A team of researchers, including members from the Optologic team, theoretically proposes a new experimental platform based on analog simulation with atom clouds to study high-harmonic generation. This ultrafast dynamic process challenges conventional computational methods. Their simulator allows for adaptation to approach a wide range of complex phenomena, opening doors to regimes that theory and direct experimentation struggle to reach.
Despite all the successes in understanding electron dynamics at their natural attosecond (one quintillion of a second) time scale, one of the fundamental processes core to this field, high-harmonic generation (HHG), raises new challenges for cold-atom simulation. It consists in a highly non-linear phenomenon where a system absorbs many photons of an incoming laser and emits a single photon of much higher energy.
Applications of High-Harmonic Generation
The unique characteristics of HHG make it an exceptional source of extreme ultraviolet radiation and consequently of attosecond pulses of light, which has important applications to various fields such as nonlinear optics or attosecond science.
Challenges in Studying HHG
The high number of variables involved hinders the study of this process, aside from its ultrafast speed. In any given material, many atoms and electrons are present, so studying most of the occurring chemical processes in all their complexity would require not only describing all these components but also their interactions with external fields and even among themselves. This task proves to be extremely challenging for any current classical computer. An alternative route involves using quantum devices, building the so-called analog simulators, whose nature allows them to better capture the complexity of the system.
Proposal of an Analog Simulator by ICFO Researchers
Now, ICFO researchers Javier Argüello, Javier Rivera, Philipp Stammer led by the ICREA Prof. at ICFO Maciej Lewenstein and in collaboration with other institutes all over the globe (Aarhus University, University of California and Guangdong Technion-Israel Institute of Technology) have proposed, in a Physical Review X Quantum publication, an analog simulator to access the emission spectrum of HHG using ultracold atomic clouds. Besides showing that an accurate replication of the key characteristics of the HHG processes in atoms was possible, they also provide details on how to implement it to specific atomic targets and discuss the main sources of errors.
The potential of analog simulation
An analog simulator allows scientists to study a complex quantum system (computationally challenging) through the control and manipulation of a much simpler one, which can be addressed experimentally. However, not every choice is valid, a connection between both systems must exist.
In this particular work, the researchers chose the complex phenomenon of high-harmonic generation to benchmark their idea. In this process, atomic bound electrons tunnel out the barrier formed by the atomic Coulomb potential and a laser electric field. Then, those free electrons accelerate, causing the emission of radiation with characteristic harmonic frequencies upon recombination with their parent ions. This emission spectrum of HHG is what the researchers aimed to recover.
On the other hand, they achieved connection to a much simpler quantum system by conveniently replacing certain components. Instead of an electron and a nuclear potential, they proposed using an atomic gas trapped by a laser beam; and instead of the incoming light and its electric field, they suggested an external magnetic gradient that could be tuned at will. It turns out that the absorption images of this engineered system coincide with the desired emission yield.
Therefore, by taking absorption images of the analog simulator, the emission spectrum of the atomic high-harmonic generation can be indirectly studied.
A new platform for ultrafast simulation
In the end, the research group has demonstrated the potential of their alternative method for addressing complex systems that would otherwise only be theoretically approximated. They proved that state-of-the-art analog simulators can retrieve the HHG emission spectrum, establishing correspondence between experimental and simulated parameters and providing an exhaustive experimental analysis.
Moreover, the platform offers twofold advantages. Firstly, scientists can easily tune the elements that emulate the incoming field and the nuclear potential. Secondly, the simulation also provides temporal magnification, allowing scientists to work in a much slower (and thus practical) frame by avoiding the attosecond time-scale.
The team emphasizes their approach’s adaptability, which is not limited to exclusively simulating HHG but could be extended to other, more exotic configurations. Specifically, ultrafast processes such as multielectronic dynamics or the reaction of matter to non-classical light could benefit the most.
