Persistent Supercurrent Atom Chip

Rubidium-87 atoms are trapped in an Ioffe-Pritchard potential generated with a persistent supercurrent that flows in a loop circuit patterned on a sapphire surface. The superconducting circuit is a closed loop made of a $100\mu \mathrm{m}$ wide molecular-beam epitaxy-grown ${\mathrm{MgB}}_2$ stripe carrying a supercurrent of 2.5 A. To control the supercurrent in the stripe, an on-chip thermal switch operated by a focused argon-ion laser is developed. The switch operates as an on/off switch of the supercurrent or as a device to set the current to a specific value with the aid of an external magnetic field. The current can be set even without an external source if the change is in the decreasing direction.

Figure 1

The two chamber vacuum system. The atoms are first trapped and precooled in the first chamber made of a glass cell. The atoms are then loaded to the magnetic quadrupole trap and transported from the glass cell to the chip chamber by moving the quadrupole trap that is mounted on a motorized linear stage.

Figure 2

The chip pattern of the persistent supercurrent atom chip made from a $1.6\mu \mathrm{m}$-thick MBE-grown ${\mathrm{MgB}}_2$ film. The inner loop (A-a: $\mathrm{\text{width}}=100\mu \mathrm{m}$) is used for a persistent current driving. The outer loop (B-b: $\mathrm{\text{width}}=200\mu \mathrm{m}$) is designed to apply radio frequency for evaporative cooling. Atoms are moved to the area marked by $T$ for trapping and to $Q$ for the distorted QMT measurement. The laser is illuminated on the appendix of the inner loop, or the area marked by $L$, when the dynamics of the thermal switch is studied.

Figure 3

(a) Pattern and density of the atoms in the QMT that are perturbed by the magnetic field induced by the persistent supercurrent on the chip (the field of view is $4\times 4\mathrm{mm}$). (b) Calculated magnetic field potential with a $5.0\times {10}^{-3}\mathrm{T}/\mathrm{cm}$ quadrupole field and $I_p=2.4\mathrm{A}$. The position of $I_p$ is indicated with an arrow. (c) Pattern and density of the atoms in the chip trap (the field of view is $2\times 2\mathrm{mm}$). The top edge of the figure is the position of the chip surface. (d) Calculated magnetic potential with $I_p=2.5\mathrm{A}$ and $B_y=1.5\times {10}^{-3}\mathrm{T}$.

Figure 4

Plot of persistent current $I_p$ left over after illuminating the switching laser pulse (0.7 W) as a function of the pulse duration time $\Delta \tau$. The inset shows the trajectory of $I_p(T)$ in the switching process. During the initial rise of the temperature, $I_p$ is constant until it reaches $I_c(T)$ (the arrow from A to B). While $T$ rises further, $I_p$ adiabatically follows $I_c(T)$ (the arrow from B to C). When the laser is switched off, the persistent current $I_p$ is preserved, and $T$ returns to the original value (the arrow from C to D).

Figure 5

Plot of the delay time $t_d$ from the onset of the laser illumination to the termination of persistent current as a function of the laser power $p$ deposited on the atom chip. The solid squares represent the experimental data when the switching laser is illuminated in the appendix of the inner loop. The solid curve is the fitting by the empirical function $t_d(p)=\frac{\alpha }{p-\beta }+\gamma$, with $\alpha =6.5\times {10}^{-3}\mathrm{J}$, $\beta =0.27\mathrm{W}$, and $\gamma =0.47\times {10}^{-3}\mathrm{s}$. The dotted curve is the experimental result when the superconductivity was broken by illuminating the outer loop at $L$ in Fig. 2.