A study of imbalanced quantum droplets

Abstract

Gases of ultracold atoms have proved to be a highly versatile platform for studying quantum physics due in part to their high controllability. One example of this control is the manipulation of interactions. A single-component Bose gas in free space is unstable to collapse under attractive contact interactions, whereas, in an attractive two-component Bose gas the collapse is arrested by the repulsion from quantum fluctuations, forming self-bound liquid droplets: quantum droplets. Quantum droplets are therefore a quantum fluid in which quantum fluctuations are not only present but integral. During formation quantum droplets preserve a constant density ratio such that the droplet can be approxi mately described by its total density rather than in terms of two independent component densities. This approximation, known as density locking, is powerful but limiting. Droplet experiments typically begin with a population imbalance during formation, meaning that imbalanced droplets may be the most prevalent. Motivated by a link with an experi mental 174YbCs mixture, which deviates from many of the assumptions built into the density-locked model, the primary focus of this thesis is: how are droplets modified by imbalances that break the assumptions of density locking? This thesis begins by showing that, up to a saturation limit, free-space droplets can lower their energy through population imbalancing but correspondingly become less stably bound. By perturbing these imbalanced droplets, breathing modes are observed to have a complex parameter space of multiple, superimposed, decaying modes with time-dependent frequencies and decay rates. All droplets are experimentally formed in harmonic traps, and so the free-space imbal anced results are extended to study how harmonic traps affect the structure and breathing modes of imbalanced droplets. Beyond the droplet formation, there is a crucial question around the stability of the imbalance during a release into free space. A key result of this thesis is that imbalances can be retained during either an instantaneous or linearly-ramped trap release. Finally, the imbalanced droplet results above are applied to explore the 174YbCs pa rameter space, by measuring droplet size, peak densities and lifetimes. A variety of dynamics are studied from droplet formation and excitations, to proposing potential ex periments serving as a platform to study the non-equilibrium dynamics of droplets.