Add MatrixGate.with_name method. (#5941)

Fixes #5938

Author: maffoo

cirq-core/cirq/ops/matrix_gates.py

@@ -55,6 +55,7 @@ def __init__(
         *,
         name: str = None,
         qid_shape: Optional[Iterable[int]] = None,
+        unitary_check: bool = True,
         unitary_check_rtol: float = 1e-5,
         unitary_check_atol: float = 1e-8,
     ) -> None:
@@ -66,6 +67,10 @@ def __init__(
             qid_shape: The shape of state tensor that the matrix applies to.
                 If not specified, this value is inferred by assuming that the
                 matrix is supposed to apply to qubits.
+            unitary_check: If True, check that the supplied matrix is unitary up to the
+                given tolerances. This should only be disabled if the matrix has already been
+                checked for unitarity, in which case we get a slight performance improvement by
+                not checking again.
             unitary_check_rtol: The relative tolerance for checking whether the supplied matrix
                 is unitary. See `cirq.is_unitary`.
             unitary_check_atol: The absolute tolerance for checking whether the supplied matrix
@@ -99,8 +104,14 @@ def __init__(
                 f'qid_shape: {self._qid_shape}\n'
             )
 
-        if not linalg.is_unitary(matrix, rtol=unitary_check_rtol, atol=unitary_check_atol):
-            raise ValueError(f'Not a unitary matrix: {self._matrix}')
+        if unitary_check and not linalg.is_unitary(
+            matrix, rtol=unitary_check_rtol, atol=unitary_check_atol
+        ):
+            raise ValueError(f'Not a unitary matrix: {matrix}')
+
+    def with_name(self, name: str) -> 'MatrixGate':
+        """Creates a new MatrixGate with the same matrix and a new name."""
+        return MatrixGate(self._matrix, name=name, qid_shape=self._qid_shape, unitary_check=False)
 
     def _json_dict_(self) -> Dict[str, Any]:
         return {'matrix': self._matrix.tolist(), 'qid_shape': self._qid_shape}

cirq-core/cirq/ops/matrix_gates_test.py

@@ -271,6 +271,16 @@ def test_named_two_qubit_diagram():
     assert expected_vertical == c.to_text_diagram(transpose=True).strip()
 
 
+def test_with_name():
+    gate = cirq.MatrixGate(cirq.unitary(cirq.Z**0.25))
+    T = gate.with_name('T')
+    S = (T**2).with_name('S')
+    assert T._name == 'T'
+    np.testing.assert_allclose(cirq.unitary(T), cirq.unitary(gate))
+    assert S._name == 'S'
+    np.testing.assert_allclose(cirq.unitary(S), cirq.unitary(T**2))
+
+
 def test_str_executes():
     assert '1' in str(cirq.MatrixGate(np.eye(2)))
     assert '0' in str(cirq.MatrixGate(np.eye(4)))