Quantum rotations in the presence of a many-body environment

The concepts of rotation and angular momentum are ubiquitous in physics, and play a role in diverse phenomena like the reactivity of molecules, accuracy of atomic clocks or spectra of stars. During the last 70 years, the quantum theory of angular momentum evolved into a powerful machinery, commonly used to describe isolated quantum systems. In realistic experiments, however, quantum systems are seldom isolated. Instead, they are disturbed by the surrounding environment be it a gas, a solution, or fluctuations of the electromagnetic field. Such an environment is capable of profoundly altering the physics of the system and can hardly be controlled experimentally. Due to the properties of quantum rotations, the angular momentum properties can be of tremendous complexity even for a few interacting particles, and often become intractable for large, many-particle systems. Recently, Schmidt and Lemeshko [Phys. Rev. Lett. 114, 203001 (2015)] introduced a new quasiparticle the `angulon which allows to drastically simplify the problem of redistribution of angular momentum in many-body systems. Introducing the angulon made it possible to describe the effects previously observed experimentally, as well as to theoretically discover novel phenomena. However, most of the physics associated with this new quasiparticle is still to be discovered. The aim of this proposal is to develop the foundations of the `angulon theory,` which would quantitatively describe the properties of quantum rotations in the presence of a many-particle environment. The focus is both on the fundamental properties of this new quasiparticle, as well as on the applications to the state-of-the-art experiments. In a broad perspective, the theory will serve as a building block to understand the transfer of orbital angular momentum in the context of condensed-matter and chemical physics.

The concepts of rotation and angular momentum are ubiquitous in physics, and play a role in diverse phenomena like the reactivity of molecules, accuracy of atomic clocks or spectra of stars. During the last 70 years, the quantum theory of angular momentum evolved into a powerful machinery, commonly used to describe isolated quantum systems. In realistic experiments, however, quantum systems are seldom isolated. Instead, they are disturbed by the surrounding environment - be it a gas, a solution, or fluctuations of the electromagnetic field. Such an environment is capable of profoundly altering the physics of the system and can hardly be controlled experimentally. Due to the properties of quantum rotations, the angular momentum properties can be of tremendous complexity even for a few interacting particles, and often become intractable for large, many-particle systems. In 2015 Schmidt and Lemeshko [Phys. Rev. Lett. 114, 203001 (2015)] introduced a new quasiparticle - the `angulon' - which allows to drastically simplify the problem of redistribution of angular momentum in many-body systems. Introducing the angulon made it possible to describe the effects previously observed experimentally, as well as to theoretically discover novel phenomena. However, back then, most of the physics associated with this new quasiparticle was still to be discovered. In the course of this FWF Project, the group of Prof. Lemeshko has developed the foundations of the `angulon theory,' which would quantitatively describe the properties of quantum rotations in the presence of a many-particle environment. The focus was both on the fundamental properties of this new quasiparticle, as well as on the applications to the state-of-the-art experiments. In a broad perspective, the developed theory will serve as a building block to understand the transfer of orbital angular momentum in the context of condensed-matter and chemical physics.