DAGDependency
class qiskit.dagcircuit.DAGDependency
Bases: object
Object to represent a quantum circuit as a Directed Acyclic Graph (DAG) via operation dependencies (i.e. lack of commutation).
The nodes in the graph are operations represented by quantum gates. The edges correspond to non-commutation between two operations (i.e. a dependency). A directed edge from node A to node B means that operation A does not commute with operation B. The object’s methods allow circuits to be constructed.
The nodes in the graph have the following attributes: ‘operation’, ‘successors’, ‘predecessors’.
Example:
Bell circuit with no measurement.
┌───┐
qr_0: ┤ H ├──■──
└───┘┌─┴─┐
qr_1: ─────┤ X ├
└───┘The dependency DAG for the above circuit is represented by two nodes. The first one corresponds to Hadamard gate, the second one to the CNOT gate as the gates do not commute there is an edge between the two nodes.
Reference:
[1] Iten, R., Moyard, R., Metger, T., Sutter, D. and Woerner, S., 2020. Exact and practical pattern matching for quantum circuit optimization. arXiv:1909.05270
Create an empty DAGDependency.
Attributes
calibrations
Return calibration dictionary.
The custom pulse definition of a given gate is of the form {'gate_name': {(qubits, params): schedule}}.
global_phase
Return the global phase of the circuit.
Methods
add_clbits
add_creg
add_op_node
add_op_node(operation, qargs, cargs)
Add a DAGDepNode to the graph and update the edges.
Parameters
- operation (qiskit.circuit.Operation) – operation as a quantum gate
- qargs (list[Qubit]) – list of qubits on which the operation acts
- cargs (list[Clbit]) – list of classical wires to attach to
add_qreg
add_qubits
copy
copy()
Function to copy a DAGDependency object. :returns: a copy of a DAGDependency object. :rtype: DAGDependency
depth
direct_predecessors
direct_predecessors(node_id)
Direct predecessors id of a given node as sorted list.
Parameters
node_id (int) – label of considered node.
Returns
direct predecessors id as a sorted list
Return type
List
direct_successors
direct_successors(node_id)
Direct successors id of a given node as sorted list.
Parameters
node_id (int) – label of considered node.
Returns
direct successors id as a sorted list
Return type
List
draw
draw(scale=0.7, filename=None, style='color')
Draws the DAGDependency graph.
This function needs pydot <https://github.com/erocarrera/pydot>, which in turn needs Graphviz <https://www.graphviz.org/>` to be installed.
Parameters
- scale (float) – scaling factor
- filename (str) – file path to save image to (format inferred from name)
- style (str) – ‘plain’: B&W graph ‘color’ (default): color input/output/op nodes
Returns
if in Jupyter notebook and not saving to file, otherwise None.
Return type
Ipython.display.Image
get_all_edges
get_edges
get_edges(src_id, dest_id)
Edge enumeration between two nodes through method get_all_edge_data.
Parameters
Returns
corresponding to all edges between the two nodes.
Return type
List
get_in_edges
get_in_edges(node_id)
Enumeration of all incoming edges for a given node.
Parameters
node_id (int) – label of considered node.
Returns
corresponding incoming edges data.
Return type
List
get_node
get_node(node_id)
Parameters
node_id (int) – label of considered node.
Returns
corresponding to the label.
Return type
node
get_nodes
get_out_edges
get_out_edges(node_id)
Enumeration of all outgoing edges for a given node.
Parameters
node_id (int) – label of considered node.
Returns
corresponding outgoing edges data.
Return type
List
predecessors
predecessors(node_id)
Predecessors id of a given node as sorted list.
Parameters
node_id (int) – label of considered node.
Returns
all predecessors id as a sorted list
Return type
List
replace_block_with_op
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.
Parameters
- 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 theDAGDependency) and will raise aDAGDependencyErrorif a cycle would be introduced. This checking comes with a run time penalty. If you can guarantee that your inputnode_blockis a contiguous block and won’t introduce a cycle when it’s contracted to a single node, this can be set toFalseto improve the runtime performance of this method.
Raises
DAGDependencyError – if cycle_check is set to True and replacing the specified block introduces a cycle or if node_block is empty.
size
successors
successors(node_id)
Successors id of a given node as sorted list.
Parameters
node_id (int) – label of considered node.
Returns
all successors id as a sorted list
Return type
List
to_retworkx
topological_nodes
topological_nodes()
Yield nodes in topological order.
Returns
node in topological order.
Return type
generator(DAGNode)