%0 Journal Article %J Nature Communications %D 2020 %T Mode-locked short pulses from an 8 um wavelength semiconductor laser %A Hillbrand, Johannes %A Opačak, Nikola %A Piccardo, Marco %A Schneider, Harald %A Gottfried Strasser %A Federico Capasso %A Schwarz, Benedikt %X Quantum cascade lasers (QCL) have revolutionized the generation of mid-infrared light. Yet, the ultrafast carrier transport in mid-infrared QCLs has so far constituted a seemingly insurmountable obstacle for the formation of ultrashort light pulses. Here, we demonstrate that careful quantum design of the gain medium and control over the intermode beat synchronization enable transform-limited picosecond pulses from QCL frequency combs. Both an interferometric radio-frequency technique and second-order autocorrelation shed light on the pulse dynamics and confirm that mode-locked operation is achieved from threshold to rollover current. Furthermore, we show that both anti-phase and in-phase synchronized states exist in QCLs. Being electrically pumped and compact, mode-locked QCLs pave the way towards monolithically integrated non-linear photonics in the molecular fingerprint region beyond 6 $μ$m wavelength. %B Nature Communications %V 11 %P 5788 %8 dec %@ 4146702019592 %G eng %U http://www.nature.com/articles/s41467-020-19592-1 %N 1 %R 10.1038/s41467-020-19592-1