Cooling of particles with internal degrees of freedom

Quantum mechanics is the most precisely tested scientific theory to date, and it has not only improved our understanding of nature but also resulted in technology that we use in our everyday lives, for example, smartphones, high-speed internet and the Global Positioning System (GPS). However, quantum mechanics makes predictions about regimes that are not yet accessible in modern physics laboratories, which leads to questions that are still unresolved. For example, do the laws of quantum mechanics hold for dense massive objects containing billions of atoms? Is quantum mechanics necessary to describe gravity at microscopic levels? Nanoparticles levitated in vacuum are a promising experimental platform for answering these questions. Such experiments may lay the foundation for future technological advances. Two crucial parameters for quantum experiments with levitated nanoparticles are the internal temperature and the amplitude of the centre-of-mass motion of the nanoparticles. In the first case, damping the translational motion is an essential step to reach the quantum regime. In the second case, the internal temperature is a perturbation source that may impede experiments in this regime. The project will investigate the damping of translational motion and the reduction of internal temperatures of particles with internal degrees of freedom, in particular, doped nanoparticles and semiconductor quantum dots. Particles with internal degrees of freedom will be trapped and cooled in a linear Paul trap, an apparatus capable of suspending charged particles by means of radiofrequency fields. Two directions will be explored in achieving simultaneous cooling of internal degrees of freedom and of the translational motion. One direction will exploit the internal structure for both cooling processes. The other direction will employ a hybrid type of cooling, where the centre-of-mass motion will be cooled by active feedback and the bulk (internal) temperature will be reduced with the help of the internal structure. Paul traps are beneficial for experiments with levitated particles because their trapping capabilities do not depend on the particles optical properties, which makes it possible to decouple the study of the cooling process from the trapping process. With the help of internal degrees of freedom, this project aims to achieve both cooling effects simultaneously for the first time, opening up possibilities for quantum experiments with new types of nanoparticles.

Quantum mechanics is the most precisely tested scientific theory to date, and it has not only improved our understanding of nature but also enabled technologies that we use in our everyday life, for example, smartphones, high-speed internet and the Global Positioning System (GPS). However, quantum mechanics makes predictions in regimes that are not yet accessible in modern physics laboratories, which leads to questions that are still unresolved. For example, do the laws of quantum mechanics hold for dense massive objects containing billions of atoms? Is quantum mechanics necessary to describe gravity at microscopic levels? Nanoparticles levitated in vacuum provide a promising experimental platform for addressing these questions. Moreover, experiments with levitated particles may lay the foundation for future technological advances. In this project, we investigated methods for controlling two key properties of levitated nanoparticles: their internal temperature and the amplitude of their centre-of-mass motion. To achieve this hybrid control of levitated particles, we worked with nanoparticles based on ytterbium-doped yttrium lithium fluoride (Yb:YLF), which allow simultaneous manipulation of both their motion and internal temperature. We developed techniques for trapping these particles in ion traps under vacuum and stabilizing their centre-of-mass motion using measurement-based electrical feedback. We also observed the first signatures of optical control of the particles' internal temperature, i.e., heating and cooling using laser light. The internal temperature can be monitored through the particles' fluorescence signal, and we measured fluorescence spectra consistent with laser cooling of their internal degrees of freedom. In addition, our experiments revealed that controlling the particles' rotation is crucial for maintaining long trapping times, which are essential for future quantum experiments. We also found that rotational motion plays a key role in the optical control of the particles' internal temperature and must be understood and controlled to achieve efficient internal cooling. Developing methods to control and stabilize this rotational motion is therefore a key next step and remains an active focus of our ongoing research. Achieving simultaneous control of motion and internal temperature of levitated particles would open new possibilities for experiments at the boundary between microscopic and macroscopic physics in quantum regime, bringing researchers closer to testing quantum mechanics in entirely new regimes.