News: CELSIUS experiment reaches BEC

On Tuesday April 25, 2006 we observed our first Bose-Einstein condensate (BEC) on a chip in the "CELSIUS" experiment. The signature was the sudden appearance of an anisotropically expanding cloud when smoothly increasing the phase-space density through forced evaporative cooling. One of our very first sightings of the condensate is shown here. It contains a few thousand atoms.
first image chip BEC in Amsterdam

Since then, we have optimized our evaporation trajectory and made various other improvements. Now we routinely reach Bose-Einstein condensation at at a temperature of around Tc = 1.5 µK, with 105 atoms (87Rb), in a cylindrically symmetric harmonic trap with an axial frequency of 35 Hz and radial frequency of 3.2 kHz. Further evaporative cooling results in a nearly pure condensate of about twenty thousand atoms. The images below (taken in August 2006) show a cloud just above the critical temperate (left), a partially condensed cloud (middle), and a nearly pure condensate (right) respectively.

cloud above Tc partially condensed cloud nearly pure condensate

Our chip consists of a silicon substrate with gold current-carrying wires on it created using vapor deposition. The wires patterns are defined using optical lithography, and the wires have a thickness of around 2 micron. In our initial experiments, we mainly use a single on-chip wire, with a Z-like shape. The central section of this Z is around 3 mm long and 125 micron wide; we typically send a current of just over 2 A through this wire.

assembly drawing assembly photo

In addition to the on-chip wires, there two orthogonal sets of three additional 'macroscopic' wires underneath the chip.They allow convenient manipulation of the atomic clouds on larger length scales. We typically run currents of up to 10 A through these wires. In the final stages of evaporation it turned out to be very helpful to use one of these wires for additional axial confinement.

The people involved are Aaldert van Amerongen, Jan-Joris van Es, Philipp Wicke (since July 2006) and Klaasjan van Druten (PI).

celsius-bec-group

With special thanks to

Funding of this project is provided by NWO, FOM and the EU Network "Atom Chips", see the funding page.