L.S. Theis

Gap-independent cooling and hybrid quantum-classical annealing

  1. L.S. Theis,
  2. Peter K. Schuhmacher,
  3. M. Marthaler,
  4. and F.K. Wilhelm
In this letter we present an efficient gap-independent cooling scheme for a quantum annealer that benefits from finite temperatures. We choose a system based on superconducting flux
qubits as a prominent example of current quantum annealing platforms. We propose coupling the qubit system transversely to a coplanar waveguide to counter noise and heating that arise from always-present longitudinal thermal noise. We provide a schematic circuit layout for the system and show how, for feasible coupling strengths, we achieve global performance enhancements. Specifically, we achieve cooling improvements of about 50% in the adiabatic and a few hundred percent in the non-adiabatic regime, respectively.
arxiv pdf

Restless Tuneup of High-Fidelity Qubit Gates

  1. M. A. Rol,
  2. C. C. Bultink,
  3. T. E. O'Brien,
  4. S.R. de Jong,
  5. L.S. Theis,
  6. X. Fu,
  7. F. Luthi,
  8. R.F.L. Vermeulen,
  9. J. C. de Sterke,
  10. A. Bruno,
  11. D. Deurloo,
  12. R. N. Schouten,
  13. F.K. Wilhelm,
  14. and L. DiCarlo
We present a tuneup protocol for qubit gates with tenfold speedup over traditional methods reliant on qubit initialization by energy relaxation. This speedup is achieved by constructing
a cost function for Nelder-Mead optimization from real-time correlation of non-demolition measurements interleaving gate operations without pause. Applying the protocol on a transmon qubit achieves 0.999 average Clifford fidelity in one minute, as independently verified using randomized benchmarking and gate set tomography. The adjustable sensitivity of the cost function allows detecting fractional changes in gate error with nearly constant signal-to-noise ratio. The restless concept demonstrated can be readily extended to the tuneup of two-qubit gates and measurement operations.
arxiv pdf