Averaging out magnetic forces with fast rf sweeps in an optical trap for metastable chromium atoms

We introduce a time-averaged trap in which the internal state of the atoms is rapidly modulated to modify the magnetic trapping potential. In our experiment, fast radio-frequency linear sweeps flip the spins of atoms at a fast rate, which averages out the magnetic forces. We use this procedure to optimize the accumulation of metastable chromium atoms in an optical dipole trap from a magneto-optical trap. The potential experienced by the metastable atoms is identical to the bare optical dipole potential, so that this procedure allows for trapping all magnetic sublevels, hence increasing by up to 80% the final number of accumulated atoms.

Figure 1

(Color online) Absorption pictures of the atoms in the mixed trap: (1) without rf; (2) with rf. OD$\left(z\right)$ represents the optical depth integrated over $X$. The sketch describes the principle of this experiment. MOT atoms decay into $D$ states from the excited state. The magnetic potential of the $D$ states is qualitatively represented for two different times $t_1$ and $t_2$.

Figure 2

(Color online) Steady-state atom number in the optical trap as a function of minimum (squares) and maximum (triangles) rf frequency in the sweep (for a mean rf power of 80 W, and a sweep rate of the rf frequency of 10 kHz). Lines are guides for the eye.

Figure 3

Steady-state atom number in the OT as a function of the mean rf power, for rf frequency sweeps between 500 kHz and 7 MHz, at a rate of 10 kHz. Inset: same, for low rf power, and similar experimental conditions. Arrow: typical rf power value above which the adiabatic criterion (2) is satisfied.

Figure 4

Steady-state atom number in the optical trap as a function of the rf sweep rate, for an average rf power of 100 W, with ${\nu }_{\text{min}}=500\text{kHz}$, and ${\nu }_{\max }=7.5\text{MHz}$. Atom numbers are normalized by reference to the OT population with no rf field.