Hans De Raedt

On the fragility of gate-error metrics in simulation models of flux-tunable transmon quantum computers

  1. Hannes Lagemann,
  2. Dennis Willsch,
  3. Madita Willsch,
  4. Fengping Jin,
  5. Hans De Raedt,
  6. and Kristel Michielsen
Constructing a quantum computer requires immensely precise control over a quantum system. A lack of precision is often quantified by gate-error metrics, such as the average infidelity
or the diamond distance. However, usually such gate-error metrics are only considered for individual gates, and not the errors that accumulate over consecutive gates. Furthermore, it is not well known how susceptible the metrics are to the assumptions which make up the model. Here, we investigate these issues using realistic simulation models of quantum computers with flux-tunable transmons and coupling resonators. We show that the gate-error metrics are susceptible to many of the assumptions which make up the model. Additionally, we find that consecutive gate errors do not accumulate linearly. Previous work showed that the gate-error metrics are poor predictors for the performance of consecutive gates. Here, we provide further evidence and a concise theoretical explanation for this finding. Furthermore, we discuss a problem that potentially limits the overall scaling capabilities of the device architecture we study in this work.
arxiv pdf

Real-time simulation of flux qubits used for quantum annealing

  1. Madita Willsch,
  2. Dennis Willsch,
  3. Fengping Jin,
  4. Hans De Raedt,
  5. and Kristel Michielsen
The real-time dynamics of systems with up to three SQUIDs is studied by numerically solving the time-dependent Schrödinger equation. The numerical results are used to scrutinize the
mapping of the flux degrees of freedom onto two-level systems (the qubits) as well as the performance of the intermediate SQUID as a tunable coupling element. It is shown that the two-level representation yields a good description of the flux dynamics during quantum annealing, and the presence of the tunable coupling element does not have negative effects on the overall performance. Additionally, data obtained from a two-level spin dynamics simulation of quantum annealing is compared to experimental data produced by the D-Wave 2000Q quantum annealer. The effects of finite temperature are incorporated in the simulation by coupling the qubit-system to a bath of spin-1/2 particles. It is shown that including an environment modeled as non-interacting two-level systems that couple only to the qubits can produce data which matches the experimental data much better than the simulation data of the isolated qubits, and better than data obtained from a simulation including an environment modeled as interacting two-level systems coupling to the qubits.
arxiv pdf