Ejection of magnetic-field-sensitive atoms from an optical dipole trap

Rubidium atoms prepared by evaporative cooling in an optical dipole trap are used in Stern-Gerlach type experiments. The analysis of the magnetic state distribution in the trap and during free fall demonstrates the possibility of ejecting all atoms with $m_F\ne 0$ from the optical dipole trap. This is achieved by applying an appropriately located inhomogeneous magnetic field. We investigate the dynamics of this cleaning process, and record the temporal history of atom positions under the combined action of magnetic field and dipole-trap potential. The experimental findings are fully supported by realistic numerical simulations of the atomic dynamics. We show that analysis of the ejected atoms provides a means for nondestructive thermometry of a trapped atom cloud.

Figure 1

The timing sequence of the Stern-Gerlach experiments. The magnetic field is switched from A to B for a time period $\Delta t$ prior to switching off the ${\text{CO}}_2$ laser.

Figure 2

(Color online) Absorption images of the cold atom cloud for a fixed free fall time of $t_{\text{fall}}=10\text{ms}$ as a function of the time $\Delta t$, the time when the magnetic-field gradient is switched on. Arrows mark the position of the dipole trap. (a) $\Delta t=0\text{ms}$, (b) $\Delta t=10\text{ms}$, and (c) $\Delta t=110\text{ms}$.

Figure 3

The dipole-trap axis (double line) is tilted by $55°$ with respect to the symmetry axis of the anti-Helmholtz coils (dashed). Magnetic-field zero is the origin of the $X$ and $Y$ axes. The gravitational axis $z$ is perpendicular to the drawing. All absorption images shown in this paper are in the $y\text{−}z$ plane.

Figure 4

(Color online) Absorption images recorded right after the ${\text{CO}}_2$-laser had been turned off. The magnetic-field gradient was turned on $\Delta t=25\text{ms}$ prior to recording these images. The trap-laser power at the end of the evaporation phase, $P_f$, is (a) 130, (b) 200, and (c) 300 mW.

Figure 5

(Color online) Absorption images for different magnetic-field switching times, $\Delta t$, recorded 1 ms after the ${\text{CO}}_2$ laser had been turned off. The time $\Delta t$ in ms is given in the top right corner of each image. The laser power at the end of the evaporation phase is $P_f=180\text{mW}$.

Figure 6

(Color online) Experimental axial positions of the cloud centers of the $m_F=0,\pm 1$ atoms along the $y$ axis as a function of $\Delta t$ are given by the thick dots. The distance scale is measured relative to the position of $m_F=0$ atoms at $\Delta t=0\text{ms}$. The dotted lines give the result of the simulation, discussed in Sec. 4A.

Figure 7

(Color online) Absorption images at various free fall times $t_{\text{fall}}$ (a) 0.5, (b) 5, and (c) 10 ms. The atoms had experienced the magnetic-field gradient in the dipole trap for a time $\Delta t=20\text{ms}$ when the trap-laser power was at $P_f=150\text{mW}$.

Figure 8

(Color online) Potential-energy contours for the three substates $m_F=\left\{0,\pm 1\right\}$ at $P_f=300\text{mW}$. Energy zero is referenced to the position $\left(-1.7,-135,-4\right)\mu \text{m}$ for $m_F=\pm 1$ and to $\left(0,0,-4\right)\mu \text{m}$ for $m_F=0$. The thick (red and blue) contours are drawn at 100 and 500 nK, respectively.

Figure 9

(Color online) Simulated images at different magnetic switching times. (a) $\Delta t=0\text{ms}$, (b) $\Delta t=10\text{ms}$, and (c) $\Delta t=110\text{ms}$. The fall time $t_{\text{fall}}=10\text{ms}$ and the atom temperature $T=340\text{nK}$ are identical to the parameters that apply for the experimental images shown in Fig. 2.

Figure 10

(Color online) Absorption images at $t_{\text{fall}}=1\text{ms}$ for different magnetic-field switching times $\Delta t$. This time is given in ms in the top right corner of each image. The final laser power is $P_f=180\text{mW}$. This is to be compared with the results in Fig. 5.

Figure 11

(Color online) The horizontal width of the ejected cloud of $m_F=-1$ atoms. The fitted FWHM values are given by the black dots for a gravitational barrier of $1.3\mu \text{K}$. The gray (red) diamonds are for a height of $3.3\mu \text{K}$. The crosses refer to experiments recorded after 5 and 7 s evaporation in the dipole trap, respectively. The gray curve shows the variation in the HO width, $a_n$.