They demonstrate that the generation of high harmonics produces quantum light

High harmonic generation (HHG) is a highly nonlinear phenomenon in which a system (e.g., an atom) absorbs many photons from a laser and emits photons of much higher energy, whose frequency is a harmonic (i.e., a multiple) of the frequency at which the incoming laser vibrates. Historically, the theoretical description of this process has been approached from a semiclassical perspective, which treats matter (the electrons of atoms) quantumly, but incident light classically. According to this approach, the emitted photons should also behave classically.

Despite this obvious theoretical discrepancy, the description was sufficient to carry out most experiments, and there was apparently no need to change the theoretical framework. Only in recent years has the scientific community begun to explore the possibility that emitted light actually exhibits quantum behavior, something that semiclassical theory might have missed. Several theoretical groups, including the Quantum Optics Theory group at the Institute of Photonic Sciences (ICFO) in Castelldefels, Barcelona, ​​have already shown that, under a completely quantum description, the high harmonic generation process emits light with quantum characteristics.

However, validation of these predictions continued to elude experimental efforts until now a team has demonstrated the quantum optical properties of high harmonic generation in semiconductors. The results of the new study are aligned with previous theoretical predictions about the generation of high harmonics.

The team that carried out this study was led by the Applied Optics Laboratory of the French National Center for Scientific Research (CNRS), in collaboration with ICREA Professor Jens Biegert of the ICFO and specialists from other institutions, including the Institute of Quantum Optics of the Leibniz University of Hannover in Germany, the Fraunhofer Institute for Applied Optics and Precision Engineering in Germany and the Friedrich Schiller University of Jena in Germany.

In their experiment, the high harmonic generation source operates at room temperature using standard semiconductors and a commercial femtosecond infrared laser. This accessibility positions high harmonic generation as a highly promising platform for generating non-classical light states, which, in turn, may pave the way toward more robust and scalable quantum devices that do not require complex cooling systems.

Artistic recreation of quantum light. (Illustration: Amazings/NCYT)

Theorists had already predicted that photons emitted through a high-harmonic generation process exhibit quantum behavior, which manifests itself in two defining characteristics: entanglement and compression.

Entanglement occurs when two particles become interconnected in such a way that measuring one instantaneously influences the result of measuring the other, regardless of the distance between them. These strong correlations defy classical intuition and can only occur in the quantum world of atoms, electrons and photons.

Compression, on the other hand, is related to the inevitable uncertainty when measuring certain pairs of properties in a quantum system: increasing the precision of the measurement of one quantity will decrease the precision of the measurement of the other. The compressed states welcome this commitment. Thus, at the cost of increasing the noise of one property of the pair, they can reduce the noise of the complementary property.

Consistent with previous theoretical predictions, the team, led by David Theidel of the Ecole Nationale Supérieure des Techniques Advances (ENSTA) in France, experimentally demonstrated the presence of both entanglement and compression in the emitted light. But how did they achieve it?

First, the researchers directed ultrafast infrared laser pulses at samples of semiconductors—gallium arsenide, zinc oxide, and silicon—to induce the generation of high harmonics. Of all the harmonics generated, they selected only two of them (the third and the fifth) using optical filters. These were sent to a detection system capable of analyzing multiple harmonics simultaneously, which was crucial in revealing the quantum behavior of light.

The first sign of quantum nature was related to compression. The team recorded that the variance in photon arrival times (and therefore the uncertainty associated with this quantity) decreased as the laser intensity increased. This reduction could only be explained by compression, providing strong evidence for this feature. Next, the team focused on entanglement. To demonstrate this, they measured the correlation between the arrival times of the photons coming from the third and fifth harmonics. The researchers consistently observed strong correlations that are prohibitive for a classical source, unequivocally indicating the presence of quantum entanglement.

These findings establish high harmonic generation as an ideal platform to produce entangled and compressed photonic systems at room temperature. “Both features are key resources for many quantum technologies, which, for example, rely on entanglement to transmit information or compression to improve the precision of measurements,” explains ICREA Professor Jens Biegert. “Ignoring quantum optical effects was preventing us from detecting non-classical features. But hopefully we will now be able to exploit the full potential of high harmonic generation for quantum information, communication and sensing applications.”

The study is titled “Evidence of the Quantum Optical Nature of High-Harmonic Generation.” And it has been published in the academic journal Physical Review X Quantum. (Source: ICFO)