All-optical bubble trap for ultracold atoms in microgravity

TL;DR

The paper addresses creating a gravity-resilient, shell-shaped trap for ultracold atoms using an all-optical approach. It introduces a doubly-dressed-state (DDS) scheme that uses spatially structured light to form a central repulsive barrier and a surrounding trapping edge, yielding a spherical bubble trap in microgravity. Analytical results connect trap geometry and timescales to laser detunings, saturation, and polarizability, and the authors apply the method to rubidium-87 with realistic parameters, proposing an experimental implementation with parabolic painted potentials and a compensation laser to extend lifetimes. This work offers a flexible platform for shell-BEC physics and microgravity experiments, with potential extensions to alkaline-earth species and broad implications for studying curved quantum gases and related many-body phenomena in 3D shell geometries.

Abstract

In this paper, we present an all-optical method to produce shell-shaped traps for ultracold atoms in microgravity. Our scheme exploits optical double dressing of the ground state to create a short range strongly repulsive central potential barrier. Combined with a long range attractive central potential, this barrier forms the shell trap. We demonstrate that a pure spherical bubble, reaching the quasi 2D regime for standard atom numbers, could be formed from two crossed beams with a parabolic profile. An analytical study shows that the relevant characteristics of the trap depend on the ratio of the ground and excited state polarisabilities and the lifetime of the excited state. As a benchmark, we provide quantitative analysis of a realistic configuration for rubidium ensembles, leading to a 250 Hz transverse confinement for a 35 $μ$m radius bubble and a trap residual scattering rate of less than 10 s$^{-1}$.

Figures (7)

• Figure 1: a) 3D representation of a bubble-shaped potential b) The three-level system is dressed by two lasers, the first one $\omega_{L,1}$ is blue-detuned on the $\ket{2} \rightarrow \ket{3}$ transition and red-detuned compared to $\ket{1} \rightarrow \ket{2}$. c) Eigen-energy profiles of $\ket{1}$ and $\ket{2}$ states along the $z$ direction when dressed by $\omega_{L,1}$ alone. The two shifts are parabolas of opposite sign. When dressed a second time by a laser radiation at $\omega_{L,2}$ this leads to d), i.e., a double well at the crossing of the double dressed state, corresponding to a sphere in 3D. The final doubly-dressed state potential comes from the relative probability for the atom to be in the state $\ket{1}$ (blue) or $\ket{2}$ (red) spatially modulated by the effective detuning $\Delta(r)$.
• Figure 2: Doubly-dressed state potential in the radial direction leading to a bubble-shaped trap with spherical symmetry in 3D. The inset shows the probability densities $|\tilde{\Psi}(r)|^2$ of the first four eigenstates without interaction and normalized in 3D in spherical coordinates.
• Figure 3: Ratio of the lightshifts $\tilde{U}_2/\abs{\tilde{U}_1}$ at low intensity $I=3800$ W.cm$^{-2}$ versus the wavelength of the first dressing laser $\lambda_{L,1}=2\pi c/\omega_{L,1}$. The two gray shaded areas show the zone where the laser is close to the two resonances $\ket{5P_{3/2}, F=3, m_F=3}\rightarrow \ket{4D_{5/2}}$ and $\ket{5P_{3/2}, F=3, m_F=3}\rightarrow \ket{4D_{3/2}}$.
• Figure 4: Kinetic, potential, interaction and total energy versus the bubble radius which is controlled by varying the radius of the parabolic beam at constant laser intensity. Wave-function probability density calculated using the numerical model for a radius $r_{\rm bubble}=4.5$ µm (b) and $r_{\rm bubble}=36$ µm (c) respectively, illustrating a cross-over from the Thomas-Fermi regime (dashed green line) to the non-interacting regime (dashed red line).
• Figure 5: Parameters of the quantum bubble trap versus the lightshift ratios: (a) Kinetic $E_{k}$, potential $E_{p}$ and interaction energy $E_{i}$; (b) product of the lifetime $\tau$ times the trap frequency $\omega$ as the figure of merit; (c) Final trap depth $\tilde{U}_t$; (d) diffusion lifetime due to spontaneous emission.