Quantum Sensors for Earth Observation

SR17_Weltraum_V8_Abstract_InfosysII.qxd 10.12.10 10:39 Seite 178 Quantum sensors for Earth observation M. Gilowski and E.M. Rasel Institut für Quantenoptik, Leibniz Universität Hannover, Welfengarten 1, 30167 Hannover, Germany Abstract mes [3-8]. This field represents an emerging We report on our development of various area of science quantum engineering, with a kinds of quantum sensors based on matter- high potential for a future technology and mul - wave interferometry for precise inertial sensing tidisciplinary applications. Thanks to an impres- of Earth`s rotation rate and gravity. By manipu- sive evolution and remarkable inventions, the lating cold or ultra cold atoms with coherent ultimate potential of matter-wave sensors is still light fields we realize several experimental plat- entirely open. For the closely related field of ato- forms for the detailed investigation of different mic clocks, the growth in performance was interferometer types and of their key technolo- exponential during the last decades! This is the gies. In an atomic gyroscope using cold Ru bi di - reason, why matter-wave sensors are conside- um atoms we demonstrate a sensitivity for red as one of the most promising fields to pro- rotations of 6x10-7 rad/s. In an atomic gravi- gress in metrology and fundamental tests. meter, which is under construction, we study Atomic gyroscopes and gravimeters provide a the source system based on ultra cold atoms in new tool for the precise detection of tiny for- a Bose-Einstein condensate. Finally, a miniatu- ces. The outstanding feature of these sensors rized and robust experiment using ultra cold is the precisely known scaling factor: there is atoms in a free fall environment is realized as a no need for calibration which predestines test-bed for matter-wave interferometry on these sensors for inertial references and for long timescales. These experiments pave the applications in the Système International. way in the direction of utilizing the technolo- The following report summarizes our activities gy of matter-wave interferometry for future and achievements in the field of matter-wave space applications. interferometry. We will describe the features of this technology by presenting three experiments which point out the development of Introduction atomic inertial sensors. The first one is an ato- Atomic quantum sensors are a key-technology mic gyroscope for the high-precision measure- for the ultra-precise monitoring of accelera- ment of rotations. The second one is a dual tions and rotations. These sensors evolved atomic gravimeter, which is under construction from a new kind of optics based on matter- and which provides a source of ultra cold waves rather than light-waves. Matter-wave atoms. The third one is a miniaturized trans- optics is still a young, but rapidly progressing portable experiment with ultra cold atoms in a science which recently generated sensational microgravity environment, which is performed Nobel-prize awarded inventions such as laser in the drop tower facility (ZARM) in Bremen. cooling and atom lasers [1,2]. These experiments are steppingstones in the The applications of atomic quantum sensors direction of the realization of future inertial are truly interdisciplinary, covering diverse and atomic sensors with sensitivities compared or important topics such as tests of fundamental even beyond current state-of-the-art devices. physics, the realization of SI-units, prospecting for resources, GALILEO technology, environment monitoring and major Earth-science the- 178 SR17_Weltraum_V8_Abstract_InfosysII.qxd 10.12.10 10:39 Seite 179 The gyroscope a subsequent 3D-MOT with a high flux of seve- The interferometric measurement of rotations ral 109 at/s. Using the moving molasses techni- is based on the Sagnac-effect [9], which indi- que 108 atoms with a forward drift velocity of cates that a phase shift is induced between 2.8 m/s and a temperature of 8 µK are laun- two interferometer paths which enclose an ched in each interferometry pulse. In a next area A, due to a rotation with the angular step the atoms are state- and velocity-selecti- velocity Ω. This phase shift is then given by vely transferred into the magnetically insensiti- dfrot=4pEAΩ/hc, where E is the energy of the ve hyperfine state F = 1, mF = 0 (where F and wave, h the Planck’s constant and c the speed mF are the quantum numbers for the total an - of light. Since this relation is also valid for light gular momentum and the Zeeman sublevel, res - as well as for matter-wave interferometers, the pectively) via a multi-stage preparation using high potential of gyroscopes based on atoms is precisely controllable laser manipulation. obvious. By comparing the phase shifts using In the interferometery section a symmetric Ram - the energies of matter and visible light, an im - sey-Bordé interferometer configuration is reali- provement in the order of 1011 for atom inter fe - zed by applying four so called p/2-light pulses rometers is in principle possible. Further more, [14]. This coherent beam splitting processes the sensor is also sensitive to accelerations [10]. are based on a Raman-transition between two In order to distinguish between phase shifts due hyperfine states which form the ground states to rotations and accelerations the sensor con- of the interferometer. With these light pulses sists of two interferometers allowing a differen- the matter-wave is split, redirected and finally tial measurement [10]. The basic schematic of recombined. In this way, the created interfero- the atomic gyroscope is sketched in Fig. 1. meter paths enclose an area of 8.6 mm2 lea- ding to a high sensitivity for the Sagnac-effect. Two identical atomic sources [13] emit atoms In this interferometer configuration phase on flat parabolic trajectories into the interfero- shifts imprinted during the interferometric meter chamber, but with opposite launch di - cycle are translated into a change of the distri- rec tions. Each source consists of a two-dimen- bution in the two ground states [14]. Thus, we sional magneto-optical trap (2D-MOT) loading finally obtain the phase shift of each interfero- Figure 1: Schematic overview of the gyroscope with its key-element sections. Laser beams for cooling and trapping as well as for the preparation, the interferometry and the detection process are represented by the arrows. More details can be found in reference [11,12]. 179 SR17_Weltraum_V8_Abstract_InfosysII.qxd 10.12.10 10:39 Seite 180 Figure 2: The Interference fringes of the two interferometers are obtained by sweeping the phase of the beam splitting light field before the last pulse is applied. Figure 3: Allan standard deviation of the combined signals of the two interferometers. meter by using a state-selective fluorescence gain the contrast of the interferometer signal. detection, applied after the interferometry se - Furthermore, a reduction of the initial tempera- quence. We detect in the gyroscope approxima - ture of the ensemble would increase the effi- tely several 107 atoms. ciency of this process and thus motivates the Typical interference patterns of the two interfe- use of ultra cold atoms (see below). The diffe- rometers of the gyroscope are shown in Fig. 2. rence in contrast can be attributed to the align- We reach a contrast in the four pulse geome- ment of the spatial interaction position, which is try of 11% and 16%. The reduced contrast different for the four beam splitting pulses for can be attributed to a reduced efficiency of the the two interferometers, respectively. beam splitting process, which depends on the one hand on the temperature and the spatial Finally, we can infer the rotation sensitivity of width of the atoms and on the other hand on the gyroscope by measuring the phase noise of the spatial intensity profile of the beam split- the two interferometers. In Fig. 3 the Allan ting light field. A further selection of velocity standard deviation for the combined signal of classes in the preparation stage will in future the two interferometers is shown. Here, the 180 SR17_Weltraum_V8_Abstract_InfosysII.qxd 10.12.10 10:39 Seite 181 rotation signal can be inferred from the sum of on matter-wave interferometry is setup for the the two interferometer signals whereas the test of the Equivalence principle. For this purpo- acceleration might be calculated from the dif- se the free fall of two atomic species, Rubidium ferential signal. It is clearly visible that vibration and Potassium, will be compared. In this experi- noise coming from accelerations is highly sup- ment the aimed shot-noise limited sensitivity is pressed due to the dual measurement scheme. 10-9 g. This corresponds to the highest sensitivi- After averaging the combined signal over 300 s ty for the test of the Equivalence principle with a phase noise of 9.5 mrad is reached. This cor - laser cooled atoms so far [12]. responds to a reached sensitivity in the gyrosco- One of the essential aspects of the dual gravi- pe of 6x10-7 rad/s which is two orders of mag- meter is the source of ultra cold atoms to reach nitude below the Earth rotation rate. the demanded accuracy. The first step in this direction was reached by creating a Bose-Ein - Currently, this sensitivity is limited by the noise stein condensate (BEC) of 87Rb atoms in a near- of the detection process as well as inertial noise. infrared single beam optical dipole trap (ODT). The implementation of an optimized detection The ODT is formed from laser light with a wa ve - sys tem and an improvement of the vibration length close to 2 µm. Fur ther more, an additional isolation are under way and will further impro- constant confinement in the axial direction of ve the sensitivity by at least one order of mag- the trap is provided by a magnetic quadrupole nitude on the short-term scale.

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