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Topological superfluidity in ultracold gas of Dysprosium atoms

Periodic Reporting for period 1 - Topological superfluidity (Topological superfluidity in ultracold gas of Dysprosium atoms)

Reporting period: 2015-05-01 to 2017-04-30

"The Nobel Prize in Physics in 2016 has celebrated the efforts of the past decades in understanding the materials with a non-trivial, so-called topological order. In mathematics topology refers to the classification of surfaces depending on properties invariant upon continuous deformations. As an illustration a ball and a torus, having different number of holes, belong to different topological classes. By analogy, topological materials possess some characteristics with an intrinsic stability against local perturbations. These unique properties might be used to dramatically decrease the coupling of the quantum system with a ""noisy"" environment, with possible application in a new generation of metrology standards and inherently error-free quantum computation.
Recently, the concept of topological matter was successfully generalized to superconductors, opening new exciting research directions. In our project, we experimentally investigate the phenomenon of superfluidity, that shares similar physics with supercondustivity, with the difference of supercurrents being formed by neutral particles (atoms) instead of paired electrons. Topological properties of the superfluid are directly connected with the type of pairing between the particles. Trivial topology, corresponding to the pairing between two fermionic particles with different internal states (for example, opposite spin directions for electrons), was experimentally realized in different systems and described within Bardeen-Cooper-Schrieffer theory. A topologically non-trivial superfluid state is more experimentally challenging and requires effective simulation of the pairing between fermionic atoms with identical spin via carefully engineered coupling between their spin and motional degrees of freedom, so-called spin-orbit coupling (SOC).
By means of atom-light interaction in our experiment we aimed to create an artificial SOC and study its key role in topological properties of materials. The main objectives of our project were to understand the underlying mechanisms of SOC and realize a topological superfluid with ensembles of Dysprosium atom at ultra-low temperatures.
We have developed and implemented the necessary toolbox for studying topological superfluid state. We performed preliminary experiments which were necessary to fine-tune our apparatus and answer related open questions in our field. These results constitute a benchmark for the experimental study of ultracold gases of magnetic Lanthanide atoms. The atom-light interactions and SOC mechanisms in our system is currently under active experimental investigation. The results will be presented most likely before the end of the year 2017."
"In the scope of this project we aimed at investigating topological superfluid states of matter with gases of Dysprosium atoms prepared at temperature close to absolute zero (-273.15°C). At this ultra-low temperatures, matter starts to reveal its quantum nature. In particular, below a certain ""critical"" temperature, the gas might undergo a phase transition into the superfluid state. Realization of the superfluid with topologicaly non-trivial properties, according to the recent proposals, requires introducing a strong and tunable SOC in the system, typically by means of laser light interacting with atoms. It turns out, however, that even with a very careful choice of coupling parameters, the unavoidable heating due to the scattering of the photons by atoms is strong enough to destroy the superfluid state way before any interesting measurements are done. Luckily, in particular case of Dysprosium atoms (Lanthanide group), the heating is about hundred times weaker, so we may create and study topological superfluid on realistic timescales.
In the initial phase of the project we have realized that a comprehensive study of SOC phenomena would first require a significant improvement on stability of our experimental apparatus. In the field of ultracold atoms we experimentally produce atomic gases at ultra-low temperatures using laser light to decelerate, trap and cool the atoms. One of the workhorses in laser cooling is the so-called magneto-optical trap (MOT), where the pre-slowed atoms are captured and further cooled with the aid of specific combination of optical and magnetic forces. The MOT of Dysprosium atoms has several non-trivial properties, which were not studied extensively as it is relatively new specie in our field. With a series of detailed experiments and development of theoretical models, we obtained at the first time a quantitative description of MOTs of the Lanthanides, which will serve as a benchmark for future experiments with this class of atoms. The results were published in the peer-reviewed journal.
In scope of this project we have implemented the setups for radio-frequency and so-called Feshbach spectroscopy (a technique widely used for probing properties of inter-atomic interactions at ultra-low temperatures) that served in numerous calibrations and fine-tunings of our system and proved to be indispensable tools. We also started testing a new high-resolution imaging system and plan to implement it in few months, completing thereby the list of minimum necessary ""hardware"" developments planned within the project. During the last phase of the project, we have started experiments on atom-light interaction with Dysprosium atoms with a prospect of implementing SOC in our setup and testing the feasibility of creating topological superfluid state. The atom-light interaction for Lanthanide atoms is very rich and almost unexplored topic and we expect the results of ongoing experiments to be published in peer-reviewed journal before the end of the year 2017.
In summary, we have progressed relatively far beyond the state-of-the-art from technical and scientific point of view.
Technically, we attained a quantitative understanding of our system and fine-tuned the apparatus to reach a perfect operational stability, which was beyond the typical planned performance. This will accelerate further experimental progress paying back the efforts on a longer timescale.
From scientific point of view, our results have filled an important gap in understanding the physics shared by the whole class of systems of ultracold Lanthanide atoms. Given the recent trend in our field of multiple newly emerging experiments on Lanthanide atoms, our results seem to consolidate the subject and to be of direct use for all similar experiments. Interestingly, our recent results have a lot in common with the old textbook material in the field of laser cooling and trapping and might stimulate further related theoretical studies. Our ongoing research on atom-light interaction with Dysprosium atoms has a large potential scientific impact as we study the regime of interactions which has not yet been extensively addressed in experiment. We are convinced that there is a lot of new physics to discover within this project.
From the socio-economic point of view, the direction of our research and its present (and potential) impact is mostly in the fundamental domain, therefore, it is very difficult to evaluate. We might give an estimate of 20 years until the first real applications of topological physics will emerge. The generality of the topology concepts, however, makes this topic universal and highly multidisciplinary which seems to be a good sign that we invest our resources into very promising and rapidly developing field.