Creation of a degenerate Bose-Bose mixture of erbium and lithium atoms

TL;DR

We realize a degenerate Bose–Bose mixture of $^{166}$Er and $^7$Li in their ground spin states by sequential laser cooling, loading into an optical dipole trap, transporting to a glass cell, and evaporative cooling of Er with sympathetic Li cooling. The Er–Li mixture exhibits highly efficient cooling thanks to a large interspecies scattering length, with a measured $a_{ErLi} \approx 100~a_0$ that supports rapid thermalization across a large mass ratio. Final states include coexisting condensates of both species, illustrating robust sympathetic cooling and stable overlap despite gravitational sag and differing trap frequencies. This platform offers access to strongly interacting, mass-imbalanced dipolar–non-dipolar quantum fluids and a route to tune interspecies interactions via Feshbach resonances for future studies of exotic quantum phases and heteronuclear molecules.

Abstract

We report the realization of a degenerate mixture of $^{166}$Er and $^{7}$Li atoms in their energetically lowest spin states. The two species are sequentially laser-cooled and loaded into an optical dipole trap, then transported to a glass cell and simultaneously evaporated to degeneracy. Er serves as the coolant for Li, and we observe efficient sympathetic cooling facilitated by a large interspecies elastic scattering cross section. Three-body losses are found to be small, making this platform promising for the study of interacting mixtures with large mass imbalance.

Figures (3)

• Figure 1: Bose-Einstein condensation of $^{166}$Er and $^7$Li. Er is cooled via forced evaporation in an optical trap, and sympathetically cools the Li cloud. The top panels show absorption images of condensed clouds and the bottom panels show the atomic density integrated along both the imaging and vertical axes, $n_{1\textrm{D}}$. The data are fit with a bimodal distribution capturing the thermal and condensed atoms, shown in dashed and solid lines respectively. (a) In the absence of Li, we produce condensates of $1.5\times 10^4$ Er atoms (time-of-flight $12~$ms). (b)-(c) Cooling an Er-Li mixture yields simultaneous condensates of both species, with $\sim 10^3$ atoms in each (time-of-flight $8~$ms and $0.8~$ms, respectively).
• Figure 2: Evolution during evaporative cooling of the phase space density at the cloud center, $\mathcal{D}$, with atom number in the central region of the optical trap, $N$. The black line indicates the threshold for condensation. Data correspond to an Er cloud in the absence of Li (blue), Er in the presence of Li (purple), and Li in the presence of Er (red). A pure erbium cloud exhibits an efficiency parameter of $\sim 3$, indicated by a dashed blue line. The addition of Li minimally affects the initial Er evaporation, but causes the efficiency to drop as the number of Er atoms decreases. This trend is well captured by a simple model (see text) shown by a dashed purple line. The observed onset of Er condensation is indicated by a purple cross. The Li cloud exhibits an increase in $\mathcal{D}$ by approximately three orders of magnitude with minimal detectable atom loss. Owing to its smaller atomic mass, Li condenses much earlier than Er (see text), indicated by a red arrow. Each point shows the average of $11$ iterations of the experiment, and error bars are statistical.
• Figure 3: Interspecies thermalization of $^{166}$Er and $^7$Li. We use a resonant light pulse with a $5~\upmu$s duration to selectively heat the Li cloud and measure the relaxation of the two species towards thermal equilibrium. By fitting the data to a simple thermalization model (see text), we infer an interspecies $s$-wave scattering length of $a_{\textrm{ErLi}} = 100 \pm 10~a_0$ at $1.3$ G.