Atom-chip Bose-Einstein condensation in a portable vacuum cell

A ${}^{87}\mathrm{Rb}$ Bose-Einstein condensate (BEC) is produced in a portable atom-chip system less than $30\times 30\times 15\mathrm{cm}$, where the ultrahigh vacuum is maintained by a small, $8\mathrm{L}∕\mathrm{s}$, ion pump and nonevaporable getter. An aluminum nitride chip with lithographically patterned copper is used to seal the vacuum system, provide the electrical feedthroughs, and create the magnetic trap potentials. All cooling and trapping processes occur $0.6–2.5\mathrm{mm}$ from ambient laboratory air. A condensate of about 2000 ${}^{87}\mathrm{Rb}$ atoms in $F=2,m_F=2$ is achieved after $4.21\mathrm{s}$ of rf forced evaporation. A magneto-optical trap lifetime of $30\mathrm{s}$ indicates the vacuum near the chip surface is about ${10}^{-10}\text{torr}$. This work suggests that a chip-based BEC-compatible vacuum system can occupy a volume of less than $0.5\mathrm{L}$.

Figure 1

Portable miniature vacuum cell for the production of a chip BEC. (a) Complete vacuum cell system, (b) detail of cell assembly.

Figure 2

The atom chip and its wire pattern. Hatch marks indicate placement of the cell (outside edges) on the chip. (a) View of the whole chip pattern; (b) center detail of wires where the BEC is produced. The $U$ wire ($I_U$, $200\mu \mathrm{m}$ wide) is used to create chip MOT, and the $Z$ wire (shown in gray, $I_Z$, $100\mu \mathrm{m}$ wide) is used to create an IP type magnetic trap by applying a $y$-bias field. The other wires seen in (a) are not used in this experiment.

Figure 3

Bose-Einstein condensation is observed. (a) Absorption images are taken after a $5\mathrm{ms}$ time of flight (TOF) with different final rf frequencies. From left to right: (1) $\nu =3.5\mathrm{MHz}$, $N=42\times {10}^3$, $T=4.4\mu \mathrm{K}$, $⟨\rho ⟩\approx 6\times {10}^{-4}$; (2) $\nu =3.0\mathrm{MHz}$, $N=20\times {10}^3$, $T=1.4\mu \mathrm{K}$, $⟨\rho ⟩\approx 7\times {10}^{-3}$; (3) $\nu =2.9\mathrm{MHz}$, $N=10\times {10}^3$, $T=515\mathrm{nK}$, $⟨\rho ⟩\approx {10}^{-1}$; (4) $\nu =2.85\mathrm{MHz}$, $N=3\times {10}^3$, $T=230\mathrm{nK}$, $⟨\rho ⟩\approx 10$. (b) TOF images of BEC cloud after release with final rf frequency $\nu =2.85\mathrm{MHz}$. From left to right: TOF (1) $t=1\mathrm{ms}$, (2) $t=3\mathrm{ms}$, (3) $t=5\mathrm{ms}$, (4) $t=7\mathrm{ms}$, and (5) $t=9\mathrm{ms}$. The nonisotropic shape during TOF is a key signature of BEC. [The images in (a) are vertically offset relative to (b) no. 3 so that they are centered.]