setup_budget

setup_budget(*, max_error_per_slice=None, max_error_total=None, num_slices=None, p_norm=1)[source]

Calculate the budget available to each slice for observable term truncation.

This method makes the construction of a TruncationErrorBudget easier for an end-user. This error budget can be provided to the backpropagate() method to enable the truncation of low-weight Pauli terms. Refer to the how-to guide for a detailed discussion on truncating terms from the output operator and bounding the incurred error.

The construction logic is as follows:

  • if max_error_per_slice is provided its value is converted to a list and used immediately for TruncationErrorBudget.per_slice_budget

  • if the above is not the case, max_error_total has to be set

  • if num_slices is not set,:attr:.TruncationErrorBudget.per_slice_budget gets set to [max_error_total] resulting in the entire budget being consumed greedily

  • however, if num_slices is provided, then TruncationErrorBudget.per_slice_budget assumes an even distribution of the maximum total error across the specified number of slices: [max_error_total / num_slices]

Finally, if max_error_total is set, this puts a hard limit on the total maximum error to be accumulated throughout the entire backpropagation. Thus, setting max_error_per_slice and max_error_total can be of value.

Note

Budget not spent during a previous iteration will carry over into subsequent iterations, meaning that the maximum budget for any slice during backpropagation may in fact exceed TruncationErrorBudget.per_slice_budget.

Parameters:

  • max_error_per_slice (float | Sequence[float] | None) – Specifies the maximum error per backpropagated slice. See above for more details.

  • max_error_total (float | None) – Specifies the total maximum error for the entire backpropagation. See above for more details.

  • num_slices (int | None) – The number of slices over which to distribute the budget. See above for more details.

  • p_norm (int) – The Lp norm of the error. This affects the gradual distribution of max_error_total in the case of num_slices also being set (see above). Refer to the how-to guide for a detailed conversation on bounding truncation error using higher Lp-norms.

Returns:

The resulting TruncationErrorBudget.

Raises:

ValueError – if max_error_per_slice and max_error_total are both None.

Return type:

TruncationErrorBudget