Molecules at almost zero temperature
Our team at the Institute for Experimental Physics at Innsbruck University has for the first time created a pure quantum gas of molecules. The starting point is a Bose-Einstein condensate (BEC) of cesium atoms (our group in Innsbruck has the only one in the world!). The atoms are glued together to form dimers in a single quantum state using resonant interactions. The temperature of the molecular cloud is almost zero: We measure temperatures below a few nanokelvin, i.e. just a few billionth of a degree above absolute zero. This is a factor of thousand below temperatures previously measured for ultracold molecules. Our results are published on August 21th in Science Express, (10.1126/science.1088876). Science 301, 1510 (2003).
Production of molecules using Feshbach resonances
We prepare the ultracold molecules by coherently transferring atoms from a BEC into a molecular bound state. The dependence of the atomic state on an external magnetic field provides a powerful tool to dramatically alter the scattering properties of two colliding atoms. By applying a homogeneous magnetic field, one can bring the unbound atomic scattering state into degeneracy with a bound molecular state, provided both states feature different Zeeman shifts. At the point of energetic degeneracy, the s-wave scattering cross-section is resonantly enhanced. This phenomenon is called Feshbach resonance. For Cesium, several resonances occur at easily accessible magnetic field strengths. Due to interactions between the colliding atoms, the potential energy curves of the molecular and atomic states do not intersect, but form an avoided crossing. By adiabatically sweeping the magnetic field from above over the resonance, one expects to convert an atomic into a molecular BEC, see figure below.
In our experiment, the starting point is a BEC of around 50.000 Cesium atoms in the hyperfine ground state with total angular momentum of 3 and a magnetic quantum number 3 in an optical dipole trap. This state features a Feshbach resonance near 20G with an estimated width of 5mG. We produce up to 3000 molecules from the atomic BEC by sweeping the magnetic field across the resonance from a higher field value with a constant rate of typically 50G/s. We determine the magnetic moment of the corresponding molecular state to a value of 0.93 Bohr's magnetons. In comparison, the magnetic moment of two atoms in the (3,3) state is 1.5 Bohr's magnetons.
The optical trap on its own is too weak to support the atoms against gravity. Thus, we apply a magnetic gradient field which is exactly calibrated as to cancel the gravitational force on our atoms. This setup provides instant Stern-Gerlach separation of molecules and atoms: Since the molecules have a smaller magnetic moment per mass, they start falling in the atomic levitation field with an acceleration of 0.38g. The two clouds are completely separated after 3ms. Alternatively, we raise the levitation gradient at the end of the ramp to the value required to levitate the molecules. In this case, the atoms are accelerated upwards at 0.61g. During the production process, the optical trap is shut off. We detect the molecules by reversing the production process, i.e. by applying a sweep over the resonance from below back to the starting point. The molecular cloud is converted into atoms and can be investigated by means of absorption imaging. The ability to levitate and directly image the molecules allows us to monitor the evolution of the molecular ensemble.
Ultralow expansion energies
We measure the expansion energies in the vertical and horizontal direction by levitating the molecules and varying the time before reconversion and imaging. From these time-of-flight images, we find energies of a few nK.
The Feshbach sweeping is a coherent process. Thus, the vanishing expansion energies together with the molecular density at the point of creation intimate that we have created a macroscopic molecular matter wave. Ongoing experiments aim at the demonstration of the coherence of the molecular cloud.
We are supported by the Austrian Science Fund (Fonds zur Förderung der wissenschaftlichen Forschung, FWF) in the frame of the Spezialforschungsbereich F15 "Control and Measurement of Coherent Quantum Systems" and by the European Union in the frame of the Cold Molecules TMR Network.
last change: 06-01-23 by JH