First condensate of cesium in the world
In the early morning hours of Oct 5th, 2002, our team at the Institute for Experimental Physics at Innsbruck University has created the first Bose-Einstein condensate (BEC) of cesium atoms in the world. Our results are published on Dec 5th in Science Express, (10.1126/science.1079699), Science 299, 232 (2003).
Optical approach to BEC
In the Innsbruck experiment, powerful lasers are used to trap atoms in their lowest internal state (F=3, mF=3). This approach overcomes inelastic loss processes that have so far prevented standard magnetic trapping procedures to reach BEC with cesium. Starting point for the evaporative cooling experiments is an optical dipole trap consisting of two crossed CO2-laser beams, each with a power of 100W. At a wavelength of 10.65m, the CO2-laser light provides a quasi-electrostatic trapping potential of 105K depth. The average trap frequency is 14Hz at a Gaussian beam waist of 6005m. To support the heavy cesium atoms against gravity in the large and shallow trap, we apply a magnetic gradient field of 31 G/cm in the vertical direction, exactly compensating the gravitational force. This levitation field is matched to the magnetic moment of the lowest Cs state. All other spin states are untrapped, which ensures perfect spin polarization in the optical trap. The CO2-laser trap is loaded from a standard magneto-optical trap via Raman sideband cooling in an optical lattice. After an initial evaporation phase of 10s at constant potential depth, it provides typically 2 x 106 polarized atoms at ~15K and a phase-space density of ~10-3.
An additional laser beam with a wavelength of 1064nm is then focused into the CO2-laser trap to a Gaussian beam waist of 305m. By ramping up the power of this beam from 0 to 90mW within 5s, we create a tight "dimple" in the trapping potential, which locally increases the phase-space density. Within this dimple, about 3 x 105 atoms are trapped at a phase-space density of ~10-2. Turning off one of the CO2 lasers removes all atoms that are not trapped in the dimple.
Evaporative cooling towards BEC now proceeds in the dimple potential. By ramping down the power P1064 of the 1064-nm beam, the total potential depth is reduced in several connected linear ramps from 155K to a variable final value within 17s. The transition to BEC is observed with 60,000 atoms at a temperature of 45nK with P1064 = 3.5mW (trap depth ~570nK). Pure condensates with 16,000 atoms are produced with P1064 reduced to 1.0mW (trap depth ~170nK). A clear transition and bimodal distributions are observed in time-of-flight images as the trap depth is ramped down. In the condensation movies below one can see the evolution of the density distribution after 50ms of expansion at a varying level of the final power of the 1064-nm laser.
Tunability of interactions
A special property of a Cs-BEC is its magnetic "tunability" at easily accessible fields. The interaction strength between atoms in the condensate is characterized by the s-wave scattering length a. In the case of Cs, this parameter depends strongly on the external magnetic field via so-called "Feshbach resonances". In our magnetic levitation trap, we are free to choose the magnetic field at the trap center without changing the shape of the trapping potential. Through this effect, we can tune the interaction from strongly attractive (negative a) to strongly repulsive (positive a).
Control of the interatomic interaction is crucial for the attainment of the BEC. For each stage of the experiment, we chose the optimum interaction for the evaporation process. The BEC is finally produced at 23G, where the scattering length is 300 times the Bohr radius a0.
In first experiments with the cesium BEC, we have demonstrated its magnetic tunability by switching to different magnetic fields. We have demonstrated imploding, exploding and "frozen" condensates. A "frozen" condensate is realized when the interatomic interaction is turned off by an appropriate choice of the magnetic field (17G). Such a condensate has no internal energy and shows an extremely slow expansion when it is released from the trap. It could therefore serve as an ideal tool for precision measurements.
Special interest in cesium
Cesium serves as our primary frequency standard, and the definition of the second (our basic unit of time) is based on cesium. The precision Cs atomic clocks could greatly benefit from the availability of a BEC. Cesium is a very heavy atom and therefore strongly influenced by relativistic effects. In a sense, the cesium atom can be used as a "microscopic elementary particle lab" for very subtle effects of great fundamental relevance.
The complete elimination of the residual thermal motion in a BEC opens up new possibilities for fundamental measurements, like precision measurements of the fine structure constant and the search for an electric dipole moment of the electron. Moreover, the large mass of cesium is of interest for ultraprecise measurements on gravitational fields.
BEC research worldwide
Bose-Einstein condensation in ultracold atomic gases was first realized in 1995 with 87Rb, 23Na, and 7Li. This pioneering work was honored with the Nobel prize 2001 in physics, awarded to Eric Cornell, Carl Wieman, and Wolfgang Ketterle. Since then, the research field has exploded with many groups working on BEC worldwide. Only a few more species could be condensed so far: Hydrogen (1998), 85Rb (2000), metastable 4He (2001), and 41K (2001). There have been several attempts to condense cesium, the Innsbruck experiment is the first one to reach this goal.
We are supported by the Austrian Science Fund (Fonds zur Förderung der
wissenschaftlichen Forschung, FWF) in the frame of the
F15 "Control and Measurement of Coherent Quantum
last change: 05-12-09 by JH