Controlling stray electric fields on an atom chip for experiments on Rydberg atoms

Experiments handling Rydberg atoms near surfaces must necessarily deal with the high sensitivity of Rydberg atoms to (stray) electric fields that typically emanate from adsorbates on the surface. We demonstrate a method to modify and reduce the stray electric field by changing the adsorbate distribution. We use one of the Rydberg excitation lasers to locally affect the adsorbed dipole distribution. By adjusting the averaged exposure time we change the strength (with the minimal value less than $0.2\mathrm{V}/\mathrm{cm}$ at $78\mu \mathrm{m}$ from the chip) and even the sign of the perpendicular field component. This technique is a useful tool for experiments handling Rydberg atoms near surfaces, including atom chips.

Figure 1

(a) Two-photon excitation scheme to the Rydberg state. (b) Example of an absorption image of an atomic cloud, trapped in a magnetic $z$-wire trap. The normalized depletion is obtained by taking the ratio of the number of atoms in the “hole” area (where the excitation lasers are focused) and the number of atoms in the “ref” area. (c) Sketch of the setup (not to scale). The high-numerical-aperture lens is coated with ITO, which allows the application of a voltage between the lens and the chip to compensate for the $z$ component of the stray electric field. In order to probe the electric field on different heights, the atomic cloud (red) can be moved up and down by varying the current through the trapping wire ($z$ wire, not shown). The chip contains a stack of layers of different materials, including a $1\text{−}\mu \mathrm{m}$ SU8 polymer layer which provides thermal insulation and leads to heating of the surface (see Sec. 4). (d) Example of a Stark map at a height of $163\mu \mathrm{m}$. For each value of the voltage between the chip and the lens, a spectrum is taken by scanning the detuning of the red laser and measuring the normalized depletion. The red line is a fit to the Stark map; the values of $E_z$ and $E_{x,y}$ are retrieved from the fit.

Figure 2

Dependence of the electric field on the distance from the chip for different duty cycles of the blue laser. The result of the measurement of $E_z$ is shown with error bars. The solid lines show the result of a single collective fit (see the text for details).

Figure 3

Simulation of the electric field $E_z$ due to a distribution of dipoles on the surface. The red curves show the dipole distribution. The gold and the cyan parts show the two outermost layers of the chip, the gold and the silica, respectively. The $E_z$ is indicated by color, blue (red) indicating the electric field pointing down (up). (a) Short duty cycle of the blue laser (see Sec. 4) and (b) adsorbate distribution with a Gaussian hole in it, corresponding to a long duty cycle of the blue laser. In the area where the blue laser (dashed line) hits the surface, a hole in the adsorbate layer occurs due to thermal and light-induced atomic desorption.

Figure 4

Plot of (a) the $x$ cut and (b) the $y$ cut of dipole distributions from the data fits for different duty cycles (see Fig. 2). The distribution consists of a two-dimensional Gaussian on a broad layer of dipoles and a saturated hole in it.