&Bullet; physics 14, s55
A new “Lensing” technique counteracts the spread of an ultracold atomic cloud in a tiny waveguide.
An ultracold atomic cloud spreads over time, just like a ray of light spreads in space, but an electromagnetic “lens” can “focus” the cloud and reduce its kinetic energy. The lowest energies ever reached – in the picokelvin (pK) range – require large facilities that allow atoms to fall freely for a few seconds. Now researchers have demonstrated a lens that works in this regime with atoms in an annular waveguide less than a millimeter in diameter  . The manipulation of these “matter waves” in small spaces is important in order to exploit their quantum effects for purposes such as high-precision navigation.
The team, led by Wolf von Klitzing from the Institute for Electronic Structure and Lasers, part of the Foundation for Research and Technology – Hellas in Greece, placed a Bose-Einstein condensate (BEC) of rubidium atoms several hundred micrometers wide in a ring-shaped electromagnetic trap , much like placing a marble in a roulette wheel. To start moving the “marble”, the team tilted the ring towards the atoms and then rotated it around its original vertical axis so that the lowest point rotated at up to 10 Hz for about 200 ms. Next, they aligned the ring and let the cloud spread freely as it sped around. Finally, the team focused the BEC by again tilting the rotating ring towards the atoms for 17 ms.
The researchers showed that the tilt angle was analogous to the focusing power of an optical lens. Too much tilt would cause the cloud to briefly shrink and then enlarge, like an overly powerful lens focusing the light in front of the intended focal plane. The ideal tilt of about 5 ° kept the cloud at a fixed size for almost a second and reduced its expansion energy to 800 pK.
David Ehrenstein is Senior Editor for Physics.
- S. Pandey et al., “Atomtronic matter wave lenses” Phys. Rev. Lett.126170402 (2021).