H. Dittus

Bose-Einstein Condensation in Microgravity

Going Down the Tube Two pillars of modern physics are quantum mechanics and general relativity. So far, both have remained apart with no quantum mechanical description of gravity available. Van Zoest et al. (p. 1540; see the Perspective by Nussenzveig and Barata) present work with a macroscopic quantum mechanical system—a Bose-Einstein condensate (BEC) of rubidium atoms in which the cloud of atoms is cooled into a collective quantum state—in microgravity. By dropping the BEC down a 146-meter-long drop chamber and monitoring the expansion of the quantum gas under these microgravity conditions, the authors provide a proof-of-principle demonstration of a technique that can probe the boundary of quantum mechanics and general relativity and perhaps offer the opportunity to reconcile the two experimentally. Studies of atomic quantum states in free-fall conditions may provide ways to test predictions of general relativity. Albert Einstein’s insight that it is impossible to distinguish a local experiment in a “freely falling elevator” from one in free space led to the development of the theory of general relativity. The wave nature of matter manifests itself in a striking way in Bose-Einstein condensates, where millions of atoms lose their identity and can be described by a single macroscopic wave function. We combine these two topics and report the preparation and observation of a Bose-Einstein condensate during free fall in a 146-meter-tall evacuated drop tower. During the expansion over 1 second, the atoms form a giant coherent matter wave that is delocalized on a millimeter scale, which represents a promising source for matter-wave interferometry to test the universality of free fall with quantum matter.

Quantum Physics Exploring Gravity in the Outer Solar System: The SAGAS Project

We summarise the scientific and technological aspects of the Search for Anomalous Gravitation using Atomic Sensors (SAGAS) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015–2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements and technologies.

Odyssey: A Solar System Mission

The Solar System Odyssey mission uses modern-day high-precision experimental techniques to test the laws of fundamental physics which determine dynamics in the solar system. It could lead to major discoveries by using demonstrated technologies and could be flown within the Cosmic Vision time frame. The mission proposes to perform a set of precision gravitation experiments from the vicinity of Earth to the outer Solar System. Its scientific objectives can be summarized as follows: (1) test of the gravity force law in the Solar System up to and beyond the orbit of Saturn; (2) precise investigation of navigation anomalies at the fly-bys; (3) measurement of Eddington’s parameter at occultations; (4) mapping of gravity field in the outer solar system and study of the Kuiper belt. To this aim, the Odyssey mission is built up on a main spacecraft, designed to fly up to 13 AU, with the following components: (a) a high-precision accelerometer, with bias-rejection system, measuring the deviation of the trajectory from the geodesics, that is also giving gravitational forces; (b) Ka-band transponders, as for Cassini, for a precise range and Doppler measurement up to 13 AU, with additional VLBI equipment; (c) optional laser equipment, which would allow one to improve the range and Doppler measurement, resulting in particular in an improved measurement (with respect to Cassini) of the Eddington’s parameter. In this baseline concept, the main spacecraft is designed to operate beyond the Saturn orbit, up to 13 AU. It experiences multiple planetary fly-bys at Earth, Mars or Venus, and Jupiter. The cruise and fly-by phases allow the mission to achieve its baseline scientific objectives [(1) to (3) in the above list]. In addition to this baseline concept, the Odyssey mission proposes the release of the Enigma radio-beacon at Saturn, allowing one to extend the deep space gravity test up to at least 50 AU, while achieving the scientific objective of a mapping of gravity field in the outer Solar System [(4) in the above list].

Mini-ASTROD: Mission concept

Advances in laser physics and its applications triggered the proposition and development of Laser Astrodynamics. Mini-ASTROD is a down-scaled version of ASTROD (Astrodynamical Space Test of Relativity using Optical Devices). This mission concept has one spacecraft carrying a payload of a telescope, six lasers, and a clock together with ground stations (ODSN: Optical Deep Space Network) to test the optical scheme and yet give important scientific results. These scientific results include a better measurement of the relativistic parameters (gamma to 1 ppm, beta to a few ppm and others with improvement), a better sensitivity (several times better) in using the optical Doppler tracking method for detecting gravitational waves, a potential of measuring the solar angular momentum via the Lense-Thirring effect and measurement of many solar system parameters more precisely. These enable us to build a more precise ephemeris and astrodynamics. The weight of this spacecraft is estimated to be about 300-350 kg with a payload of about 100-120 kg. The spacecraft goes into an inner solar orbit with several options. One option is with period 304 days as for the inner spacecraft of the standard two-spacecraft ASTROD mission concept and it takes about 900 days to reach the other side of the Sun relative to the Earth. Another option is to launch with initial period about 290 days and to pass by Venus twice to receive gravity-assistance for achieving shorter periods. For a launch on November 15, 2008, after two encounters with Venus, the orbital period can be shortened to 165 days. After about 400 days from launch, the spacecraft will arrive at the other side of the Sun and the relativistic parameter gamma can be determined to 1 ppm. We discuss the payload configuration and outlook for technological developments to reach the mission goals, and summarize the conclusions and recommendations of the first and second organizational meeting for the Mini-ASTROD study.

Application of LTS-SQUIDs for testing the weak equivalence principle at the Drop Tower Bremen

Abstract Free fall tests to prove the weak equivalence principle were rarely be done in history. Presently, very precise fall tests in the 10−13 range are possible and under preparation to be carried out on Drop Tower Bremen during free fall over 109 m. A level of accuracy of 10−18 will be achieved in the current satellite test of the equivalence principle space mission of NASA/ESA. Both kinds of experiments require position detectors with an extremely high resolution to measure infinitesimal displacements of freely falling test masses. On the basis of the LTS SQUID system of the Jena University an experimental setup was developed containing a pair of superconducting levitated test masses installed in a vacuum chamber at 4.2 K. The resolution of the SQUID position detector was measured to be as high as 4×10−14 m/ Hz . This whole apparatus was successfully tested and dropped at the Drop Tower Bremen providing a free fall height of 109 m corresponding to a flight time of 4.7 s. Recent results of this measurements are described in this work.

Application of high performance LTS SQUID systems in gravitational experiments

The design of a Galilean-type free-fall experiment to test Einsteins equivalence principle at an improved level of sensitivity is described. Two test bodies of different material fall down from a height of 109 m inside of an evacuated drop tube (Drop Tower Facility Bremen, Germany). Their possible relative displacement is measured using LTS DC SQUID based position sensors. Experiences and results of this experiment meet directly the current STEP project of NASA/ESA to test the Equivalence Principle in space at a level of one part in 10/sup 18/. Several issues of the developed measuring system are discussed, above all, the performance of two types of SQUID position sensors. Furthermore some recent results of free fall tests of components of the measuring system at the Bremen Drop Tower are presented.

Testing Einstein's equivalence principle at Bremen Drop Tower using LTS SQUID technique

Free fall tests to prove the Weak Equivalence Principle (WEP) were rarely be done in history. Although they seem to be the natural experiments to test the equivalence of inertial and gravitational mass, best results for WEP-proofs could be attained with torsion pendulum tests to an accuracy of 10/sup -12/ because these pendulum tests are long term periodic experiments, Otherwise, free fall tests on Earth can be carried out only for seconds causing certain principle limitations. Nevertheless, very precise fall tests in the 10/sup -13/ range are possible and under preparation to be carried out on Drop Tower Bremen during free fall over 109 m. A level of accuracy of 10/sup -18/ will be achieved in the current STEP (Satellite Test of the Equivalence Principle) space mission of NASA/ESA. Both kinds of experiments require position detectors with an extremely high resolution to measure infinitesimal displacements of freely falling test masses. On the basis of the LTS SQUID system of the Jena University an experimental set-up was developed containing a pair of superconducting levitated test masses installed in a vacuum chamber at 4.2 K. The resolution of the SQUID position detector was measured to be as high as 4/spl times/10/sup -14/ m//spl radic/Hz. This whole apparatus was successfully tested and dropped at the Drop Tower Bremen providing a free fall height of 109 m corresponding to a flight time of 4.7 s. Recent results of this measurements are described in this work.

Drop tower tests of the Equivalence Principle

Abstract Up to now an experimental proof of a violation of the Weak Principle of Equivalence (WEP) has been missing. Experiments carried out differ mainly in the applied measurement technique and can be classified into groups: (1) torsion balance experiments and (2) Galilean (free fall) experiments. With torsion balance experiments, the WEP is proven on the 10 −12 level. We discuss the possibility to carry out experiments to test the universality of free fall, the WEP, on Drop Tower Bremen with higher accuracy and report on experimental results.

RUBIDIUM BOSE–EINSTEIN CONDENSATE UNDER MICROGRAVITY

Weightlessness promises to substantially extend the science of quantum gases toward presently inaccessible regimes of low temperatures, macroscopic dimensions of coherent matter waves, and enhanced duration of unperturbed evolution. With the long-term goal of studying cold quantum gases on a space platform, we currently focus on the implementation of an 87Rb Bose–Einstein condensate (BEC) experiment under microgravity conditions at the ZARM drop tower in Bremen (Germany). Special challenges in the construction of the experimental setup are posed by a low volume of the drop capsule (< 1 m3) as well as critical vibrations during capsule release and peak decelerations of up to 50 g during recapture at the bottom of the tower. All mechanical and electronic components have thus been designed with stringent demands on miniaturization, mechanical stability and reliability. Additionally, the system provides extensive remote control capabilities as it is not manually accessible in the tower two hours before and during the drop. We present the robust system and show results from first tests at the drop tower.

ATOMIC QUANTUM SENSORS IN SPACE

In this article we present actual projects concerning high resolution measurements developed for future space missions based on ultracold atoms at the Institut fur Quantenoptik (IQ) of the University of Hannover. This work involves the realization of a Bose–Einstein condensate in a microgravitational environment and of an inertial atomic quantum sensor.