Breaking New Ground! When quantum optics meets attosecond science -

Non-perturbative interactions (i.e., so-called interactions that are too strong to be described by perturbation theory) between light and matter have been the subject of numerous studies. However, the role of the quantum properties of light in these interactions and the resulting phenomena have not been widely explored.

Now, researchers at the Technion-Israel Institute of Technology (TIT) have proposed a new theory that describes the physics underlying non-perturbative interactions driven by quantum light. Their theory, published in the journal Nature Physics, could guide future experiments to probe strong-field physical phenomena, as well as the development of new quantum technologies.

“High-harmonic generation driven by quantum light”

This recently published paper is the result of close collaboration between three different research groups at the Technion-Israel Institute of Technology, led by principal investigators Prof. Ido Kaminer, Prof. Oren Cohen and Prof. Michael Krueger. Students Alexey Gorlach and Matan Even Tsur, who are co-first authors of the paper, led the research with the support and advice of Michael Birk and Nick Rivera.

"This has been an important scientific journey for us." Prof. Kaminer and Gorlach sued, "We started thinking about Higher Harmonic Generation (HHG) and its quantum characterization back in 2019. At that time, light in all HHG experiments was explained by classical methods, and we wanted to find out when quantum physics started to play a role in it."

"Frankly, what bothered us was that several of the phenomena underlying physics were each explained by completely different theories, and therefore it was impossible to connect them. For example, the theory on which HHG is based contradicts the theory that is usually used to calculate spontaneous radiation - both are explained on different bases."

Higher harmonic generation and its quantum characterization

HHG is a highly nonlinear physical process that involves a strong interaction between light and matter. Specifically, when a strong light pulse acts on matter, the matter emits so-called high harmonics that drive the strong light pulse.

Higher harmonic generation (HHG) driven by quantum light states: implications for extended spectral cutoff. Schematic of an emitting system, such as a gas cell, driven by bright light for high harmonic generation.The HHG spectrum depends strongly on the quantum state of the driving field. For example, when the system is driven by a bright squeezed vacuum state (shown in green), it emits more harmonics than when it is illuminated by classical coherent light (shown in red), even if the field has the same average intensity, the same frequency, and the same polarization.

For several years, Prof. Kaminer and his research group have been working to devise a single framework based on quantum theory to comprehensively explain all photonics phenomena, including higher harmonics. Their first paper, published in Nature Communications in 2020, presented a proposed version of this unified framework, analyzing HHGs in the language of quantum optics.

The 2020 article shows that high harmonic generation (HHG) is an extremely nonlinear effect that produces coherent broadband radiation and pulse durations reaching attosecond time scales. The team proposes a fully quantum theory of extreme nonlinear optics that predicts quantum effects that alter the spectral and photonic statistics of HHG, thus departing from all previous approaches.

Prof. Kaminer and Prof. Gorlach explain, "This research helps to open up the currently emerging field of quantum HHG. However, all HHG experiments are driven by classical laser fields. It did not even seem possible that any quantum light would be strong enough to produce HHG. however, Prof. Maria Chekhova's research shows that it is possible to produce sufficiently strong quantum light in a form known as a bright squeezed vacuum. This has inspired our new research."

As part of the new research, Prof. Kaminer, Gorlach and their colleagues devised a complete framework describing the strong-field physical processes driven by quantum light. To theoretically validate their framework, they applied it to HHG to predict how this process would change when driven by quantum light.

"We found that, contrary to expectations, many of the important features (such as intensity and spectrum), change as a result of using a driving light source with different quantum photon statistics. The paper we wrote also predicts experimentally feasible scenarios that cannot be explained by any other method than considering photon statistics. These upcoming experiments will have an even greater impact and importance to this rising field of strong-field quantum optics."

First theory of non-perturbative processes driven by quantum light

So far, the work carried out by this team of researchers has been purely theoretical: their paper presents the first theory of a non-perturbative process driven by quantum light, as well as theoretically demonstrating that the quantum state of light affects measurable quantities such as the emission spectrum.

"Our theory works by splitting the driving light into one of two representations, called the generalized Glauber distribution or the Husimi distribution, and then simulating the parts of the distribution separately using the time-varying Schrödinger equation (TDSE), a traditional simulation of the HHG field, and then combining the simulation results are combined to produce an overall result."

Spectra of high harmonic generation in different driving light states

"What makes this work powerful and useful - for arbitrary quantum states of light and arbitrary emitter systems - is precisely that we have combined the standard tools of the physics community with such a quantum optical computational scheme."

The new theory derived could soon inform research in different areas of physics. Indeed, their paper envisions generalizing the idea to a variety of non-perturbative processes beyond HHGs that can be driven by non-classical light sources.

This theoretical prediction could soon be tested and validated in an experimental setting. For example, the team's theory could be directly applied to the generation of attosecond pulses via HHGs, a process that could underpin the operation of quantum sensing and quantum imaging techniques.

"Another ambitious goal is to generalize the developed theory beyond HHG and to study quantum effects in various materials driven by intense light, which links our new advances in quantum optics to the frontier of condensed matter physics."

Despite the foreshadowing, Nature was extremely complimentary of the results, "We believe that the paper by Gorlach and colleagues actually marks the beginning of a new subfield of high-intensity quantum optics. Short-wavelength and X-ray quantum optics will be generated from transparent materials and metals as well as atomic and molecular gases. One can study the quantum response in all subfields of high-field physics, including tunneling ionization, subthreshold ionized electrons, and electrons emitted from metal tips. All of these can be powered by quantum light or classical light or a combination of both."

"We expect that X-rays and high-energy electrons with energies up to a thousand electron volts will be able to carry quantum statistics. These new and diverse quantum sources will greatly expand the impact and applications of quantum optics."

参考链接：[1]https://www.nature.com/articles/s41467-020-18218-w[2]https://phys.org/news/2023-09-theory-strong-field-non-perturbative-physics-driven.html[3]https://www.nature.com/articles/s41567-023-02160-x