Accumulates operations affecting the given pair of qubits.

        The scan terminates when it hits the end of the circuit, finds an
        operation without a known matrix, or finds an operation that interacts
        the given qubits with other qubits.

        Args:
            circuit: The circuit to scan for operations.
            index: The index to start scanning forward from.
            qubits: The pair of qubits we care about.

        Returns:
            A tuple containing:
                0. The operations.
                1. The moment indices those operations were on.
                2. A matrix equivalent to the effect of the scanned operations.

def _scan_two_qubit_ops_into_matrix(
            self,
            circuit: circuits.Circuit,
            index: Optional[int],
            qubits: Tuple[ops.Qid, ...]
    ) -> Tuple[List[ops.Operation], List[int], np.ndarray]:
        """Accumulates operations affecting the given pair of qubits.

        The scan terminates when it hits the end of the circuit, finds an
        operation without a known matrix, or finds an operation that interacts
        the given qubits with other qubits.

        Args:
            circuit: The circuit to scan for operations.
            index: The index to start scanning forward from.
            qubits: The pair of qubits we care about.

        Returns:
            A tuple containing:
                0. The operations.
                1. The moment indices those operations were on.
                2. A matrix equivalent to the effect of the scanned operations.
        """

        product = np.eye(4, dtype=np.complex128)
        all_operations = []
        touched_indices = []

        while index is not None:
            operations = list({circuit.operation_at(q, index) for q in qubits})
            op_data = [
                self._op_to_matrix(op, qubits)
                for op in operations
                if op is not None
            ]

            # Stop at any non-constant or non-local interaction.
            if any(e is None for e in op_data):
                break
            present_ops = [op for op in operations if op]
            present_op_data = cast(List[np.ndarray], op_data)

            for op_mat in present_op_data:
                product = np.dot(op_mat, product)
            all_operations.extend(present_ops)

            touched_indices.append(index)
            index = circuit.next_moment_operating_on(qubits, index + 1)

        return all_operations, touched_indices, product