Recent Trends on Quantum Fluctuations in Ultra-Cold Quantum Gases

Dear Colleagues,

Quantum fluctuations (QF) lie at the heart of the Heisenberg uncertainty principle, and they represent a tiny shift in the ground-state properties of a simple quantum system. For instance, the beyond-mean-field energy corrections of a Bose gas, were found at the end of the fifties by Lee, Huang and Yang (LHY). This term stems from QF, and it is usually neglected in the weakly interacting regime. However, as the complexity of the quantum system increases, these fluctuations can be sizable with respect to other energy scales, bringing thus nontrivial consequences.

Ultracold quantum gases serve as a platform to create and engineer complex systems formed by different kinds of atoms. In these systems interactions can be tuned at will, allowing thus a high control of desirable energy scales. Prominent candidates are atoms with a high magnetic dipolar moment and quantum mixtures. QF has given rise to new states of matter with fined-tune interactions such as ultra dilute-liquid self-bound droplets, bubbles, supersolids, and polaronic effects beyond the mean-field paradigm. For instance, in highly magnetic atomic species such as Dysprosium or Erbium, a competition between the mean-field energy scales associated with the short and long-range interaction yields an enhancement of a repulsive beyond the mean-field term, forming ultra dilute quantum droplets, arresting the mean-field collapse. A similar phenomenon is observed in non-dipolar mixtures, yet a two-component mixture is needed to trigger a competition between the net mean-field energy and the QF term. Likewise to the dipolar case and for specific parameters, the system stabilizes, forming droplets and bubbles or even a gas ruled only by QF-the LHY gas. On the other hand, supersolidity also appears as a direct manifestation of the enhancement of  QF in the neighborhood of phase a transition. Here a global phase-coherent array of dipolar quantum droplets acquires a spatial modulation similar to a crystal. Highly imbalanced atomic species permit the formation of polarons, where single particles are screened by the low-energy excitation of a quantum gas. These quasiparticles serve as a non-demolition probe of QF in their host environment, for different impurity-bath couplings. Consequences of beyond mean-field terms are the formation of many-body bound states, the interplay between few and many-body physics.

Understanding QF and how they arise in different quantum systems is essential for future technologies, such as gravitational wave detectors where these fluctuations are unavoidable in experimental measurements.

The special issue aims to bring together contributions from the theoretical and experimental points of view related to quantum fluctuations in ultracold gases. In this Special Issue, original research articles, communications and reviews are welcome. Research areas may include (but are not limited to) the following:

• Quantum mixtures and polaron physics
• Ultradilute quantum droplets and bubbles
• superfluidity and supersolidty
• Quantum sensing and metrology
• Excitions and polariton
• Casimir effect

We look forward to receiving your contributions.

Dr. Luis Aldemar Peña Ardila
Dr. Cesar Cabrera
Guest Editors

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