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Dynamics and Thermodynamics of Ultradilute Liquids

Periodic Reporting for period 1 - UltraLiquid (Dynamics and Thermodynamics of Ultradilute Liquids)

Reporting period: 2019-09-01 to 2021-08-31

One of the most exciting recent discoveries in ultracold mixtures of two bosonic atomic ensembles is the experimental realization in 2018 of novel liquids featuring densities and temperatures which are eight to ten orders of magnitude smaller than water. Nonetheless, the thermodynamics of such quantum liquids remains unexplored due to large three-atom losses, which hinder their experimental study. In one spatial dimension, losses are limited thus providing enhanced stability to the system.

That greater stability is achieved even in a single-component system, where the understanding of thermodynamic properties is still an open issue in our field. The solution to such a central problem is of paramount importance to shed light also on the properties of ultradilute liquids. A system of bosons moving along one dimension looks deceivingly simple, but it is actually an extremely sophisticated system due to the intricate interplay of thermal motion, collisions, and quantum statistics. Importantly, this problem is not a merely academic one since these systems have been experimentally realized since 2004 with ultracold gases.

A problem that remained open until now was the understanding of the microscopic mechanisms ruling the thermodynamic behavior of these extremely quantum systems. In addition, thermodynamic quantities already showed an anomaly in their temperature dependence, resembling the presence of a phase transition. However, a peculiarity of one-dimensional geometry is that phase transitions cannot occur, making the issue extremely puzzling. Finally, the complete in-depth understanding of microscopic correlation properties was still missing.

In this project, we have investigated the thermal properties of ultracold bosonic gases and liquids in a one-spatial dimension. Besides the solution of the challenging problems listed above, we have discovered radically new quantum regimes and phenomena and proposed how they can be explored in cutting-edge experiments.

Ultracold gases and liquids allow for far-reaching investigations of quantum many-body effects and high-resolution measurements inconceivable so far. These promise innovative applications in quantum metrology and sensing. Finally, the understanding of thermal properties, enhanced by the quantum simulation of very different systems ensured by ultracold ensembles, is relevant for the development of future quantum technologies, innovative materials, high-critical-temperature superconductors, and quantum computers.
Our project is focused to the key understanding of thermal properties in ultracold bosonic gases and liquids in one dimension, where the stability is enhanced due to limited losses.

We discovered the new mechanisms of evaporation and dynamical instability which drive the thermal liquid-to-gas transition in bosonic mixtures. At high densities, atoms are found to evaporate from the surface of the liquid. At low densities, instead, a dynamical instability takes place and the liquid disappears completely at the critical temperature. Both mechanisms can be observed and their onset may be adjusted at will tuning the interatomic interaction strengths. We built the complete and rich phase diagram of the mixture (see Figure attached) and we characterized the main thermodynamic properties of the liquid. We suggested innovative precise methods to measure the temperature in ultracold liquids, which are based on the direct measurement of the thermodynamic properties and on the interaction dependence of the liquid-to-gas critical temperature.

We have solved the open problem of the understanding of the thermal effects in a single-component gas from different perspectives.

Firstly, we provided an explanation of the microscopic effects determining the different thermodynamic behaviors for weak and strong interactions.

Secondly, we explained the non-monotonic behavior in the temperature dependence of the thermodynamic quantities by proposing the novel Dark-Soliton anomaly. This new anomaly can be used as a simulator of other anomalies in condensed matter, atomic and solid-state physics. It can be used as a new precise tool for the measurement of temperature and signals dramatic changes of the quantum regime and in the excitation spectrum. We also predicted novel quantum regimes and provided the most detailed theory of thermodynamics so far, holding at any temperature and interaction strength.

Finally, we have characterized all the main microscopic correlation properties for any interaction strength and temperature values.

Throughout, we built on local and international collaborations and benefited from a strong synergy with other research lines, active at the host institution. All results have been disseminated through press releases and peer-reviewed prestigious publications which are freely available to the community on the arXiv repository. We have presented our work at many international conferences, workshops, other events and institutions. We have talked about our research to school students and general audience in different events, some of them focused on women in science issues. We have highlighted our activities on our websites and several social media.
Our project has made important contributions to the state-of-the-art, including the first characterization of the thermodynamic behavior of liquids in an ultracold mixture, the complete understanding of the associated thermal liquid-to-gas phase transition, and novel proposed experimental methods to measure the temperature in liquids. In a one-dimensional Bose gas, we understood the microscopic effects ruling its thermodynamic behavior, we discovered new quantum regimes, and we provided complete knowledge of microscopic correlation functions. In addition, we introduced the first anomaly in the ultracold atom field and an innovative concept of quantum simulation at the thermodynamic level. Throughout, we paid particular attention to current experiments and the possible implementation of all our theoretical ideas is currently under discussion with several international experimental groups belonging to our broad scientific network. Our results have made a fundamental contribution towards bringing the novel field of ultracold liquids to full scientific maturity.

In the future, our project can have a social impact by deepening our understanding of quantum physics and by feeding through in the long-term emerging associated technologies. Our results have opened up several promising research lines, including the possibilities of precise measurements of temperature with important foreseen applications in quantum metrology and sensing.
Anomalies are present in many areas ranging from condensed matter, atomic, many-body, solid-state physics to material science. The new quantum simulation based on the dark-soliton anomaly could be fundamental for applied research in material science and quantum technology.
Typical phase diagram at a fixed temperature of a 1D Bose-Bose mixture