Fix to WeightedPauliOperator (#891)

CHANGELOG.md

@@ -48,7 +48,9 @@ Fixed
 
 -   Boolean logic circuit construction (#819)
 -   Measurement on actual devices for Amplitude Estimation algorithms (#841) (#842)
--   Supported constant values on the left-hand side of constraints and variables on the right-hand side of constraints for the DOcplex translator (#750)
+-   Supported constant values on the left-hand side of constraints and variables on the right-hand
+    side of constraints for the DOcplex translator (#750)
+-   WeightedPauliOperator constructor simplification bug (#891)
 
 [0.6.5](https://github.com/Qiskit/qiskit-aqua/compare/0.6.4...0.6.5) - 2020-03-16
 =================================================================================

qiskit/aqua/operators/weighted_pauli_operator.py

@@ -365,9 +365,8 @@ def simplify(self, copy=False):
                         break
             for idx in indices:
                 new_idx = old_to_new_indices[idx]
-                if new_idx is not None:
+                if new_idx is not None and new_idx not in new_indices:
                     new_indices.append(new_idx)
-            new_indices = list(set(new_indices))
             if new_indices and not found:
                 new_basis.append((basis, new_indices))
         op._basis = new_basis

test/aqua/operators/test_weighted_pauli_operator.py

@@ -578,6 +578,39 @@ def test_evolve(self, expansion_mode, evo_time, num_time_slices):
             self.log.debug('The fidelity between matrix and circuit: %s', f_mc)
             self.assertAlmostEqual(f_mc, 1)
 
+    def test_simplification(self):
+        """ Test Hamiltonians produce same result after simplification by constructor """
+        q = QuantumRegister(2, name='q')
+        qc = QuantumCircuit(q)
+        qc.rx(10.9891251356965, 0)
+        qc.rx(6.286692023269373, 1)
+        qc.rz(7.848801398269382, 0)
+        qc.rz(9.42477796076938, 1)
+        qc.cx(0, 1)
+
+        def eval_op(op):
+            from qiskit import execute
+            backend = BasicAer.get_backend('qasm_simulator')
+            evaluation_circuits = op.construct_evaluation_circuit(qc, False)
+            job = execute(evaluation_circuits, backend, shots=1024)
+            return op.evaluate_with_result(job.result(), False)
+
+        pauli_string = [[1.0, Pauli.from_label('XX')],
+                        [-1.0, Pauli.from_label('YY')],
+                        [-1.0, Pauli.from_label('ZZ')]]
+        wpo = WeightedPauliOperator(pauli_string)
+        expectation_value, _ = eval_op(wpo)
+        self.assertAlmostEqual(expectation_value, -3.0, places=2)
+
+        # Half each coefficient value but double up (6 Paulis total)
+        pauli_string = [[0.5, Pauli.from_label('XX')],
+                        [-0.5, Pauli.from_label('YY')],
+                        [-0.5, Pauli.from_label('ZZ')]]
+        pauli_string *= 2
+        wpo2 = WeightedPauliOperator(pauli_string)
+        expectation_value, _ = eval_op(wpo2)
+        self.assertAlmostEqual(expectation_value, -3.0, places=2)
+
 
 if __name__ == '__main__':
     unittest.main()