Fix flipping of gate direction in transpiler for CZ gate

qiskit/transpiler/passes/utils/gate_direction.py

@@ -20,7 +20,7 @@
 
 from qiskit.circuit import QuantumRegister
 from qiskit.dagcircuit import DAGCircuit
-from qiskit.circuit.library.standard_gates import RYGate, HGate, CXGate, ECRGate, RZXGate
+from qiskit.circuit.library.standard_gates import RYGate, HGate, CXGate, CZGate, ECRGate, RZXGate
 
 
 class GateDirection(TransformationPass):
@@ -78,6 +78,11 @@ def __init__(self, coupling_map, target=None):
         self._ecr_dag.apply_operation_back(HGate(), [qr[0]], [])
         self._ecr_dag.apply_operation_back(HGate(), [qr[1]], [])
 
+        self._cz_dag = DAGCircuit()
+        qr = QuantumRegister(2)
+        self._cz_dag.add_qreg(qr)
+        self._cz_dag.apply_operation_back(CZGate(), [qr[1], qr[0]], [])
+
     @staticmethod
     def _rzx_dag(parameter):
         _rzx_dag = DAGCircuit()
@@ -138,6 +143,8 @@ def run(self, dag):
                 if (physical_q0, physical_q1) not in cmap_edges:
                     if node.name == "cx":
                         dag.substitute_node_with_dag(node, self._cx_dag)
+                    elif node.name == "cz":
+                        dag.substitute_node_with_dag(node, self._cz_dag)
                     elif node.name == "ecr":
                         dag.substitute_node_with_dag(node, self._ecr_dag)
                     elif node.name == "rzx":
@@ -169,6 +176,16 @@ def run(self, dag):
                             "The circuit requires a connection between physical "
                             "qubits %s and %s for cx" % (physical_q0, physical_q1)
                         )
+                elif node.name == "cz":
+                    if (physical_q0, physical_q1) in self.target["cz"]:
+                        continue
+                    if (physical_q1, physical_q0) in self.target["cz"]:
+                        dag.substitute_node_with_dag(node, self._cz_dag)
+                    else:
+                        raise TranspilerError(
+                            "The circuit requires a connection between physical "
+                            "qubits %s and %s for cz" % (physical_q0, physical_q1)
+                        )
                 elif node.name == "ecr":
                     if (physical_q0, physical_q1) in self.target["ecr"]:
                         continue

releasenotes/notes/fix-flipping-cz-gate-fd08305ca12d9a79.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fix a problem in the `GateDirection` transpiler pass for the CZ gate. The CZ is symmetric, so flipping the qubit arguments is allowed to match the directed coupling map.
+

test/python/transpiler/test_gate_direction.py

@@ -15,6 +15,7 @@
 from math import pi
 
 from qiskit import ClassicalRegister, QuantumRegister, QuantumCircuit
+from qiskit.compiler import transpile
 from qiskit.transpiler import TranspilerError
 from qiskit.transpiler import CouplingMap
 from qiskit.transpiler.passes import GateDirection
@@ -245,6 +246,21 @@ def test_preserves_conditions(self):
 
         self.assertEqual(circuit_to_dag(expected), after)
 
+    def test_regression_gh_8387(self):
+        """Regression test for flipping of CZ gate"""
+        qc = QuantumCircuit(3)
+        qc.cz(1, 0)
+        qc.barrier()
+        qc.cz(2, 0)
+
+        coupling_map = CouplingMap([[0, 1], [1, 2]])
+        _ = transpile(
+            qc,
+            basis_gates=["cz", "cx", "u3", "u2", "u1"],
+            coupling_map=coupling_map,
+            optimization_level=2,
+        )
+
 
 if __name__ == "__main__":
     unittest.main()