Fix oracle_evaluation for PhaseOracle

qiskit/circuit/library/phase_oracle.py

@@ -79,12 +79,15 @@ def synthesizer(boolean_expression):
     def evaluate_bitstring(self, bitstring: str) -> bool:
         """Evaluate the oracle on a bitstring.
         This evaluation is done classically without any quantum circuit.
+
         Args:
-            bitstring: The bitstring for which to evaluate.
+            bitstring: The bitstring for which to evaluate. The input bitstring is expected to be
+                in little-endian order.
+
         Returns:
             True if the bitstring is a good state, False otherwise.
         """
-        return self.boolean_expression.simulate(bitstring)
+        return self.boolean_expression.simulate(bitstring[::-1])
 
     @classmethod
     def from_dimacs_file(cls, filename: str):

releasenotes/notes/fix_phaseoracle_evaluate-52be05221d2761aa.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fix a bug where the bit-order in
+    :meth:`qiskit.circuit.library.PhaseOracle.evaluate_bitstring` did not agree with the 
+    order of the measured bitstring. This fix also affects the execution of Grover's algorithm
+    if the oracle is specified as :class:`~qiskit.circuit.library.PhaseOracle`, which now
+    correctly identifies the correct bitstring.

test/python/algorithms/test_grover.py

@@ -243,6 +243,15 @@ def test_max_probability(self):
         result = grover.amplify(problem)
         self.assertEqual(result.max_probability, 1.0)
 
+    def test_oracle_evaluation(self):
+        """Test oracle_evaluation for PhaseOracle"""
+        oracle = PhaseOracle("x1 & x2 & (not x3)")
+        problem = AmplificationProblem(oracle, is_good_state=oracle.evaluate_bitstring)
+        grover = Grover(quantum_instance=self.qasm)
+        result = grover.amplify(problem)
+        self.assertTrue(result.oracle_evaluation)
+        self.assertEqual("011", result.top_measurement)
+
 
 if __name__ == "__main__":
     unittest.main()