Optimized magneto-optical trap for experiments with ultracold atoms near surfaces

We present an integrated wire-based magneto-optical trap for the simplified trapping and cooling of large numbers of neutral atoms near material surfaces. With a modified $\mathrm{U}$-shaped current-carrying $\text{Cu}$ structure we collect more than $3\times {10}^8$ ${}^{87}\mathrm{Rb}$ atoms in a mirror magneto-optical trap without using quadrupole coils. These atoms are subsequently loaded to a $\mathrm{Z}$-wire trap where they are evaporatively cooled to a Bose-Einstein condensate close to the surface.

Figure 1

Vector plots of different field configurations. The solid (dashed) lines indicate the axes of the approximated (ideal) quadrupole fields. (a) Ideal quadrupole field, (b) regular $\mathrm{U}$-wire quadrupole field with untilted bias field, (c) optimized $\mathrm{U}$-wire quadrupole field. The bottom parts of (a)–(c) show a cross section of the wire (black), the substrate, and the surface above it (gray). (d) Angular deviations from the ideal quadrupole axes are plotted as a function of the distance from the reflecting surface along the two $45°$ light beam paths [dashed lines in (b) and (c)]. The solid (dashed) lines correspond to the regular (optimized) $\mathrm{U}$-wire configuration. The zero point of the position axes is chosen to be the center of the quadrupole field (field zero). The broad wire $\mathrm{U}$ clearly approximates the ideal quadrupole field better throughout a larger spatial region than the thin wire U. The parameters chosen in these examples were $I_{U-wire}=55\mathrm{A}$, $B_{\Vert }=14.5\mathrm{G}$ $\left(12.8\mathrm{G}\right)$ in the plane parallel to the wire and $B_{\perp }=0\mathrm{G}$ $\left(3.0\mathrm{G}\right)$ perpendicular to the wire for the regular (optimized) $\mathrm{U}$.

Figure 2

(a) Atom chip assembly: The $\mathrm{U}$-wire structure for the MOT and an additional structure containing $\mathrm{Z}$-shaped wires in several sizes are connected to high-current vacuum feedthroughs. The atom chip is mounted directly on top of these wire structures. (b) Photograph of the wire structures fitted into a MACOR ceramics holder. (c) Results of a numerical calculation of the current-density distribution in the $\mathrm{U}$ wire. Dark (light) shades correspond to high (low) current densities. The thick connecting leads ensure a homogenous fanning out of the current through the central plate as it is needed to improve the quadrupole field for the MOT.

Figure 3

Top: The number of atoms is plotted vs the tilting angle between the bias field and the plane of the broad $\mathrm{U}$-shaped wire. For these measurements the $\mathrm{U}$ current was $55\mathrm{A}$, the bias field strength $13\mathrm{G}$. Bottom: corresponding vector field plots for three different angles ($0°$, $13°$, and $26°$) as indicated by the arrows. The maximum number of atoms is trapped at a bias field angle of $13°$ where the shape of the field is closest to an ideal quadrupole field. Note that this optimal trap position is not centered above the wire center.