelity under non-ideal conditions.
arxiv pdf

Universal non-adiabatic control of small-gap superconducting qubits

  1. Daniel L. Campbell,
  2. Yun-Pil Shim,
  3. Bharath Kannan,
  4. Roni Winik,
  5. Alexander Melville,
  6. Bethany M. Niedzielski,
  7. Jonilyn L. Yoder,
  8. Charles Tahan,
  9. Simon Gustavsson,
  10. and William D. Oliver
Resonant transverse driving of a two-level system as viewed in the rotating frame couples two degenerate states at the Rabi frequency, an amazing equivalence that emerges in quantum
mechanics. While spectacularly successful at controlling natural and artificial quantum systems, certain limitations may arise (e.g., the achievable gate speed) due to non-idealities like the counter-rotating term. Here, we explore a complementary approach to quantum control based on non-resonant, non-adiabatic driving of a longitudinal parameter in the presence of a fixed transverse coupling. We introduce a superconducting composite qubit (CQB), formed from two capacitively coupled transmon qubits, which features a small avoided crossing — smaller than the environmental temperature — between two energy levels. We control this low-frequency CQB using solely baseband pulses, non-adiabatic transitions, and coherent Landau-Zener interference to achieve fast, high-fidelity, single-qubit operations with Clifford fidelities exceeding 99.7%. We also perform coupled qubit operations between two low-frequency CQBs. This work demonstrates that universal non-adiabatic control of low-frequency qubits is feasible using solely baseband pulses.
arxiv pdf