Bose Einstein condensates in microgravity
C. Grzeschik, M. Schiemangk, A. Dinkelaker, M. Krutzik, and A. Peters
Cooling, trapping and manipulating of neutral atoms has become one of the most exciting fields in the modern physics. Continuous improvement towards achievement of ever lower temperatures temporarily culminated in the experimental realisation of Bose-Einstein condensation (BEC) in 1995. Since that time further effords to reduce the energy of trapped atoms resulted in reaching a low-temperature record of 500 pK. Corresponding average energy per atom at this temperature equals the gravitational potential energy of a single Rb atom at a hight of 5 nm, much smaller than a typical physical dimension of a condensate. Therefore, in the low-temperature regime, earth gravity presents a major perturbation to the system.
Weightlessness offers a significant potential to extend the physics of degenerate quantum gases in new directions. First of all, in the absence of disturbing gravitational force, it is possible to "open" adiabatically the trapping potential without the need of any levitational fields compensating gravity. Reducing confinement of the trap, on one hand enlarges the physical extension of the ground state. On the other hand, lowered ground state energy and expected corresponding temperature in the fK range represent a gain of up to three orders of magnitude as compared to present experiments. Another important point is a significantly extended time of free and unperturbed evolution of a condensate released from the trap, which is crucial for precision interferometric measurements with coherent dilute matter waves.
Last but not least, microgravity is a suitable environment to investigate mixtures of cold gases, since atoms with different masses do not experience different gravitational forces and can be equally well held by the trapping potential.
Microgravity At The Drop Tower in Bremen
QUANTUS (Quantengase Unter Schwerelosigkeit), founded at the beginning of 2004, is a collaboration of several German universities. Our long-term goal is to estabilish an experimental platform in space to allow investigation of a cold quantum matter in free fall for unlimited time. At the present we focus on the implementation of an 87Rb BEC experiment at the ZARM (Center of Applied Space Technology and Microgravity) drop tower in Bremen.
The drop tower environment is in several aspects simmilar to that of a space platform. On one side it offers an excellent acceleration suppression down to the microgravity level of 10-6 g, which is four orders of magnitude better than during a parabolic flight of a zero-g aircraft. On the other side, strict requirements concerning, among others, the limited volume and low power consumption of the drop capsule, have to be fulfilled. Thus, special technical challenges in construction of the experimental setup make it to a large extent different from common earth-bound setups. The following points have been crucial for design of all mechanical, optical and electronical components:
- Low weight
- Low power consumption
- High mechanical stability
- Short time for performing the experiment
- Remote control
One of the most challenging tasks during the construction of the setup was to squeeze the whole laboratory caboodle into the slim capsule. We did it – we built the most consolidated and the heaviest capsule that has ever flown in Bremen.
The complete lasersystem for the QUANTUS project has been built in our group and in the Institute für Laserphysik in Hamburg. Close collaboration, sometimes with a slight tendency for a competition ;) has resulted in development of a compact and robust setup. The lasers have been drop-tower-tested already at the end of 2005. Since that time they have been uninterrupted in use to pre-cool the Rb atoms in the laboratory in Hannover and currently being dropped again with the whole apparatus.
In autumn 2007 we succeeded in realization of the first freely falling Bose-Einstein Condensate. Since that time the BEC-experiment has been dropped nearly 200 times and it is still alive! We optimized the experimental parameters with regard to number of condensed atoms (N=10000) and low steepness of the trapping potential (trapping frequency in 10Hz range). The condensate was released from this shallow trap and expanded freely for up to 1 second. Both, the low trapping frequency and the unprecedented time of the free evolution are not achievable in any earthbound laboratory.
The results are published in the Science Magazine: Bose-Einstein Condensation in Microgravity
Atom interferometry as the coherent manipulation of matter waves is proven to be a very promising tool for performing high precision measurements, e.g. to measure inertial forces such as gravity or rotations. The spectrum of applications of these quantum sensors covers a broad area from geodesy, through gravimetry and metrology up to addressing the fundamental questions in physics, as for instance testing the Einstein equivalence principle in the quantum domain. In preparation of the first interferometry experiments with condensed Rb atoms in microgravity we are working on the controlled transfer of the atoms into a magnetic insensitive state and the realization of different interferometer topologies. The manipulation of the matter waves is realized with Bragg diffraction as a coherent beam splitting and recombination process. With a Mach-Zehnder configuration we will be able to measure spatial and temporal coherence of the condensate in the extended parameter regime available during the free fall, which means large distances between the diffracted wave packets and long timescales within the sequence.
The second generation apparatus combines an extremely compact design and extends the physics observable in microgravity. The experiment is designed to use 87Rb and 41K as degenerate quantum gases in order to carry out experiments on tests of the Einstein equivalence principle in the quantum domain. A major experimental challenge is to design a catapult capable experimental setup, which have to withstand 30g accelerations during the catapult launch. In particular the laser system consists of hybrid integrated master-oscillator power amplifiers and highly miniaturized, spectroscopy stabilized master lasers with micro-bench technology. All optical and electronic components have been designed with stringent demands on mechanical stability and reliability. Catapult mode offers the possibility to prepare and manipulate the matter wave packets during 9 seconds of microgravity, which substantially increases the interrogation time and therefore the sensitivity of the interferometer. Ultimately, even longer timescales of microgravity will be accessible in future campaigns on sounding rockets or space-borne platforms.