New states of matter with fine-tuned interactions: quantum droplets and dipolar supersolids

Abstract

Quantum fluctuations can stabilize Bose–Einstein condensates (BEC) against the mean-field collapse. Stabilization of the condensate has been observed in quantum degenerate Bose–Bose mixtures and dipolar BECs. The fine-tuning of the interatomic interactions can lead to the emergence of two new states of matter: liquid-like self-bound quantum droplets and supersolid crystals formed from these droplets. We review the properties of these exotic states of matter and summarize the experimental progress made using dipolar quantum gases and Bose–Bose mixtures. We conclude with an outline of important open questions that could be addressed in the future.

Beyond-mean-field crossover from one dimension to three dimensions in quantum droplets of binary mixtures

“…The name droplet is given in analogy to liquid droplets which have similar properties such as a constant density profile although their stabilization mechanism is different. Quantum droplets were also observed in dipolar condensates where the magnetic interaction competes with the usual contact interaction [13] (see Refs [14,15] and references therein for recent reviews on quantum droplets). Experimentally, the droplets are observed at high densities (typically ∼ 10 21 m −3 ).…”

Self-Evaporation Dynamics of Quantum Droplets in a 41K-87Rb Mixture

“…These findings have triggered an intense research activity on droplets properties (see, e.g., the recent reviews in Refs. [16,17]) and on the so-called Lee-Huang-Yang fluid [18][19][20], and have motivated further studies in low-dimensional systems [21,22] and also beyond Petrov's theory [23][24][25][26][27].…”

Proof of universality in one-dimensional few-body systems including anisotropic interactions

“…Now we prove that in the limit of a vanishing binding energy E (2) 0 of the heavy-light ground state, short-range potentials with both negative (v 0 F(0) < 0) and vanishing (F(0) = 0) integral over space yield the same universal solutions for the two-body ground state as for the contact interaction. This universality is shown not only for the binding energy, but also for the corresponding wave function.…”

“…In contrast to purely attractive potentials, which are ubiquitous in quantum physics, interactions whose attractive and repulsive parts cancel each other are only scarcely discussed. Nevertheless, the latter allow for bound states [1], and interest in such potentials ramped up within the last years with the ability to realize them in systems of ultracold dipoles [2]. This is supported by recent analysis for these potentials on the formation of few-and many-body bound states in arrays of one-dimensional tubes [3] or in terms of beyond-mean-field contributions in reduced dimensions [4].…”

“…Effectively, we have used here the fact that in momentum space the potential is more slowly varying around p = 0 compared to the wave function, which becomes more localized as E (2) → 0 − , or q 0 → 0 accordingly. This argument is equivalent to the picture in coordinate representation that in the limit E (2) → 0 − , the bound state wave function becomes more extended with respect to the fixed range of the potential.…”