Stable Neutral Atom Trap with a Thin Superconducting Disc

A stable magnetic quadrupole trap for neutral atoms on a superconducting Nb thin-film disc is demonstrated. The quadrupole field is composed of the magnetic field that is generated by vortices on the disc introduced by cooling the disc across the transition temperature with a finite field and an oppositely directed uniform field applied after cooling. The trap is stable when all trapping processes are performed above the dendritic instability temperature $T_a$. When the field intensity is changed below this temperature, the quadrupole field collapses and the trap disappears. The initial vortex density decreases even when the external field is changed at a temperature $T>T_a$. However, the vortex density is stabilized at an equilibrium density, whereas at $T<T_a$, it almost completely disappears. A stable trap can be formed, even when the initial vortices are introduced through a dendritic avalanche.

Figure 1

An absorption image of the trapped atomic cloud. The bottom of the granular part is the surface of Nb film. $B_i=1.5\mathrm{mT}$, $B_m=1.25\mathrm{mT}$, and $T_0=7.4\mathrm{K}$.

Figure 2

The trap height as a function of $B_i/B_m$. $B_m=0.92\mathrm{mT}$ was fixed. The temperature was $T_0=7.4\mathrm{K}$. ■: measured trap height. ○: theoretical height when the flux magnitude before cooling was preserved until the time of the trap observation. The inset shows the sequence of the operation in the $B\mathrm{\text{−}}T$ plane.

Figure 3

Trap height versus applied field $B_m$ at various temperatures $T_m$. The trap height decreases with $B_m$. If $T_m>T_a$, the height remains at a finite value. If $T_m<T_a$, the trap suddenly vanishes when $B_m$ exceeds a certain value. Zero height means that trapping was not observed 2 s after the introduction of the atomic cloud. $B_i=1.5\mathrm{mT}$ for all points. The inset shows the sequence of operation.

Figure 4

The boundary between the trapping and no-trapping regions. The boundary was not a definite line and could be defined up to $\pm 0.2\mathrm{K}$. Open symbols show the case when trapping was observed, and filled symbols indicate that there was no trapping. The figure includes points with different initial field $B_i$. We found practically no variation.

Figure 5

(a) Trap height versus ramp-down or -up temperature of the initial field $B_i$. The filled circles show the trap height when the disc was cooled with the applied field $B_i=1.5\mathrm{mT}$. The open circles show the height when the disc was cooled without a field. Zero height means that trapping was not observed 2 s after the loading of the atomic cloud. The inset shows the sequence of operation. Dark gray (blue) arrows show the sequence when the disc is cooled across $T_c$ with a finite field, $B_i\ne 0$, (filled circles in the graph). Light gray (orange) arrows show the sequence when $T_c$ is crossed without field, $B_i=0$ (open circles). Bottom figures are atomic clouds obtained with $B_i=1.5\mathrm{mT}$ and $T_m=6.0\mathrm{K}$. (b) is at 2 s and (c) at 10 ms after the loading of atomic cloud. (b) is 5 times intensified in density scale.