VF2PostLayout

class qiskit.transpiler.passes.VF2PostLayout(*args, **kwargs)

Bases: AnalysisPass

A pass for choosing a Layout after transpilation of a circuit onto a Coupling graph, as a subgraph isomorphism problem, solved by VF2++.

Unlike the VF2Layout transpiler pass which is designed to find an initial layout for a circuit early in the transpilation pipeline this transpiler pass is designed to try and find a better layout after transpilation is complete. The initial layout phase of the transpiler doesn’t have as much information available as we do after transpilation. This pass is designed to be paired in a similar pipeline as the layout passes. This pass will strip any idle wires from the circuit, use VF2 to find a subgraph in the coupling graph for the circuit to run on with better fidelity and then update the circuit layout to use the new qubits. The algorithm used in this pass is described in arXiv:2209.15512.

If a solution is found that means there is a lower error layout available for the circuit. If a solution is found the layout will be set in the property set as property_set['post_layout']. However, if no solution is found, no property_set['post_layout'] is set. The stopping reason is set in property_set['VF2PostLayout_stop_reason'] in all the cases and will be one of the values enumerated in VF2PostLayoutStopReason which has the following values:

• "solution found": If a solution was found.
• "nonexistent solution": If no solution was found.
• ">2q gates in basis": If VF2PostLayout can’t work with basis

By default this pass will construct a heuristic scoring map based on the the error rates in the provided target (or properties if target is not provided). However, analysis passes can be run prior to this pass and set vf2_avg_error_map in the property set with a ErrorMap instance. If a value is NaN that is treated as an ideal edge For example if an error map is created as:

from qiskit.transpiler.passes.layout.vf2_utils import ErrorMap
 
error_map = ErrorMap(3)
error_map.add_error((0, 0), 0.0024)
error_map.add_error((0, 1), 0.01)
error_map.add_error((1, 1), 0.0032)

that represents the error map for a 2 qubit target, where the avg 1q error rate is 0.0024 on qubit 0 and 0.0032 on qubit 1. Then the avg 2q error rate for gates that operate on (0, 1) is 0.01 and (1, 0) is not supported by the target. This will be used for scoring if it’s set as the vf2_avg_error_map key in the property set when VF2PostLayout is run.

Initialize a VF2PostLayout pass instance

Parameters

• target (Target) – A target representing the backend device to run VF2PostLayout on. If specified it will supersede a set value for properties and coupling_map.
• coupling_map (CouplingMap) – Directed graph representing a coupling map.
• properties (BackendProperties) – The backend properties for the backend. If readout_error() is available it is used to score the layout.
• seed (int) – Sets the seed of the PRNG. -1 Means no node shuffling.
• call_limit (int) – The number of state visits to attempt in each execution of VF2.
• time_limit (float) – The total time limit in seconds to run VF2PostLayout
• strict_direction (bool) – Whether the pass is configured to follow the strict direction in the coupling graph. If this is set to false, the pass will treat any edge in the coupling graph as a weak edge and the interaction graph will be undirected. For the purposes of evaluating layouts the avg error rate for each qubit and 2q link will be used. This enables the pass to be run prior to basis translation and work with any 1q and 2q operations. However, if strict_direction=True the pass expects the input DAGCircuit object to run() to be in the target set of instructions.
• max_trials (int) – The maximum number of trials to run VF2 to find a layout. A value of 0 (the default) means ‘unlimited’.

Raises

TypeError – At runtime, if neither coupling_map or target are provided.

Attributes

is_analysis_pass

is_transformation_pass

Methods

name

run