Bose-Einstein condensates under microgravity: On 23rd January 2017, a team of German scientists under the scientific supervision of Leibniz Universität Hannover conducted interferometry experiments with "decelerated" ultra-cold matter waves, known as Bose-Einstein condensates (BECs), for the first time. By cooling using laser light and other atomic-optical methods, rubidium atoms were brought to an ultra-low energy state corresponding to less than a millionth of a degree above absolute zero. In these conditions, a cloud of several ten thousands of atoms forms a new state of matter - a Bose-Einstein condensate. One particular characteristic of this state is its macroscopic wave character - the ultra-cold atomic wave shows interference phenomena similar to those of laser light. This superposition of waves can in principle be used for making precision measurements in atomic-optical instruments such as matter-wave interferometers.
Many technical challenges had to be met on the way to the first experiment in space with BECs, as these matter waves react sensitively to environmental influences such as vibration and magnetic fields. For this reason, their coherent development can be observed for only a short period in free fall on earth. The aim of the MAIUS 1 rocket mission was thus to demonstrate and investigate for the first time the viability of a BEC using miniaturised and robust apparatus in space. Ultimately, about one hundred experiments on methods for manipulating quantum matter and BEC interferometry were conducted during the rocket flight. For this, conditions of microgravity like those reached during the rocket flight are required.
The German Aerospace Centre (Deutsches Zentrum für Luft- und Raumfahrt; DLR) rocket MAIUS 1 (Matter-Wave Interferometry in Microgravity) was launched at 03:30 Central European Time (CET) on 23rd January from the Swedish Esrange Space Centre near Kiruna, north of the Arctic Circle, and reached a height of 243 km. A team of scientists, engineers and technicians from eleven German universities, including Holger Ahlers, Dennis Becker, Maike Lachmann, Dr. Thijs Wendrich and Dr. Stephan Seidel from Leibniz Universität Hannover, and other research facilities as well as the Swedish launch-site operator monitored the autonomously operating payload and the rocket flight from the ground. The coasting phase above about 100 kilometres to the top of the trajectory and back provided six minutes of experimental time in microgravity.
The immediate analysis of the data transmitted during the flight was a cause for celebration, when the researchers recognised that one of the key targets of the experiment had been achieved: they had managed for the first time to create BECs in space. Using a system of parachutes, the technologically and scientifically extremely sophisticated payload landed also as planned in the deep snow of northern Sweden. After years of intensive preparation for the mission, the evaluation of the many experiments will keep the scientists in suspense over the next few weeks. The aim was to observe the phase transition during condensation and to analyse the evolution of the BECs during extended free fall by means of atom interferometry. The insights gained will go into the development of new methods necessary for future space missions.
Conventional apparatus takes up to one minute to generate Bose-Einstein condensates and usually fills an entire laboratory. A crucial requirement for the MAIUS mission is thus to be able to create BECs with atom chips, with which atoms can be decelerated in the smallest possible space. Atom chips enable the generation of magnetic fields to trap the atoms with the help of planar electric circuits in the smallest possible space on a substrate. They were developed in particular by Nobel-Prize winner Theodor Hänsch, Jakob Reichel and Jörg Schmiedmayer. They were the starting point for the BEC interferometers based on atom chips that are being investigated at the Institut für Quantenoptik, Leibniz Universität Hannover, in the group headed by Prof. Dr. Ernst M. Rasel and Prof. Dr. Wolfgang Ertmer, and in close cooperation with the theoretician Prof. Dr. Wolfgang Schleich in Ulm. With the atom chip used on MAIUS-1, BECs of several hundred thousand atoms can be generated in less than two seconds. Experiments in the Bremen drop tower enabled German scientists to explore methods for BEC interferometry under microgravity, thus paving the way for the MAIUS mission. In a terrestrial laboratory setup, BECs can be maintained and manipulated in free fall for only about a hundred milliseconds. For this, a free fall of the condensed atomic cloud is used in an evacuated apparatus over a few centimetres - the atoms experience a short period of microgravity here. In the last few years, German scientists have managed to generate and investigate BECs in microgravity over a period of some two seconds in the Bremen drop tower. The free fall of a special apparatus called QUANTUS was created in the evacuated tower tube over a distance of around 110 metres. The free fall of the MAIUS-1 rocket lasting several minutes on a long ballistic trajectory opened up a much longed-for greater scope for penetrating into new experimental territory.
The attraction of maintaining a BEC for as long as possible also has an important application aspect. The sensitivity of an atom interferometer increases quadratically with the free fall time of BECs in such a measuring device. It is thus not surprising that longer space satellite missions are being considered, which would use quantum matter to examine more precisely than before Einstein's principle of equivalence of gravitational and inert mass - a cornerstone of our physical understanding of the world. The use of quantum sensors in satellites for more precise geodesy and navigation is also being discussed.
The MAIUS-1 project is scientifically supervised by Leibniz Universität Hannover in cooperation with Humboldt-Universität and Ferdinand-Braun-Institut in Berlin, ZARM at Universität Bremen, Johannes Gutenberg-Universität Mainz, Universität Hamburg, Universität Ulm and Technische Universität Darmstadt. Further research partners are the Institute of Space Systems at the German Aerospace Centre (DLR) in Bremen, the Institute of Space Operations and Astronaut Training - in particular the Mobile Rocket Base MORABA - in Oberpfaffenhofen and the DLR Simulation and Software Technology Facility in Braunschweig. Students, many young scientists and engineers, and university professors from different disciplines worked together on all subsystems of the payload, from the atom chip apparatus, laser, electronics, data storage, magnetic shielding, and batteries to the flight software. The rocket was launched in a two-stage configuration with solid-fuel engines produced in Brazil. DLR-MORABA was responsible for the launch campaign.
The project was supported by DLR Space Administration in Bonn, with funding from the Federal Ministry of Economics and Technology. After a detailed analysis of the experimental data, a further mission is planned for 2018. Here, the Bose-Einstein condensates of two atomic species (rubidium and potassium) will be investigated in an interferometer, an essential step on the way to testing Einstein's equivalence principle with matter waves. In addition, the MAIUS rocket mission and the QUANTUS experiments in the Bremen drop tower under the long-standing supervision of Rainer Kuhl and his successor Thomas Driebe from DLR Space Administration are an important link for the experiments planned in cooperation with NASA from 2017. German scientists will play an important role in these experiments into ultra-cold atoms, which are to be conducted in the International Space Station.
