In a theoretical and experimental study published in Nature Physics, an international team of researchers demonstrates the generation of highly non-classical states of light in intense laser–atom interactions.
Advances in Laser Technology and Light-Matter Interaction
Over the past four decades, astounding advances have been made in the field of laser technologies and the understanding of light-matter interactions in the non-linear regime. Consequently, scientists have been able to carry out extremely complex experiments related to, for example, ultra-fast light pulses in the visible and infrared range. Furthermore, they have accomplished crucial milestones such as using a molecule’s own electrons to image its structure, to see how it rearranges and vibrates or breaks apart during a chemical reaction.
The development of high-power lasers allowed scientists to study the physics of ultra-intense laser-matter interactions which, in its standard version, treats ultra-strong ultra-short driving laser pulses only from a classical point of view. The famous theory coined as the “simple man’s model” or the “three-step model” – which had its 25th anniversary in 2019 – dealt with the interaction of an electron with its parent nucleus sitting in a strong laser field environment, and elegantly described it according to classical and quantum processes. However, due to the fact that these laser pulses are highly coherent and contain huge numbers of photons, the description of the interaction in the strong field has so far been incomplete, because it treated the atomic system in a quantum way but the electromagnetic field in a classical way.
Generation of Non-Classical States of Light through Intense Laser-Atom Interactions
In the current landscape, the description of the most relevant processes in ultra-intense laser-matter physics, such as high-harmonic generation, above-threshold ionization, laser-induced electron diffraction, and sequential and non-sequential multi-electron ionization, often overlooks the quantum-fluctuation effects of the laser electric field, let alone the magnetic fields. Nevertheless, the quantum nature of the entire electromagnetic field is inherently present in these processes. Consequently, a natural question arises: does this quantum nature manifest itself in observable ways? Furthermore, under what specific circumstances does this manifestation occur?
In the recent study published in Nature Physics, OPTOlogic researchers at ICFO led by ICREA Prof. Maciej Lewenstein, Max Born Institute in Berlin, in collaboration with the PhD researcher Theocharis Lamprou, led by the Research Director Paraskevas Tzallas, from FORTH, have reported on the theoretical and experimental demonstration that intense laser–atom interactions may lead to the generation of highly non-classical states of light.
Researchers obtained such results by using high-harmonic generation in atoms, where they up-convert large numbers of photons from an infrared driving laser pulse into higher-frequency photons in the extreme ultraviolet spectral range. The quantum electrodynamical theory formulated in this study predicts that a coherent initial state of the driving laser remains coherent but experiences an amplitude shift after interacting with the atomic medium.
Similarly, the harmonic modes’ quantum states become coherent with small coherent amplitudes. However, researchers can condition the driving laser pulse’s quantum state to account for this interaction, transforming it into an optical Schrödinger cat state. This state represents a quantum superposition of two distinct coherent states of light: the initial laser state and the amplitude-reduced coherent state resulting from the interaction with the atoms.
Experimental Verification and Future Implications of Non-Classical Light States
The researchers experimentally accessed the full quantum state of this laser pulse using quantum state tomography. This process requires a coherent reduction of the light’s amplitude to only a few photons on average before measuring all quantum properties of the state.
The results of this study pave the way for investigations into controlling non-classical states of ultra-intense light and exploiting conditioning approaches on physical processes relevant to high-harmonic generation. This research will hopefully create a novel and unexpected link between ultra-intense laser-matter physics, attoscience, quantum information science, and quantum technologies.
Cited article: M. Lewenstein, M. F. Ciappina, E. Pisanty, J. Rivera-Dean, P. Stammer, Th. Lamprou and P. Tzallas. Generation of optical Schrödinger “cat” states in intense laser-matter interactions, Nature Physics, 2021.
