Persistent Supercurrent Atom Chip
Tetsuya Mukai1, Christoph Hufnagel1, and Fujio Shimizu2
1Physical Science Laboratory,
2The University of Electro-Communications/NTT Research Professor
Neutral single atoms are promising candidate for producing quantum devices. An important technology for developing these quantum devices is a precise control of the external motion of each single atom. In general, room temperature (300 K) atoms are moving with relatively high speed at around 1,000 km/h, but with laser cooling techniques, atoms can be quickly decelerated to velocities slower than the walking speed of a human being, i.e., lower than 100 K in temperature. At such a low temperature gravitational acceleration is significant and atoms start falling down just like a macroscopic object.
Atoms with non-zero magnetic moment have weak field-seeking states. They can be trapped with a magnetic potential which competes against the gravity. The magnetic potential can be easily generated by a current. It is widely used for trapping large number of atoms. However, trapping single atoms with a magnetic field generated by a current has never been successful because of the short lifetime caused by the current noise. This is significant in a short atom-surface distance which is suitable for making strong confining single atom traps.
Recently, we have succeeded in developing a persistent supercurrent atom chip and trapping of atoms in the vicinity of a solid surface with a practically noise free magnetic field which is highly expected to trap single atoms. As plotted in Fig.1, 0.5 million rubidium atoms are trapped about 300 µm away from the atom chip surface, on which a trap is generated by driving a persistent current of 2.5 Ampere through a MgB2 superconducting loop circuit on a sapphire substrate. Apart from trapping atoms with a persistent supercurrent, we have also succeeded in controlling the persistent current with an on-chip thermal switch driven by a laser (Fig. 2) .
In future we will miniaturize the atom chip pattern and try to make single atom traps with a persistent supercurrent on a solid surface as a resource for quantum devices.
This research is partially supported by the Japan Science and Technology Agency CREST "Creation of new technology aiming for the realization of quantum information processing systems".
 T. Mukai, C. Hufnagel, et al., Phys. Rev. Lett. 98 (2007) 260407.
Fig. 1. Trapped atoms: (a) experiment and (b) calculation.
Fig. 2. Persistent current control.
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