Meissner Effect in Superconducting Microtraps

We report on the realization and characterization of a magnetic microtrap for ultracold atoms near a straight superconducting Nb wire with circular cross section. The trapped atoms are used to probe the magnetic field outside the superconducting wire. The Meissner effect shortens the distance between the trap and the wire, reduces the radial magnetic-field gradients, and lowers the trap depth. Measurements of the trap position reveal a complete exclusion of the magnetic field from the superconducting wire for temperatures lower than 6 K. As the temperature is further increased, the magnetic field partially penetrates the superconducting wire; hence the microtrap position is shifted towards the position expected for a normal-conducting wire.

Figure 1

Front view (a) and top view (b) of the experimental system. A single vacuum chamber contains a standard room-temperature trap setup (to the right) and a superconducting microtrap on a helium flow cryostat (to the left). Atoms are initially prepared in MOT and magnetic trap, and subsequently transported to the superconducting microtrap by means of optical tweezers. The optical tweezers are translated over 44 mm with an air-bearing translation stage outside the vacuum chamber.

Figure 2

Superconducting microtrap. The trap is generated below the Nb wire by applying a current $I_{\mathrm{Nb}}$ and a bias field ${\mathit{B}}_B$ in the $x$ direction. The Nb wire with circular cross section of diameter $125\mu \mathrm{m}$ is clamped in a slit of a gold-plated copper holder which is attached to the cryostat. It can be cooled to variable temperatures down to 4.2 K. A small cut in the copper holder gives free access to the Nb wire at the central part of the microtrap. A pair of kapton-insulated offset wires along the $x$ direction, just above the Nb wire, are used for the axial confinement. Gravity is along the $y$ direction.

Figure 3

(a) Absorption image of the atom cloud at the central region of the magnetic trap. The dashed lines represent the longitudinal axis of the trap and the Nb wire surface. (b) Optical density of one of the transverse sections of the atom cloud. The best-fit curve $\rho (y)$ is also plotted.

Figure 4

(a) Distance between the trap center and the wire center as a function of the applied current $I_{\mathrm{Nb}}$ for six different temperatures. $|{\mathit{B}}_B|=0.64\mathrm{mT}$ and $B_0={10}^{-4}\mathrm{T}$. The normal-conducting case is represented as a dashed curve. The error bars, which result from the fitting procedure illustrated in Fig. 3, are smaller than the symbols and therefore not represented. (b) Best-fit radius as a function of the Nb wire temperature. The error bars represent the 95% confidence interval. (c),(d) Modulus of the magnetic-field profiles calculated for $I_{\mathrm{Nb}}=0.45\mathrm{A}$ for the normal-conducting and the superconducting cases, respectively. The comparison shows that the Meissner effect strongly distorts the bias field while leaving the field of $I_{\mathrm{Nb}}$ unchanged. This results in a lower trap depth and a shorter distance to the Nb wire. The realization of the microtrap requires that the bias field is opposite in direction to ${\mathit{B}}_{\mathrm{Nb}}$. The additional offset field $B_0={10}^{-4}\mathrm{T}$ along the $z$ direction changes the magnetic profile from linear to parabolic (dotted curve).