Shell-shaped Bose-Einstein Condensates Advance Understanding of Atomic and Astrophysical Fluids

Shell-shaped Bose-Einstein condensates represent a fascinating intersection of ultracold atomic physics and the study of fluids in extreme environments, offering insights relevant to astrophysical phenomena. These hollow quantum fluids provide a unique platform for exploring how geometry and topology influence collective behaviour in curved spaces.

Brendan Rhyno, from the Institute of Quantum Optics at Leibniz Universität Hannover, Kuei Sun of Washington State University Tri-Cities, and Jude Bedessem and Smitha Vishveshwara from the University of Illinois at Urbana-Champaign, lead a comprehensive investigation into the behaviour of these unique condensates. Their work synthesises two decades of theoretical development by the Vishveshwara group and collaborators, together with recent experimental advances, to establish a unified understanding of shell-shaped Bose-Einstein condensates.

This research combines long-standing theoretical insights with recent experimental breakthroughs, notably the creation of ultracold atomic shells under microgravity conditions aboard the International Space Station. By analysing the evolution of a condensate from a filled sphere to a hollow shell, the authors demonstrate that microgravity is essential to prevent gravitational collapse and preserve shell integrity. Their study provides a detailed overview of the dynamics, thermodynamics, and collective excitations of these systems, revealing how closed surface topology governs vortex behaviour and produces a universal dip in excitation spectra during the hollowing transition. These findings deepen our understanding of quantum fluids in curved geometries and may inform models of related phenomena in condensed matter physics and astrophysics.

Ultracold Atom Trapping and Bose-Einstein Condensates

This research builds upon the foundation of Bose-Einstein condensates, created and manipulated to study fundamental quantum phenomena. Scientists employ various techniques to trap and control ultracold atoms, including magnetic traps, optical lattices, and crucially, radio frequency (RF)-dressed adiabatic potentials. These RF-dressed potentials are particularly versatile, allowing for the creation of tunable trapping potentials, including those necessary for forming two-dimensional and shell-shaped geometries. A significant focus lies on performing BEC experiments in microgravity, utilising drop towers, parabolic flights, and the International Space Station to eliminate gravitational effects and accurately model theoretical predictions. Researchers also investigate dual-species mixtures of atoms to create more complex BEC systems and explore novel quantum behaviours.

Shell-Shaped BECs Exhibit Universal Spectral Dip

Scientists have extensively studied shell-shaped Bose-Einstein condensates, unique quantum fluids exhibiting properties relevant to both ultracold atomic physics and astrophysics. They demonstrated that evolving a BEC from a filled sphere to a hollow shell requires microgravity conditions to counteract gravitational deformation. Analysis of collective modes revealed a universal dip in frequency spectra and reconfiguration due to inner-surface excitations, serving as robust signatures of the hollowing-out transition. The team investigated vortex physics within these shell-shaped BECs, discovering that the closed surface topology enforces vortex-antivortex pair configurations, and that rotation can stabilize these pairs.

The critical rotation rate correlates directly to shell thickness, offering a potential method for experimental thickness determination. Investigations into the thermodynamic properties of low-density BEC bubbles computed the critical temperature across various geometries, and tracked the evolution of temperature and condensate fraction during isentropic expansions, observing a loss of space density. The bubble trap potential, achieved through time-dependent RF-dressed adiabatic potentials, enables a smooth evolution between filled-sphere and hollow-shell geometries by tuning the effective detuning, allowing for detailed study of the transition.

Shell Condensate Evolution and Vortex Stability

This research presents a comprehensive investigation into shell-shaped Bose-Einstein condensates, building upon two decades of theoretical work and recent experimental advances. Scientists have explored the evolution of these condensates from filled spheres to hollow shells, demonstrating the necessity of microgravity conditions to prevent gravitational collapse during this transformation. A key finding is the presence of a universal dip in the frequency spectra and reconfiguration of collective modes as the condensate hollows out, providing robust signatures of this geometric transition. The team also examined vortex dynamics within these shell-shaped BECs, revealing that the closed surface topology leads to stable vortex-antivortex pairs, which can be maintained through rotation.

Furthermore, researchers investigated the interplay between shell inflation and the phase transition of the condensate, finding that adiabatic expansions result in condensate depletion, a loss of phase-space density. To model these dynamic processes, scientists developed a time-dependent method capable of capturing the evolution of the condensate in both adiabatic and non-adiabatic regimes. This research provides a foundational understanding of fluids in curved geometries and opens new avenues for exploring fundamental physics in unique and challenging environments.