High harmonic generation as a new source of squeezed quantum light

A team of researchers theoretically proves that the emitted light after a high harmonic generation (HHG) process is not classical, but quantum and squeezed. The study unveils the potential of HHG as a new source of bright entangled and squeezed light, two inherent quantum features with several cutting-edge applications within quantum technologies. High harmonic generation is a highly non-linear phenomenon where a system (for example, an atom) absorbs many photons of an incoming laser and emits a single photon of much higher energy.This process is crucial for attoscience (the science of the ultrafast processes), since it generates attosecond pulses of ultraviolet light, an essential ingredient for many applications within the field. In this regime, HHG experiments can be explained by means of semi-classical theory with great success: matter (the electrons of the atoms) is treated quantum-mechanically, while the incoming light is treated classically. According to this approach, unsurprisingly the emitted light turns out to be classical, something which was in agreement with all previous observations. However, physicists tend to feel uncomfortable when using two different theories (quantum and classical) to describe the same phenomenon. During the last years, the efforts to understand HHG from a full quantum optical perspective have kept growing, but a more general description to show different aspects of the quantum nature of the outgoing radiation remained an elusive milestone. Now, ICFO researchers, led by Prof. ICREA Maciej Lewenstein, have theoretically described high-harmonic generation using just quantum physics and, for the first time, they have found squeezing and entanglement features simultaneously in the emitted light. The study, published in Physical Review Letter, explains why previous classical descriptions were not in disagreement with the observations and, at the same time, unveils a new method to generate quantum optical resources with squeezing and massive entanglement in a new bright frequency regime, two features of current technological interest.