Trapping of Rb Atoms by ac Electric Fields

We demonstrate trapping of an ultracold gas of neutral atoms in a macroscopic ac electric trap. Three-dimensional confinement is obtained by switching between two saddle-point configurations of the electric field. Stable trapping is observed in a narrow range of switching frequencies around 60 Hz. The dynamic confinement of the atoms is directly visualized at different phases of the ac switching cycle. We observe about ${10}^5$ Rb atoms in the $1{\mathrm{mm}}^3$ large and several microkelvins deep trap with a lifetime of approximately 5 s.

Figure 1

Cross section of the ac trap. The two end cap and two ring electrodes of semicircular shape are arranged for axial symmetry along the $z$ axis (vertical) and mirror symmetry with respect to the $z=0$ plane. The distance between the end caps is 6 mm, the inner diameter of the rings is 6.7 mm, the other dimensions are indicated in millimeters. The voltage applied is indicated on each electrode for the $\rho$-focusing (left) and $z$-focusing (right) configurations. The calculated electric field strengths close to the trap center are shown in the contour plots (color scale in $\mathrm{kV}/\mathrm{cm}$). The saddle point is vertically displaced due to the gravity-compensating quadrupolar field component.

Figure 2

Absorption images of the Rb cloud at different times within the 61st switching cycle, i.e., at the beginning, middle, and end of the $\rho$-focusing [(a)–(c)] and at the middle and end of the $z$-focusing [(d),(e)] phase. The corresponding times are indicated in the switching pattern; the switching frequency is 60 Hz, giving a period of $T=16.7\mathrm{ms}$. In (b) and (d) the atoms are at the turning points of the micromotion. The image area is $2.2\times 2{\mathrm{mm}}^2$, such that the field of view covers exactly the space between the two ring electrodes. Each image is obtained by averaging 100 pictures and the corresponding optical density is indicated by the color scale.

Figure 3

Observed number of atoms after 1.0 s of ac trapping as a function of the switching frequency. Each point is the average of 60 experimental runs. Note that the range of switching frequencies suitable for trapping is rather narrow.

Figure 4

Absorption images after different free ballistic expansion times of 0.1 ms [(a),(d)], 4 ms [(b),(e)], and 7 ms [(c),(f)]. The ac trap is switched off at the end of the $z$-focusing phase for (a)–(c) and at the end of the $\rho$-focusing phase for (d)–(f). For each measurement the trapping time is 1.0 s and the switching frequency is 60 Hz. All images have the same technical characteristics as in Fig. 2.