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Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Optical Metrology

QUANTUS - Bose Einstein condensates in microgravity

A. Dinkelaker, C. Grzeschik, J. Pahl, M. Schiemangk, M. Krutzik, and A. Peters

 

Introduction

Cooling, trapping and manipulation of neutral atoms has become one of the most exciting fields in modern physics. A major milestone in this field was the cooling of atoms to degeneracy in 1995: the realisation of Bose-Einstein condensation (BEC). Since then, BECs with different atomic species and ever lower energy scales have been achieved, while research of quantum gases and their applications is ongoing. Atom interferometry (AI) is one promising application. It is the coherent manipulation of matter waves and a candidate for high precision quantum sensors, e.g. to measure inertial forces such as gravity or rotations.

Photo: CuttyP at the German language Wikipedia, CC license
To further push the frontiers of physics and enable new ways of studying degenerate quantum gases, BEC and AI experiments can be performed in microgravity. First of all, in the absence of disturbing gravitational force, there is no need for any atomic-mass dependent levitational fields. This allows for shallower and more symmetric trapping geometries. Secondly, the time of free and unperturbed evolution of a BEC released from the trap can be extended significantly, which is crucial for precision interferometric measurements with matter waves.

Different platforms offer microgravity environments, both on ground and in space, e.g. satellites, research rockets, the International Space Station (ISS), parabolic flights with a zero-g aircraft and drop towers such as the ZARM drop tower in Bremen, where the QUANTUS experiments are performed. The drop tower environment is similar to that of a space platform in several points. It offers excellent low residual accelerations down a level of 10-6 g while setting strict requirements concerning e.g. experiments volume, mass and power consumption and mechanical stability. The complete experiment has to fit inside a drop capsule, which is then either released from a height of 110 m, or catapulted inside the drop tower, giving experiment times in microgravity of 4.5 and 9 seconds, respectively, before finally being captured inside a styrofoam bath resulting in accelerations of up to 50 g.

 

QUANTUS-1: First BEC in Microgravity

The first rubidium BEC in microgravity ever has been observed within the first generation experiment QUANTUS-1 (Zoest et al.). A complete lab experiment was reduced in size, to fit into the drop capsule. An atom chip allows for the creation of steep magnetic traps for fast BEC creation at low power consumption.

First atom interferometry experiments were conducted on ground and in microgravity after a technological upgrade. Spatial fringes, arising from the interference of two overlapping BECs at the output ports of an asymmetric Mach-Zehnder matter wave interferometer, have been studied (Müntinga et al., Ahlers et al.). The reduction of the mean kinetic energy of the atomic ensemble, allowing a prolonged observation, has been achieved using a magnetic lens.

While QUANTUS-1 is no longer operated in drop mode, recent experiments on ground are centered around inertial measurements of the gravitational acceleration using Bragg diffraction and Bloch oscillations (Abend et al.).

 

 

 

QUANTUS-2: Dual-Species Atom Interferometry in Microgravity

The second generation rubidium experiment (depicted right) features a 2D magneto-optical trap for a high atomic flux (Rudolph et al.) and will be upgraded to a dual-species experiment with potassium. Thanks to an even more compact setup, the experiment can be operated in catapult mode of the drop tower, thus doubling the microgravity time to 9 seconds. The experiments capabilities were demonstrated by creating four rubidium BECs in a row during one single catapult flight (picture below).

The reduction of the expansion rate of the BEC was achieved by a tailored magnetic lens, reducing the effective temperature below 120 pK, which is as  as of now the lowest expansion rate ever achieved in three dimensions.

After upgrading QUANTUS-2 with potassium, two component mixtures at ultra-low energy levels will be investigated. Differential atom interferometry experiments with two atomic species on timescales of few seconds will allow for a test of the Einstein equivalence principle. Thus, QUANTUS-2 serves as a pathfinder for future experiments on a satellite platform.

 

Four Bose-Einstein condesates created sequentially during a
9 s lasting catapult flight. The absorption images have been
taken after increasing time of freeevolution, thus showing the
free expansion of the wave packet.

 

Laser System Development

One of the key technologies for BEC creation and experiments with cold atoms is the laser system, as light is necessary to cool, trap, manipulate and detect atoms, and to act as a beam splitter for atom interferometry. At Humboldt-Universität zu Berlin, we have designed and assembled such a laser system in order to provide light at the required frequency, beam shape and intensity.

The laser sources themselves consist of micro-integrated semiconductor diode lasers, which are developed at the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik as part of the Joint Lab activities that connect our institutes. Both, the lasers as well as the whole laser system have to be compact and robust; images of a laser and of the dual-species QUANTUS-2 laser system are shown below.

 

Left: micro-integrated diode laser module produced at the Ferdinand-Braun-
Institut. Right: dual-species laser system designed and set up within our group
incorporating the laser modules.

 

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