Tunable Miscibility in a Dual-Species Bose-Einstein Condensate

We report on the observation of controllable phase separation in a dual-species Bose-Einstein condensate with ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$. Interatomic interactions between the different components determine the miscibility of the two quantum fluids. In our experiments, we can clearly observe immiscible behavior via a dramatic spatial separation of the two species. Furthermore, a magnetic-field Feshbach resonance is used to change them between miscible and immiscible by tuning the ${}^{85}\mathrm{Rb}$ scattering length. The spatial density pattern of the immiscible quantum fluids exhibits complex alternating-domain structures that are uncharacteristic of its stationary ground state.

Figure 1

(a) Schematic drawing of the optical trap laser beam and the magnetic-field coils. The curvature of the magnetic field enhances confinement along the axial direction ($z$) of the optical trap. The center position of the trapping potential along its axis is influenced by the tilt of the optical trap relative to gravity and the relative position of the optical trap beam focus and the coils. Optical trap tilt and misalignment of the beam focus and the coils have been exaggerated for clarity. Absorption images are acquired along the axis that is into the page. (b),(c) Absorption images of single-species ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ condensates demonstrate a smooth density profile, consistent with the shape of the trapping potential. Prior to imaging, the gas is released from the optical trap and expands for 20 ms; the magnetic field remains on for the first 10 ms of expansion to prevent collapse of the ${}^{85}\mathrm{Rb}$ gas.

Figure 2

Absorption images at two different ${}^{85}\mathrm{Rb}$ scattering lengths that display (a),(b) immiscible and (c),(d) miscible behavior. Image (c) has been scaled by a factor of 5 for clarity. Each image in the pairs (a),(b) and (c),(d) was acquired with different condensates after creation under the same conditions. The sum of each pair should be interpreted as the total density of the expanded gas. (e),(f) The measured two-dimensional optical density (OD) was summed in the remaining radial direction to produce a cross section. The ${}^{85}\mathrm{Rb}$ scattering length was $51a_0$ for images (a) and (b) and it was $780a_0$ for images (c) and (d). The number of ${}^{85}\mathrm{Rb}$ (${}^{87}\mathrm{Rb}$) atoms is $4.5\times {10}^4$ ($9\times {10}^4$).

Figure 3

Measured axial position of the ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ gases as a function of the parameter $\Delta$. We report the uncertainty in the position as the shot-to-shot reproducibility of our measurements. When $\Delta >0$, the measured difference in the position of each gas is in good agreement with that expected for single-species condensates. The number of ${}^{85}\mathrm{Rb}$ [${}^{87}\mathrm{Rb}$] atoms is $5.0(5)\times {10}^4$ [$1.2(2)\times {10}^5$].

Figure 4

(a)–(e) Absorption images of ${}^{85}\mathrm{Rb}$ (left column) and ${}^{87}\mathrm{Rb}$ (right column) condensates demonstrating the immiscibility of the quantum gases. For clarity, the OD of each ${}^{87}\mathrm{Rb}$ image was scaled by a factor of 0.5, and the size of each image is $330\mu \mathrm{m}\times 150\mu \mathrm{m}$. The number of ${}^{85}\mathrm{Rb}$ particles varies from approximately $1.2\times {10}^4$ in (a) to $2.2\times {10}^4$ in (e); the ${}^{87}\mathrm{Rb}$ number varies from $1.4\times {10}^5$ in (a) to $3\times {10}^3$ in (e). For comparison, images of single-species ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ condensates are shown in Figs. 1b, 1c. (f) Absorption images following recompression of the optical trap that demonstrate a more symmetrical radial distribution.