qiskit/releasenotes/notes/0.25/add-dag-causal-cone-5a19311...

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---
features:
- |
Added :meth:`.DAGCircuit.quantum_causal_cone` to obtain the causal cone of a qubit
in a :class:`~.DAGCircuit`.
The following example shows its correct usage::
from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister
from qiskit.circuit.library import CXGate, CZGate
from qiskit.dagcircuit import DAGCircuit
# Build a DAGCircuit
dag = DAGCircuit()
qreg = QuantumRegister(5)
creg = ClassicalRegister(5)
dag.add_qreg(qreg)
dag.add_creg(creg)
dag.apply_operation_back(CXGate(), qreg[[1, 2]], [])
dag.apply_operation_back(CXGate(), qreg[[0, 3]], [])
dag.apply_operation_back(CZGate(), qreg[[1, 4]], [])
dag.apply_operation_back(CZGate(), qreg[[2, 4]], [])
dag.apply_operation_back(CXGate(), qreg[[3, 4]], [])
# Get the causal cone of qubit at index 0
result = dag.quantum_causal_cone(qreg[0])