A team from the Autonomous University of Madrid has developed a proposal for a tunable light source that emits individual photons in the terahertz (THz) frequency regime. This frequency regime is crucial for accessing vibrational and rotational level transition frequencies in molecules, as well as transitions of individual and collective excitations in semiconductor materials. This opens up unique spectroscopy and microscopy applications in multiple areas, ranging from food science, medical diagnostics, and biology to broadband communication or security. However, THz quantum technology is in a much more nascent stage than its counterparts in the visible, near-infrared, or microwave regions. In the visible spectrum, it has been demonstrated that quantum light offers significant technological advantages, such as metrological precision at the Heisenberg limit, alternative paradigms of quantum computing, or protection against espionage in remote communications. Through the development of THz quantum technology, these advances could be transferred and exploited in areas where THz radiation is highly relevant. Furthermore, it would offer a compromise between the microwave regime, which requires cooling to millikelvin temperatures and involves significant scalability challenges, and the optical regime, where materials are highly absorbent and require nanometric precision in manufacturing. In this work, published in the journal PRX Quantum, a proposal for quantum light in the THz range has been designed. The design is based on a single polar quantum emitter, with a transition energy in the visible spectrum and a permanent dipolar moment. The combination of these two characteristics allows for the generation, through excitation with an intense optical laser, of hybrid states of light and matter that exhibit permitted transitions in the THz range. These transitions result in the emission of individual photons, which are captured with high efficiency using plasmonic cavities in the THz range. Crucially, these transitions are controllable by laser excitation, allowing for the adjustment of both the intensity of the light source in the THz range and its quantum fluctuations. This result represents the first proposal for a source of individual photons in the THz range compatible with experimentally accessible parameters in present-day laboratories. This represents a step forward in the development of future quantum technologies that exploit the unique advantages of the terahertz frequency regime. [Full article]
