Professor Krzysztof Sacha


Time Crystals

Due to mutual interaction atoms can self-organize and form a space crystal. In 2012 Frank Wilczek asked the question if a similar phenomenon can occur in the time domain, i.e. if a many-body system can self-organize in time and spontaneously start a periodic motion. Original Wilczek idea turned out to be impossible to realize, however, it became an inspiration to other scientists.

In 2015 the first article on the so-called discrete time crystals was published, Sacha, Phys. Rev. A, where it was shown that periodically driven interacting atoms can spontaneously self-organize and start moving with a period twice longer than the driving period. A year later independent ideas on discrete time crystals were published. In 2017 two groups performed experiments where discrete time crystals were realized. The results were announced in Nature magazine, J. Zhang et al. and S. Choi i et al, see also News and Views in Nature and News of Polish Sicience or a lecture in CFT PAN. and in Vlinus, Physics World 2020

In our group we have been carrying the research on time crystals from the very beginning. It is already known that time crystals, similarly as space crystals, can be conductors or insulators, Sacha, Sci. Rep. 2015.

A book on "Time Crystals" is available
and lecture series on "TIME CRYSTALS" - YouTube.

Cold Quantum Gases

Development in trapping and cooling of dilute atomic gases allows experimentalists to obtain Bose-Einstein condensates of bosonic atoms and also to reach transition to a superfluid state in cold fermionic gases. In our group we do theoretical research in such degenerate atomic gases. Our interests concern: collective excitation in atomic gases (such as solitons and vortices), cold gases in optical lattice potentials (i.e. realizations of solid state models in atomic gases), analysis of a measurement problem in the quantum many body systems, possibilitiesof realization of field theory models.

Ionization in Strong Laser Fields

Recent development in the laser technology allow experimental studies of high order harmonic generation, above threshold ionization and multi-electron effects, such as a non-sequential double ionization. While the single ionization of atoms or molecules, as well as the high order harmonic generation can be described within a single active electron model, such an approximation in the case of multiple ionization and laser intensity below the saturation value gives ionization rates that are much smaller than experimentally observed, indicating that interactions between electrons are important. Having that in mind, in our group we consider the multiple ionization of atoms and molecules both from a classical and a quantum point of view.