Option for skipping OptimizeSwapBeforeMeasure when not all the wires are measured.

qiskit/transpiler/passes/optimization/optimize_swap_before_measure.py

@@ -24,7 +24,13 @@ class OptimizeSwapBeforeMeasure(TransformationPass):
 
     Transpiler pass to remove swaps in front of measurements by re-targeting
     the classical bit of the measure instruction.
+
+    If `all_measurement` is `True` (default is `False`)`, the SWAP to be removed
+    has to be measure on both wires. Otherwise, it stays.
     """
+    def __init__(self, all_measurement=False):
+        self.all_measurement = all_measurement
+        super().__init__()
 
     def run(self, dag):
         """Run the OptimizeSwapBeforeMeasure pass on `dag`.
@@ -39,8 +45,10 @@ def run(self, dag):
         for swap in swaps[::-1]:
             final_successor = []
             for successor in dag.successors(swap):
-                final_successor.append(successor.type == 'out' or (successor.type == 'op' and
-                                                                   successor.op.name == 'measure'))
+                is_final_successor = successor.type == 'op' and successor.op.name == 'measure'
+                if not self.all_measurement:
+                    is_final_successor = is_final_successor or successor.type == 'out'
+                final_successor.append(is_final_successor)
             if all(final_successor):
                 # the node swap needs to be removed and, if a measure follows, needs to be adapted
                 swap_qargs = swap.qargs

qiskit/transpiler/preset_passmanagers/level3.py

@@ -177,7 +177,7 @@ def _opt_control(property_set):
 
     _reset = [RemoveResetInZeroState()]
 
-    _meas = [OptimizeSwapBeforeMeasure(), RemoveDiagonalGatesBeforeMeasure()]
+    _meas = [OptimizeSwapBeforeMeasure(all_measurement=True), RemoveDiagonalGatesBeforeMeasure()]
 
     _opt = [
         Collect2qBlocks(),

releasenotes/notes/OptimizeSwapBeforeMeasure_preserves_unitary_when_there_are_no_measurments-4c8a989c4428b600.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes #4911. Level 3 now calls
+    :class:`qiskit.transpiler.passes.optimization.optimize_swap_before_measure.OptimizeSwapBeforeMeasure`
+    with a special parameter to avoid removing SWAP gates when not followed by measurment on both wires.

test/python/transpiler/test_optimize_swap_before_measure.py

@@ -218,6 +218,86 @@ def test_optimize_overlap_swap(self):
 
         self.assertEqual(expected, after)
 
+    def test_all_measurement(self):
+        """OptimizeSwapBeforeMeasure(all_measurement=True) on total measurment
+            qr0:--X-----m--
+                  |     |
+            qr1:--X--m--|--
+                     |  |
+            cr :-----0--1--
+        """
+        qr = QuantumRegister(2, 'qr')
+        cr = ClassicalRegister(2, 'cr')
+        circuit = QuantumCircuit(qr, cr)
+        circuit.swap(qr[0], qr[1])
+        circuit.measure(qr[1], cr[0])
+        circuit.measure(qr[0], cr[1])
+
+        expected = QuantumCircuit(qr, cr)
+        expected.measure(qr[0], cr[0])
+        expected.measure(qr[1], cr[1])
+
+        pass_manager = PassManager()
+        pass_manager.append(
+            [OptimizeSwapBeforeMeasure(all_measurement=True), DAGFixedPoint()],
+            do_while=lambda property_set: not property_set['dag_fixed_point'])
+        after = pass_manager.run(circuit)
+
+        self.assertEqual(expected, after)
+
+    def test_all_measurement_skip(self):
+        """OptimizeSwapBeforeMeasure(all_measurement=True) on no total measurment
+            qr0:--X-----
+                  |
+            qr1:--X--m--
+                     |
+            cr0:-----.--
+        """
+        qr = QuantumRegister(2, 'qr')
+        cr = ClassicalRegister(1, 'cr')
+        circuit = QuantumCircuit(qr, cr)
+        circuit.swap(qr[0], qr[1])
+        circuit.measure(qr[1], cr[0])
+
+        pass_manager = PassManager()
+        pass_manager.append(
+            [OptimizeSwapBeforeMeasure(all_measurement=True), DAGFixedPoint()],
+            do_while=lambda property_set: not property_set['dag_fixed_point'])
+        after = pass_manager.run(circuit)
+
+        self.assertEqual(circuit, after)
+
+    def test_all_measurement_mixed(self):
+        """OptimizeSwapBeforeMeasure(all_measurement=True) on mixed measurment
+            qr0:--X-----------
+                  |
+            qr1:--X--X-----m--
+                     |     |
+            qr2:-----X--m--|--
+                        |  |
+            cr :--------0--1--
+        """
+        qr = QuantumRegister(3, 'qr')
+        cr = ClassicalRegister(2, 'cr')
+        circuit = QuantumCircuit(qr, cr)
+        circuit.swap(qr[0], qr[1])
+        circuit.swap(qr[1], qr[2])
+        circuit.measure(qr[2], cr[0])
+        circuit.measure(qr[1], cr[1])
+
+        expected = QuantumCircuit(qr, cr)
+        expected.swap(qr[0], qr[1])
+        expected.measure(qr[1], cr[0])
+        expected.measure(qr[2], cr[1])
+
+        pass_manager = PassManager()
+        pass_manager.append(
+            [OptimizeSwapBeforeMeasure(all_measurement=True), DAGFixedPoint()],
+            do_while=lambda property_set: not property_set['dag_fixed_point'])
+        after = pass_manager.run(circuit)
+
+        self.assertEqual(expected, after)
+
 
 if __name__ == '__main__':
     unittest.main()