try an ancilla

cirq-core/cirq/sim/simulation_state.py

@@ -173,6 +173,13 @@ def kronecker_product(self: TSelf, other: TSelf, *, inplace=False) -> TSelf:
         args._set_qubits(self.qubits + other.qubits)
         return args
 
+    def add_qubits(self: TSelf, qubits: Sequence['cirq.Qid']) -> TSelf:
+        new_space = type(self)(qubits=qubits)  # type: ignore
+        return self.kronecker_product(new_space, inplace=True)
+
+    def remove_qubits(self: TSelf, qubits: Sequence['cirq.Qid']) -> TSelf:
+        return self.factor(qubits, inplace=True)[1]
+
     def factor(
         self: TSelf, qubits: Sequence['cirq.Qid'], *, validate=True, atol=1e-07, inplace=False
     ) -> Tuple[TSelf, TSelf]:
@@ -298,11 +305,16 @@ def strat_act_on_from_apply_decompose(
     operations, qubits1, _ = _try_decompose_into_operations_and_qubits(val)
     assert len(qubits1) == len(qubits)
     qubit_map = {q: qubits[i] for i, q in enumerate(qubits1)}
+    ancillas = list(set([q for op in operations for q in op.qubits if q not in qubits1]))
+    for q in ancillas:
+        qubit_map[q] = q
+    args.add_qubits(ancillas)
     if operations is None:
         return NotImplemented
     for operation in operations:
         operation = operation.with_qubits(*[qubit_map[q] for q in operation.qubits])
         protocols.act_on(operation, args)
+    args.remove_qubits(ancillas)
     return True
 
 

cirq-core/cirq/sim/simulation_state_test.py

@@ -101,3 +101,31 @@ def test_field_getters():
     args = DummySimulationState()
     assert args.prng is np.random
     assert args.qubit_map == {q: i for i, q in enumerate(cirq.LineQubit.range(2))}
+
+
+@pytest.mark.parametrize('exp', [-3, -2, -1, 0, 1, 2, 3])
+def test_ancilla(exp):
+    class AncillaX(cirq.Gate):
+        def __init__(self, exponent=1):
+            self._exponent = exponent
+
+        def num_qubits(self) -> int:
+            return 1
+
+        def _decompose_(self, qubits):
+            ancilla = cirq.NamedQubit('Ancilla')
+            yield cirq.X(ancilla) ** self._exponent
+            yield cirq.CX(ancilla, qubits[0])
+            yield cirq.X(ancilla) ** -self._exponent
+
+    q = cirq.LineQubit(0)
+    test_circuit = cirq.Circuit(AncillaX(exp).on(q))
+    control_circuit = cirq.Circuit(cirq.XPowGate(exponent=exp).on(q))
+
+    test_sv = cirq.final_state_vector(test_circuit)
+    control_sv = cirq.final_state_vector(control_circuit)
+    assert np.allclose(test_sv, control_sv)
+
+    test_dm = cirq.final_density_matrix(test_circuit)
+    control_dm = cirq.final_density_matrix(control_circuit)
+    assert np.allclose(test_dm, control_dm)