Apparatus for quantum-mixture research in microgravity
Baptist Piest, Jonas Böhm, Timothé Estrampes, Priyanka Guggilam, Annie Pichery, Paweł Arciszewski, Wolfgang Bartosch, Sören Boles, Klaus Döringshoff, Michael Elsen, Ortwin Hellmig, Christian Kürbis, Dorthe Leopoldt, Gabriel Müller, Alexandros Papakonstantinou, Christian Reichelt, André Wenzlawski, Thijs Wendrich, Éric Charron, Christoph Lotz, Achim Peters, K. Sengstock, Andreas Wicht, Patrick Windpassinger, Jens Große, Naceur Gaaloul, Ernst Maria Rasel
Abstract Experiments with ultracold quantum gases are a rapidly advancing research field with many applications in fundamental physics and quantum technology. Here, we report on a high-flux generation of Bose-Einstein condensate mixtures of 41 K and 87 Rb, using a fully integrated sounding rocket setup. We compare the release and the free expansion of the quantum mixtures obtained with the apparatus placed on ground or in free fall in an Einstein-Elevator. The release dynamics are governed by the intra- and interspecies interactions as well as the decaying magnetic field during the release. The latter can be minimized by a dedicated switch-off protocol of the trap generating currents where an exact model enabled us to characterize the interaction effects. Our results establish a new benchmark for generating ultracold mixtures on mobile platforms, with direct relevance for future experiments on interacting quantum gases and tests of the equivalence principle in space.
Abstract Experiments with ultracold quantum gases are a rapidly advancing research field with many applications in fundamental physics and quantum technology. Here, we report on a high-flux generation of Bose-Einstein condensate mixtures of 41 K and 87 Rb, using a fully integrated sounding rocket setup. We compare the release and the free expansion of the quantum mixtures obtained with the apparatus placed on ground or in free fall in an Einstein-Elevator. The release dynamics are governed by the intra- and interspecies interactions as well as the decaying magnetic field during the release. The latter can be minimized by a dedicated switch-off protocol of the trap generating currents where an exact model enabled us to characterize the interaction effects. Our results establish a new benchmark for generating ultracold mixtures on mobile platforms, with direct relevance for future experiments on interacting quantum gases and tests of the equivalence principle in space.
Experiments with ultracold quantum gases are a rapidly advancing research field with many applications in fundamental physics and quantum technology. Here, we report on a high-flux generation of Bose-Einstein condensate mixtures of 41 K and 87 Rb, using a fully integrated sounding rocket setup. We compare the release and the free expansion of the quantum mixtures obtained with the apparatus placed on ground or in free fall in an Einstein-Elevator. The release dynamics are governed by the intra- and interspecies interactions as well as the decaying magnetic field during the release. The latter can be minimized by a dedicated switch-off protocol of the trap generating currents where an exact model enabled us to characterize the interaction effects. Our results establish a new benchmark for generating ultracold mixtures on mobile platforms, with direct relevance for future experiments on interacting quantum gases and tests of the equivalence principle in space.
Extended free fall enables experiments with quantum gases unbiased by gravity. Apart from a 10m-fountain, we have access to various platforms comprising the Bremen drop- and gravi-tower, the Einstein Elevator, sounding rockets as well as NASA‘s cold atom laboratory (CAL) in orbit, all offering experiments with different durations and repetition rates. Atom chips turned out to successfully create Rubidium Bose-Einstein condensates on these platforms. Recently we have lowered the expansion energies of Bose-Einstein condensates to an unprecedented level corresponding to 38pK which makes them an excellent resource for interferometry. We exploit the latter to study delta-kick collimation as well to extend interferometry times in free fall. Atom chips also allow to investigate quantum mixtures in absence of bouyancy. For this purpose, we have been creating potassium 41 BECs by sympathetic cooling with rubidium. These experiments were continued to space by sounding rocket missions and the CAL. In future, the recently inaugurated Einstein elevator as well as gravitower will be important platforms to increase the experimental cycles to study quantum gas mixtures and high-precision interferometry as well as to test fundamental physics such as Einstein’s principle of equivalence and the nature of dark energy on ground in free fall.
Ultra-cold quantum gases in space promise to boost the sensitivity of matter-wave interferometers. Applications of the latter extend from fundamental physics over the use in navigation to interdisciplinary applications such as geodesy, e.g. satellite gravimetry [1, 2]. Exploiting quantum gases for high-precision interferometry places high demands on their control and manipulation. We take benefit of various microgravity platforms such as the Bremen drop tower [3], the Einstein elevator in Hannover [4], sounding rockets [5,6] and the international space station [7] to advance the necessary methods. The DLR-mission MAIUS-1 demonstrated Bose-Einstein condensation and performed first interferometry experiments [4]. NASA’s Cold Atom Laboratory continues this research in orbit on the ISS [7]. In addition, atom interferometry is pursued in highly dynamic environments such as parabolic flights [8]. Starting from a rubidium Bose-Einstein condensate, recently lowest expansion energies have been achieved by us in the Bremen drop tower as required for extending atom interferometry over several seconds [9]. Extending these methods to quantum mixtures not only opens up new physics in the absence of buoyancy, but also faces challenges regarding their use for interferometry. Interferometers based on two chemical elements have been proposed for quantum tests of the equivalence principle on the ISS as well as on satellites. Currently we prepare a sounding rocket mission to investigate the simultaneous generation and manipulation of potassium and rubidium condensates [10]. Together with CAL [7], these experiments will prepare the DLR-NASA multi-user facility BECCAL for research on quantum gas mixtures and interferometry [11] as well as enhance the readiness level of methods required for STE-QUEST [12], a proposal for a satellite mission currently studied in an modified version within ESA’s VOYAGE 2050 program [13].
One of the major goals of fundamental physics is to unify general relativity, which describes macroscopic phenomena driven by the influence of gravity, with quantum mechanics, which focuses on effects at microscopic scales. Ensembles of cold atoms, as massive quantum objects, lie at the crossroads of both theories and appear as a test object of choice. They can be used to test theories that postulate a violation of Einstein's Equivalence Principles, in particular a violation of the Universality of Free Fall (UFF). Recent proposals suggest using mixtures of Bose-Einstein condensates (BEC) as sources for precision atom interferometry to perform UFF. These have the potential to match the precision of the best tests with classical test masses performed during the MICROSCOPE mission, and may even provide better results in the long term. The realization of experiments in microgravity, where atoms can float for long periods of time, allows longer interrogation times, thus increasing the performance of matter-wave sensors. To optimize the implementation of UFF tests, one needs exquisite control of the atoms due to stringent requirements on the error budget. In this work, we focus on the design of the input state with control over the position and velocity of the atom clouds, as well as their size evolution. The experiments studied here are designed with atom chip setups that manipulate atoms with magnetic traps. Most of the applications presented are experiments performed in the NASA Cold Atom Laboratory (CAL) aboard the International Space Station as part of the Consortium for Ultracold Atoms in Space (CUAS). This multi-user BEC machine allows the manipulation of single species BEC at its installation as well as dual species mixtures after upgrades. Following this chronology, we first study the dynamics of single species BEC and then extend the work to the manipulation of an interacting mixture of two BECs. The first step after calibrating the chip model is to design a fast and robust transport protocol to move the atoms away from the atom chip. We present and use a Shortcut-To-Adiabaticity (STA) protocol, based on reverse engineering, to transport the BEC and meet the requirements of position control at the sub-\textmu m level and velocity control at the hundreds of \textmu m/s level. The free expansion of the atom cloud with its inherent atomic density drop makes signal detection difficult. By analogy with light, it is possible to collimate the atom cloud with atomic lenses using the Delta-Kick Collimation (DKC) technique. Application to CAL resulted in expansion energies in the tens of pK level. To simulate the imaging process and to support the data analysis, theoretical models are presented that take into account the resolution effects of the camera and the frame transformations associated with the orientation of the camera or the orientation of the trapping potential with respect to the atom chip. Space allows the operation of Bose-Einstein condensate mixtures under miscibility conditions not possible on the ground. The colocation of the trap center for the different species in microgravity can lead to different topologies of the trap ground state. Moreover, the interaction energy between the species, which is almost negligible in the ground state, plays a significant role in the dynamics of the mixture during its transport. However, the simulation of the dynamics of interacting dual-species BEC mixtures is computationally challenging, particularly due to the long expansion times. In this work, scaling techniques to overcome these limitations are presented and illustrated in the case of space experiments in CAL and aboard sounding rockets. Such scaled-grid approaches make it possible to simulate long transports with free expansion times on the order of seconds, which would not be feasible with a fixed-grid approach on reasonable time scales, not to mention the problems of memory usage.
The capability to reach ultracold atomic temperatures in compact instruments has recently been extended into space. Ultracold temperatures amplify quantum effects, while free-fall allows further cooling and longer interactions time with gravity - the final force without a quantum description. On Earth, these devices have produced macroscopic quantum phenomena such as Bose-Einstein condensation (BECs), superfluidity, and strongly interacting quantum gases. Quantum sensors interfering the superposition of two ultracold atomic isotopes have tested the Universality of Free Fall (UFF), a core tenet of Einstein's classical gravitational theory, at the 10−12 level. In space, cooling the elements needed to explore the rich physics of strong interactions and preparing the multiple species required for quantum tests of the UFF has remained elusive. Here, utilizing upgraded capabilities of the multi-user Cold Atom Lab (CAL) instrument within the International Space Station (ISS), we report the first simultaneous production of a dual species Bose-Einstein condensate in space (formed from 87Rb and 41K), observation of interspecies interactions, as well as the production of 39K ultracold gases. We have further achieved the first space-borne demonstration of simultaneous atom interferometry with two atomic species (87Rb and 41K). These results are an important step towards quantum tests of UFF in space, and will allow scientists to investigate aspects of few-body physics, quantum chemistry, and fundamental physics in novel regimes without the perturbing asymmetry of gravity.
Quantum sensors based on the interference of cold atoms have advanced to the forefront of precision measurements in geodesy, metrology and tests of fundamental physics. The ultimate potential of these devices is realized using quantum degenerate atoms in extended free fall. This can be achieved on microgravity platforms such as drop towers, parabolic flights, ballistic rockets, satellites and space stations. The transition to mobile and robust devices that can withstand the demands of these environments comes with many challenges. Quantum sensors need to be scaled down and integrated without compromising their performance. In fact, they need to significantly outpace conventional instruments, since microgravity time is an expensive resource and limited to a few seconds at a time on the most accessible platforms. This thesis describes the construction, qualification and operation of a miniaturized ultracold atom experiment that meets these challenges. The QUANTUS-2 apparatus features a payload weight of 147 kg and a payload volume of 0.3 m^3. It generates Bose-Einstein condensates of 4×10^5 ^87Rb atoms every 1.6 seconds, a flux of ultra-cold atoms that is on par with the best lab-sized devices. Ensembles of 1×10^5 atoms can be created at a 1 Hz rate. It is currently the fastest machine of its kind and achieves the highest atom number of any atom chip setup. The apparatus continuously withstands peak accelerations of up to 45 g during microgravity campaigns at the drop tower facility in Bremen, Germany. Here, the payload has accrued 208 drops and 9 catapult launches over 24 month. The setup is the first atom optics experiment to stand up to the technical demands of catapult operation. Four condensates can be created and observed consecutively during nine seconds of free fall in a single catapult launch. In total, the experiment has been suspended in microgravity for over 17 minutes. With the record source performance, the repetition rate for microgravity experiments with ultra-cold atoms was increased by a factor of four compared to previous devices. The total atom number was increased by a factor of 40, vastly improving the signal to noise ratio for absorption images of spatially extended clouds. The ensembles can be prepared consistently over many weeks of drop tower operation. The variance of the mean center of mass velocity in two observable directions is 7.3 μm/s and 6.9 μm/s. Magnetic lensing techniques were employed to manipulate the expansion of the ensembles. First results yield a residual expansion rate in three dimensions of σ_v = 116.9 ± 13.9 μm/s, which implies a three-dimensional effective temperature of T = 47.6 ± 11.3 pK at an average condensate atom number of N = 93000. These values constitute the best collimation of any atomic ensemble and the most promising source for atom interferometry reported to date. Optimizing the current lensing sequence will reduce the expansion rate further to effective temperatures in the femtokelvin regime. The level of control demonstrated over the condensates is highly relevant for the advancement of matter-wave optics and quantum sensors. Controlling the motion and size of atomic clouds is intrinsically tied to many systematic effects in high precision measurements. QUANTUS-2 will provide a platform to explore and mitigate these limitations on unprecedented time scales of up to seven seconds of free evolution.