fixed docstrings

qiskit/transpiler/passes/optimization/dynamic/replace_swap_with_zero_state.py

@@ -0,0 +1,63 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2017, 2019.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Replace Swap gate in case of qubit in zero state with dynamic protocol."""
+
+from qiskit.circuit import ClassicalRegister, QuantumCircuit, QuantumRegister, Reset
+from qiskit.circuit.library import CXGate, HGate, Measure, SwapGate
+from qiskit.circuit.controlflow.if_else import IfElseOp
+from qiskit.dagcircuit import DAGCircuit, DAGInNode
+from qiskit.transpiler.basepasses import TransformationPass
+
+
+class ReplaceSwapWithZeroState(TransformationPass):
+    """Replace Swap gate in case of qubit in zero state with
+    dynamic protocol."""
+
+    def run(self, dag):
+        """Run the ReplaceSwapWithZeroState pass on `dag`.
+
+        Args:
+            dag (DAGCircuit): the DAG to be optimized.
+
+        Returns:
+            DAGCircuit: the optimized DAG.
+        """
+        swaps = dag.op_nodes(SwapGate)
+        for swap in swaps:
+            predecessor = next(dag.predecessors(swap))
+            if isinstance(predecessor, DAGInNode) or isinstance(predecessor, Reset):
+                zero_qubit = predecessor.wire.index
+                for qarg in swap.qargs:
+                    if qarg.index != zero_qubit:
+                        data_qubit = qarg.index
+
+                mini_dag = DAGCircuit()
+                qreg = QuantumRegister(2)
+                creg = ClassicalRegister(1)
+                mini_dag.add_qreg(qreg)
+                mini_dag.add_creg(creg)
+
+                mini_dag.apply_operation_back(CXGate(), [qreg[1], qreg[0]])
+                mini_dag.apply_operation_back(HGate(), [qreg[1]])
+                mini_dag.apply_operation_back(Measure(), [qreg[1]], [creg[0]])
+
+                true_body = QuantumCircuit(qreg)
+                true_body.z(0)
+                true_body.x(1)
+
+                mini_dag.apply_operation_back(
+                    IfElseOp((creg[0], 1), true_body), [qreg[0], qreg[1]], [creg[0]]
+                )
+                dag.substitute_node_with_dag(swap, mini_dag, wires=[qreg[0], qreg[1], creg[0]])
+
+        return dag