Flux-Bias Offsets

Description

In an optimal QPU calibration, annealing an unbiased chain produces spin-state statistics that are equally split between spin-up and spin-down. When plotted against the \(h\) values, this even distribution results in a sigmoid curve that passes through the point of origin (0,0); see Figure 90. However, qubits in a chain with strong negative \(J\) values experience a \(J\)-induced bias—an offset magnetic field that is potentially \(s\)-dependent. This field shifts the sigmoid curve of plotted \(h\) values from its ideal path. To compensate, chains using strong negative \(J\) couplings must be recalibrated to remove the bias from the chain and reduce the incidence of analog errors that may be associated with minor-embedding.

Graph showing ideal and offset h sigmoid

Recalibration involves applying per-qubit flux-bias offsets to nudge the plotted \(h\) sigmoid to its ideal position. The optimal offset value for a chain depends on the qubits and couplers involved and on the chain coupling strength.

[1]For information on the measured performance improvements resulting from this feature, see Virtual Graphs for High-Performance Embedded Topologies, D-Wave White Paper Series, no. 14-1020A, 2017. This and other white papers are available from https://www.dwavesys.com/resources/publications.

Release Introduced

Solver API Release 3.0, January 2018

Properties and Parameters

User parameter:

  • flux_biases—Parameter that takes a list of flux-bias offset values with which to calibrate a chain.

Solver properties:

  • extended_j_range—Property that shows the extended range of values possible for the coupling strengths (quadratic coefficients), \(J\), for the solver.
  • per_qubit_coupling_range—Property that identifies the limits on the total coupling strength possible for a qubit.

See Also