def do_gate_matrix(self, matrix: np.ndarray,
                       qubits: Sequence[int]) -> 'AbstractQuantumSimulator':
        """
        Apply an arbitrary unitary; not necessarily a named gate.

        :param matrix: The unitary matrix to apply. No checks are done
        :param qubits: A list of qubits to apply the unitary to.
        :return: ``self`` to support method chaining.
        """
        unitary = lifted_gate_matrix(matrix=matrix, qubit_inds=qubits, n_qubits=self.n_qubits)
        self.density = unitary.dot(self.density).dot(np.conj(unitary).T)
        return self