Combined chips for atom optics

We present experiments with Bose-Einstein condensates on a combined atom chip. The combined structure consists of a large-scale “carrier chip” and smaller “atom-optics chips,” containing micron-sized elements. This allows us to work with condensates very close to chip surfaces without suffering from fragmentation or losses due to thermally driven spin flips. Precise three-dimensional positioning and transport with constant trap frequencies are described. Bose-Einstein condensates were manipulated with submicron accuracy above atom-optics chips. As an application of atom chips, a direction sensitive magnetic-field microscope is demonstrated.

Figure 1

(a) Three wire configuration on a chip used for a magnetic waveguide potential. The central conductor QP2 carries current opposite to the outer conductors QP1 and QP3. A waveguide is formed above the chip, as indicated by the magnetic-field lines. Marked also is the thickness of the substrate $s_0=250\mu \mathrm{m}$ as used for the carrier chip in our experiment. (b) Conductor geometry for a microtrap with three-dimensional confinement. A waveguide potential is generated by the conductors QP1, QP2, and QP3. The magnetic field of a pair of perpendicular conductors T1 and T5 provide axial confinement with nonvanishing offset field. An equidistant set of perpendicular conductors (“transport conductors”) can be used for an axial displacement. The geometry of conductors is marked: the distance between the QP wires $d_{\mathrm{QP}}$ is $750\mu \mathrm{m}$ and the distance between the transport wires is $d_T=650\mu \mathrm{m}$. (c) Translation of the trap. If the current in T1 and T5 is replaced by a current in T2 and T6, the trap is axially displaced by the distance of $d_T=650\mu \mathrm{m}$.

Figure 2

Position of the waveguide in the $x,y$ plane. Solid lines are trajectories for a constant ratio of $\gamma =I_{\mathrm{QP}1}∕I_{\mathrm{QP}3}$ and increasing $I_{\mathrm{QP}2}$. Dashed lines are trajectories for constant $I_{\mathrm{QP}2}$ and varying $\gamma =I_{\mathrm{QP}1}∕I_{\mathrm{QP}3}$. The sum of the currents $I_{\mathrm{QP}1}+I_{\mathrm{QP}3}=2\mathrm{A}$ was kept constant in this plot.

Figure 3

(Left) The axial magnetic potential for five displacements (0, 162.5, 325, 487.5 and $650\mu \mathrm{m}$) during transport over $d_T=650\mu \mathrm{m}$ (solid line). Matching the currents in T1, T2, T4, and T5 [Eq. (11)] results in a smooth transport without changing the trap frequencies. (Right) Absorption images of thermal clouds for the corresponding displacements. The dashed line shows the chip surface. In this experiment the currents were driven linearly, resulting in a variation of the axial potential (dashed potential curves on the left).

Figure 4

Carrier chip. (a) Schematic plot of the dual-layer atom chip with conductors at both surfaces of the substrate and contact pads on the top. (b) Central part of the carrier chip. Indicated are the conductors QP1, QP2, and QP3 on the top and the transport wires T1–T8 on the back surface of the chip. The eight transport wires are periodically repeated underneath the chip. Electrical connection between the separate blocks of T1–T8 is achieved by running the wires through laser cut holes from underneath to the top surface of the chip and back. The additional wire pattern between QP1 and QP2, as well as between QP2 and QP3 are not used for the experiments described in this article. These can be used for further microtraps as described elsewhere [16].

Figure 5

(Color online) (a) Combined chip. Atom-optics chips with micron scaled conductor patterns are attached to the surface of the carrier chip. (b) Microscope image of a meandering conductor structure deposited on the atom-optics chip on the left. Conductors of $1\mu \mathrm{m}$ width are separated by gaps of $1\mu \mathrm{m}$. (c) Schematics and connection scheme of the nested meander patterns M1 and M2. The simplified geometry (bottom) is used for calculating magnetic fields in the center area of the structure.

Figure 6

Position of a thermal cloud during transport over a distance of 1.95 mm. The transport starts at $t=0$. The atoms are accelerated to a velocity of $v_T=2.6\mathrm{mm}∕\mathrm{s}$ over 500 ms. For the next 250 ms the atoms move at constant velocity $v_T$ before they are decelerated within another 500 ms until they come to rest at $t=1.25\mathrm{s}$. The solid line shows the expected position, calculated from the currents in the wires. The inset image shows the number of atoms estimated from the absorption images changing in time.

Figure 7

Eliminating axial center-of-mass oscillations of a Bose-Einstein condensate. Axial position of a condensate, initially oscillating with $\omega =2\pi \times (16.4\pm 0.3){\mathrm{s}}^{-1}$ and $A=(36.8\pm 1.6)\mu \mathrm{m}$, after 25 ms time of flight. The center-of-mass motion is stopped by a phase-matched displacement of the trap center using the conveyor.

Figure 8

Positioning condensates on a meandering wire pattern. Plot of the position of Bose-Einstein condensates after 25 ms ballistic expansion for currents $I_M=+1\mathrm{mA}$ (solid squares) and $I_M=-1\mathrm{mA}$ (open circles) in the meandering current path, relative to the position for zero current. The current was applied during the first 5 ms of the time of flight. The data are compared to the numerical integration of the equation of motion, based on the known structure of the meander pattern (solid and dashed lines).

Figure 9

(a) Magnetic-field microscope. Positions $z_0$ (open circles) and $z_i$ (filled squares) of the Bose-Einstein condensate for zero and for $1.2\mathrm{mA}$ current, respectively, in a thin conductor perpendicular to the waveguide. (b) $z$ component of the magnetic field of the thin conductor, as integrated from the measured gradient according Eq. (17). The data are in good agreement with the axial field component calculated for a finite wire oriented parallel to the $x$ axis.