Direct measurement of an ultrafast sub-bandwidth-limited signal in mid-infrared quantum cascade lasers Barbara Schneider, Alexander Dikopoldsev, Philipp Täschler, Filippos Kapsalidis, Mattias Beck and Jérôme Faist EPJ Web Conf., 287 (2023) 07027 Published online: 18 October 2023 DOI: 10.1051/epjconf/202328707027