Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)
Space-Qualified Ridge Waveguide Optical Amplifiers for Ultra-Cold Atom Experiments on the ISS
Karl Häusler, Jan Markus Baumann, Carsten Netzel, Anna Mogilatenko, Johannes Glaab, Jos E. Boschker, Jörg Fricke, A. Maaßdorf, H. Wenzel, Andrea C. Knigge, Ahmad Bawamia, Andreas Wicht
The realization of Bose-Einstein Condensates (BECs) is of great interest for both fundamental research and emerging applications such as high-precision quantum sensors, atom interferometers, gravimeters, and “atom lasers” exploiting the coherence of atomic waves [1]. To investigate fundamental principles - e.g. the equivalence of heavy mass and inertial mass - BECs have been created and examined in weightlessness in the Bremen drop tower in 2007 (QUANTUS) and on board a sounding rocket in 2017 (MAIUS) [2]. Building on these missions, further atom-optical experiments are planned for the International Space Station (ISS). All these experiments rely on coherent, monochromatic, frequency-stabilized laser light for cooling and manipulating cold atoms.
The realization of Bose-Einstein Condensates (BECs) is of great interest for both fundamental research and emerging applications such as high-precision quantum sensors, atom interferometers, gravimeters, and “atom lasers” exploiting the coherence of atomic waves [1]. To investigate fundamental principles - e.g. the equivalence of heavy mass and inertial mass - BECs have been created and examined in weightlessness in the Bremen drop tower in 2007 (QUANTUS) and on board a sounding rocket in 2017 (MAIUS) [2]. Building on these missions, further atom-optical experiments are planned for the International Space Station (ISS). All these experiments rely on coherent, monochromatic, frequency-stabilized laser light for cooling and manipulating cold atoms.
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].
Owing to the low-gravity conditions in space, space-borne laboratories enable experiments with extended free-fall times. Because Bose–Einstein condensates have an extremely low expansion energy, space-borne atom interferometers based on Bose–Einstein condensation have the potential to have much greater sensitivity to inertial forces than do similar ground-based interferometers. On 23 January 2017, as part of the sounding-rocket mission MAIUS- 1, we created Bose–Einstein condensates in space and conducted 110 experiments central to matter-wave interferometry, including laser cooling and trapping of atoms in the presence of the large accelerations experienced during launch. Here we report on experiments conducted during the six minutes of in-space flight in which we studied the phase transition from a thermal ensemble to a Bose–Einstein condensate and the collective dynamics of the resulting condensate. Our results provide insights into conducting cold-atom experiments in space, such as precision interferometry, and pave the way to miniaturizing cold-atom and photon-based quantum information concepts for satellite-based implementation. In addition, space-borne Bose–Einstein condensation opens up the possibility of quantum gas experiments in low-gravity conditions1,2.
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.
Ultra-cold atoms have become indispensable in quantum sensing, quantum computing, and quantum communication applications, as well as in fundamental sciences. In quantum sensing, they enable precision measurements with outstanding sensitivities that can be of great benefit in navigation, geodesy, and fundamental physics applications. The sensitivity of the atom interferometers used herefore typically scales with the square of the interrogation time. Therefore, in atom interferometric measurements, one seeks ensembles that feature a particularly low internal kinetic energy and can thus be detected after a long time-of-flight. The lowest values are achieved using Bose-Einstein condensates in conjunction with delta-kick collimation, a technique for the collimation of the atomic wave function. Bose-Einstein condensation can be achieved using a multi-stage laser cooling scheme, followed by evaporative cooling, which finally brings the temperature down to the critical temperature, typically in the nanokelvin regime. In addition to the low thermal spread, microgravity enables particularly long observation times, as it avoids limitations of the interrogation time due to vacuum chamber constraints, which are present when falling freely under the influence of gravity. Additionally, microgravity enables measurements that are not possible under normal gravity conditions, such as buoyancy-free miscibility studies, truly uniform potentials, and advanced cold atom quantum memories, which are projected to feature unprecedented time-bandwidth products. For the preparation of a Bose-Einstein condensate, two techniques are widely used in science, namely magnetic traps and optical dipole traps. For microgravity missions, magnetic traps on atom chips have been used in nearly all setups so far due to their particularly low SWAP budget and high cold atom flux. In contrast, ground-based experiments often prefer optical traps due to their higher versatility, full optical access, and the absence of surfaces in proximity to the atoms, which is fundamental for precision measurements. Additionally, optical traps enable the manipulation of intra- and inter-species scattering lengths using Feshbach resonances. Compared to atom chip traps, they also feature an intrinsically better symmetry of the trapping potential, in theory leading to fewer aberrations in matter-wave optics. The latter could enable the generation of ensembles in the yet unreached femtokelvin regime using delta-kick collimation with fewer perturbations. This thesis is about the realization of a 1064nm optical dipole trap set up for the microgravity environment of the Bremen Drop Tower. It aims to highlight the advantages of optical traps in conjunction with microgravity and prepare optical trapping technology for use in future microgravity and space missions. The thesis describes the experimental setup, the direct loading process, the characterization of the optical potential, and the optimization and investigation of the evaporative cooling in the optical dipole trap. For the evaporation process, the influence of gravity is comprehensively investigated through analytical calculations and microgravity tests. Bose-Einstein condensation is demonstrated, proving the achievement of condensation through absorption imaging of the spatial distribution, tests on the free evolution of the ensemble, and investigation of the phase-space densities achieved. Up to (74 ± 4)% condensation of an ensemble featuring (30 000 ± 13 000) atoms at a kinetic temperature of (37 ± 3) nK is demonstrated. The evaporation shows high efficiency down to a 3 s evaporation time, while a critical phase-space density can be achieved within (1.29 ± 0.07) s using an even faster evaporation ramp. This opens the door to fundamental physics experiments on Bose-Einstein condensates in microgravity, as well as applications pursuing atomic ensembles with exceptionally low expansion velocities.
On 23rd of January 2017 the first Bose-Einstein Condensate (BEC) in Space was created on-board the sounding rocket mission MAIUS-1. The successful launch marks a major advancement in the effort of performing matter wave interferometry with BECs on space vehicles. Its high BEC-flux enables more than 100 experiments during flight, characterizing the creation of BECs in space, their free evolution, state preparation, and the creation of cold atoms in highly dynamic environments. MAIUS-1 opens a new path towards space borne inertial sensing employing interferometers with high accuracy and sensitivity. Two follow-up missions will investigate dual-species interferometry. Recently several missions were proposed ranging from tests of the universality of free fall to gravimetry. Due to their small initial size and low expansion rates BECs are the ideal source for such an interferometric measurement. The findings of the mission will contribute to the NASA CAL project and BECCAL (NASA and DLR). This research is funded by DLR under grant 50WP1435.
On 23rd of January 2017 the first Bose-Einstein Condensate (BEC) in Space was created on-board the sounding rocket mission MAIUS-1. The successful launch marks a major advancement in the effort of performing matter wave interferometry with BECs on space vehicles. Its high BEC-flux enables more than 100 experiments during flight, characterizing the creation of BECs in space, their free evolution, state preparation, and the creation of cold atoms in highly dynamic environments. MAIUS-1 opens a new path towards space borne inertial sensing employing interferometers with high accuracy and sensitivity. Two follow-up missions will investigate dual-species interferometry. Recently several missions were proposed ranging from tests of the universality of free fall to gravimetry. Due to their small initial size and low expansion rates BECs are the ideal source for such an interferometric measurement. The findings of the mission will contribute to the NASA CAL project and BECCAL (NASA and DLR). This research is funded by DLR under grant 50WP1435.