Pass to remove diagonal gates before measurement

CHANGELOG.rst

@@ -85,6 +85,8 @@ Added
   wires instead of named bits.
 - Added a ``OptimizeSwapBeforeMeasure`` pass that removes the swap gates when they
   are followed by a measurement instruction, moving the latter to the proper wire. (#1890)
+- Added a ``RemoveDiagonalGatesBeforeMeasure`` pass that removes the diagonal gates when they
+  are followed by a measurement instruction. (#2208)
 - Added a ``CommutativeCancellation`` pass that cancels self-inverse gates and combines
   rotations about the Z axis, leveraging previously-found gate commutation relations. (#2012)
 - Added a ``Collect2qBlocks`` pass that analyzes the circuit for uninterrupted sequences

qiskit/dagcircuit/dagcircuit.py

@@ -1089,6 +1089,17 @@ def predecessors(self, node):
 
         return self._multi_graph.predecessors(node)
 
+    def quantum_predecessors(self, node):
+        """Returns list of the predecessors of a node that are
+        connected by a quantum edge as DAGNodes."""
+
+        predecessors = []
+        for predecessor in self.predecessors(node):
+            if isinstance(self._multi_graph.get_edge_data(predecessor, node, key=0)['wire'][0],
+                          QuantumRegister):
+                predecessors.append(predecessor)
+        return predecessors
+
     def ancestors(self, node):
         """Returns set of the ancestors of a node as DAGNodes."""
         if isinstance(node, int):

qiskit/transpiler/passes/__init__.py

@@ -37,5 +37,6 @@
 from .mapping.noise_adaptive_layout import NoiseAdaptiveLayout
 from .mapping.basic_swap import BasicSwap
 from .mapping.lookahead_swap import LookaheadSwap
+from .remove_diagonal_gates_before_measure import RemoveDiagonalGatesBeforeMeasure
 from .mapping.stochastic_swap import StochasticSwap
 from .mapping.legacy_swap import LegacySwap

qiskit/transpiler/passes/remove_diagonal_gates_before_measure.py

@@ -0,0 +1,44 @@
+# -*- coding: utf-8 -*-
+
+# Copyright 2019, IBM.
+#
+# This source code is licensed under the Apache License, Version 2.0 found in
+# the LICENSE.txt file in the root directory of this source tree.
+
+
+"""
+Transpiler pass to remove diagonal gates (like RZ, T, Z, etc) before
+a measurement. Including diagonal 2Q gates.
+"""
+
+from qiskit.circuit import Measure
+from qiskit.extensions.standard import RZGate, ZGate, TGate, SGate, TdgGate, SdgGate, U1Gate,\
+    CzGate, CrzGate, Cu1Gate, RZZGate
+from qiskit.transpiler.basepasses import TransformationPass
+
+
+class RemoveDiagonalGatesBeforeMeasure(TransformationPass):
+    """Remove diagonal gates (like RZ, T, Z, etc) before a measurement.
+    Including diagonal 2Q gates."""
+
+    def run(self, dag):
+        """Return a new circuit that has been optimized."""
+        diagonal_1q_gates = (RZGate, ZGate, TGate, SGate, TdgGate, SdgGate, U1Gate)
+        diagonal_2q_gates = (CzGate, CrzGate, Cu1Gate, RZZGate)
+
+        nodes_to_remove = set()
+        for measure in dag.op_nodes(Measure):
+            predecessor = dag.quantum_predecessors(measure)[0]
+
+            if predecessor.type == 'op' and isinstance(predecessor.op, diagonal_1q_gates):
+                nodes_to_remove.add(predecessor)
+
+            if predecessor.type == 'op' and isinstance(predecessor.op, diagonal_2q_gates):
+                successors = dag.quantum_successors(predecessor)
+                if all([s.type == 'op' and isinstance(s.op, Measure) for s in successors]):
+                    nodes_to_remove.add(predecessor)
+
+        for node_to_remove in nodes_to_remove:
+            dag.remove_op_node(node_to_remove)
+
+        return dag

test/python/test_dagcircuit.py

@@ -168,6 +168,24 @@ def test_quantum_successors(self):
         self.assertEqual(successor_cnot[0].type, 'out')
         self.assertIsInstance(successor_cnot[1].op, Reset)
 
+    def test_quantum_predecessors(self):
+        """The method dag.quantum_predecessors() returns predecessors connected by quantum edges"""
+        self.dag.apply_operation_back(Reset(), [self.qubit0], [])
+        self.dag.apply_operation_back(CnotGate(), [self.qubit0, self.qubit1], [])
+        self.dag.apply_operation_back(Measure(), [self.qubit1, self.clbit1], [])
+
+        predecessor_measure = self.dag.quantum_predecessors(
+            self.dag.named_nodes('measure').pop())
+        self.assertEqual(len(predecessor_measure), 1)
+        cnot_node = predecessor_measure[0]
+
+        self.assertIsInstance(cnot_node.op, CnotGate)
+
+        predecessor_cnot = self.dag.quantum_predecessors(cnot_node)
+        self.assertEqual(len(predecessor_cnot), 2)
+        self.assertEqual(predecessor_cnot[1].type, 'in')
+        self.assertIsInstance(predecessor_cnot[0].op, Reset)
+
     def test_get_gates_nodes(self):
         """The method dag.gate_nodes() returns all gate nodes"""
         self.dag.apply_operation_back(HGate(), [self.qubit0], [])