- DAGDependency.replace_block_with_op(node_block, op, wire_pos_map, cycle_check=True)¶
Replace a block of nodes with a single node.
This is used to consolidate a block of DAGDepNodes into a single operation. A typical example is a block of CX and SWAP gates consolidated into a LinearFunction. This function is an adaptation of a similar function from DAGCircuit.
It is important that such consolidation preserves commutativity assumptions present in DAGDependency. As an example, suppose that every node in a block [A, B, C, D] commutes with another node E. Let F be the consolidated node, F = A o B o C o D. Then F also commutes with E, and thus the result of replacing [A, B, C, D] by F results in a valid DAGDependency. That is, any deduction about commutativity in consolidated DAGDependency is correct. On the other hand, suppose that at least one of the nodes, say B, does not commute with E. Then the consolidated DAGDependency would imply that F does not commute with E. Even though F and E may actually commute, it is still safe to assume that they do not. That is, the current implementation of consolidation may lead to suboptimal but not to incorrect results.
node_block (List[DAGDepNode]) – A list of dag nodes that represents the node block to be replaced
op (qiskit.circuit.Operation) – The operation to replace the block with
wire_pos_map (Dict[Qubit, int]) – The dictionary mapping the qarg to the position. This is necessary to reconstruct the qarg order over multiple gates in the combined single op node.
cycle_check (bool) – When set to True this method will check that replacing the provided
node_blockwith a single node would introduce a cycle (which would invalidate the
DAGDependency) and will raise a
DAGDependencyErrorif a cycle would be introduced. This checking comes with a run time penalty. If you can guarantee that your input
node_blockis a contiguous block and won’t introduce a cycle when it’s contracted to a single node, this can be set to
Falseto improve the runtime performance of this method.
DAGDependencyError – if
cycle_checkis set to
Trueand replacing the specified block introduces a cycle or if