Quantum-limited frequency fluctuations in a terahertz laser

Quantum cascade lasers can be considered the primary achievement of electronic band structure engineering, showing how artificial materials can be created through quantum design to have tailor-made properties that are otherwise non-existent in nature. Indeed, quantum cascade lasers can be used as powerful testing grounds of the fundamental physical parameters determined by their quantum nature, including the intrinsic linewidth of laser emission, which in such lasers is significantly affected by the optical and thermal photon number generated in the laser cavity. Here, we report experimental evidence of linewidth values approaching the quantum limit in far-infrared quantum cascade lasers. Despite the broadening induced by thermal photons, the measured linewidth results narrower than that found in any other semiconductor laser to date. By performing noise measurements with unprecedented sensitivity levels, we highlight the key role of gain medium engineering and demonstrate that properly designed semiconductor-heterostructure lasers can unveil the mechanisms underlying the laser-intrinsic phase noise, revealing the link between device properties and the quantum-limited linewidth.