def multiply(self, other):
        """Return the QuantumChannel self + other.

        Args:
            other (complex): a complex number.

        Returns:
            SuperOp: the scalar multiplication other * self as a SuperOp object.

        Raises:
            QiskitError: if other is not a valid scalar.
        """
        if not isinstance(other, Number):
            raise QiskitError("other is not a number")
        return SuperOp(other * self._data, self.input_dims(),
                       self.output_dims())