15.02.2010
When a dilute gas is cooled close to absolute zero the motion of the gas particles have to be described by quantum mechanics: The particles start to behave as waves. If the de-Broglie wavelength reaches the mean inter particle distance, a phase transition to a new state can occur: Bosonic particles accumulate in the ground state of the system forming a Bose-Einstein condensate. Bose-Einstein condensates have been used in the past years for a variety of measurements of fundamental quantum mechanics as well as a model system for solid state physics and for quantum information.
A Bose-Einstein condensate of alkaline earth atoms has additional features: its narrow clock transitions can be used in precision measurements. A possible application could be, e.g., extremely sensitive sensors for gravity. In the PTB the possibilities to use the two alkaline earth elements Calcium and Strontium as optical clocks are investigated.
We have, for the first time, produced a BEC of alkaline earth atoms: After a two stage magneto-optical trap we load 4 .106 40Ca atoms at a temperature of 20 µK into a crossed dipole trap. By evaporative cooling we reach quantum degeneracy within 1.5 s with 105 atoms at a temperature of 200 nK. From this we can cool 2.104 atoms to a pure condensate.
Fig.: Measured density distribution of the calcium atoms. In the center of a thermal cloud the peak of a Bose-Einstein condensate is visible