QUANTUS-2: High resolution interferometry with ultra-cold mixtures in microgravity
The experimental realization of a Bose-Einstein condensate (BEC) in 1995 has opened up a plethora of new possibilities for cold atom research. Today, exciting new phenomena such as quantum phase transitions, superfluidity and matter wave interference are being studied in a multitude of lab-based experiments. The extremely low energies achieved in a typical earth-bound laboratory have motivated us to continue the path towards lower energy scales by overcoming earth-bound laboratory restrictions. This has led us to realize a BEC of 10000 87Rb atoms in microgravity, during the free fall of the QUANTUS-1 experiment. The experimental results establish the feasibility of cold atom research in microgravity. Ultra-large condensates (~1.5 mm) have been observed after free evolution times of 1 second, a timescale that is inaccessible in ground-based devices [Science 328 (5985): 1540-1543].
There are several other advantages of performing an experiment in microgravity or space. In particular, a microgravity environment allows for mass independent confining potentials which are crucial for studies on mixtures of several atomic species such as degenerate Bose-Fermi gases.
In our effort to continuously push the existing frontiers further, we realized a new setup which fulfills the criteria of being more compact, having better optical access, higher numbers of atoms, employing multiple atomic species and being able to use the catapult mode of the drop tower, doubling the time of microgravity to 9.3 seconds. The experiment employs Rubidium-87 and different isotopes of potassium as degenerate Bose and Fermi gases, respectively, and can be used to carry out experiments on high resolution interferometry, Bose-Fermi mixtures and tests of the weak equivalence principle in the quantum domain.
(see also Tom Scott visiting QUANTUS-2)
The whole setup is housed in a catapult mode drop tower capsule (see Fig.1). The payload volume of the capsule is only 950mm in height times a diameter of 650mm. All necessary components for the experiment, namely - from top - the laser system (first platform), the electronics and computer control system (2nd&3rd platform), the vacuum chamber (4th platform) and the power supply (not shown) are fitted into this confined volume. The total weight of the setup is 245.8 kg, 98.8 kg of which is owed to the application specific structure (stringers and platforms). Hence, the payload mass amounts to 147 kg including a two layer MuMetal shield around the vacuum chambers (42.6 kg). The power consumption of the entire setup in operation is 363.9 W. Hence, it can easily be run on commercially available accumulators for several hours at a time.
While lightweight materials would consequently be favoured, the apparatus also has to withstand intense forces during launch and impact, that are reaching amplitudes of up to 45g. Consequently all parts of the experiment need to be very rigid to endure a large number of launches without displacement.
The laser system of QUANTUS-2 was developed in two steps. The first generation system (see Fig.2) has been operating the experiment in ground based prestudies in Hannover, but is also compact and robust enough to withstand the emerging forces of the drop mode of the drop tower. The entire laser system is mounted on a sinlge platform of the capsule with a total heigt of 8 cm and is able to operate the 2D+-MOT, the 3D-MOT and provide light for optical pumping, detection and a pair of phase locked laser beams for high precision Raman spectroscopy with rubidium.
The second generation laser system (see Fig.3) is additionally able to drive Raman and Bragg transitions for rubidium and potassium. It is further miniaturized and has separately been tested to also operate in the catapult mode of the drop tower. This laser system has mainly been developed and fabricated by our partners at the Humboldt-Universität zu Berlin. This laser systems uses for the first time the newly developed miniaturized MOPA laser sources of the Ferdinand-Braun-Institut and has been implemented into the QUANTUS-2 experiment in 2013.
QUANTUS-2 employs the first double MOT system in microgravity consisting of a two and a three dimensional magneto optical trap (see Fig.4). This vacuum setup consists of two chambers separated by a differential pumping stage, allowing for a pressure difference of up to three orders of magnitude. A high vacuum (HV) area (2D chamber) is used for the atomic source and is operated slightly below the room temperature vapor pressure of rubidium at 10−7mbar. It generates a pre-cooled beam of atoms towards an ultra-high vacuum (UHV) chamber. The UHV region (3D chamber) is used to capture the atoms, cool them to degeneracy and perform atom interferometry.
The highly miniaturized chamber is entirely fabricated out of non-magnetic materials avoiding residual magnetisations, which would lead to unwanted effects disturbing the interferometric measurements. To further reduce parasitic effects of magnetic stray fields two cylindrical mu-metal shields are wrapped around the experimental setup.
Three layer atom chip setup
The heart of the experiment chamber is the further optimised atom chip (see Fig.5) built in cooperation with Jakob Reichel. The atom chip setup consists of three layers ranging from mesoscopic structures in the millimeter regime to wires of a thickness of 50µm.
The first layer holds the largest, mesoscopic structures which are constructed from Kapton isolated 0.9mm diameter copper wires. These are used in the generation of the quadrupole field for the 3D-MOT with a U-shaped layout. Additionally, three individual copper wires form an H-shaped structure to generate a Ioffe-Pritchard type potential, that is used in the first magnetic trap. The second layer (base chip) features intermediate sized gold wires of 0.5mm width, electroplated onto a 35mm × 35mm Aluminum nitride substrate. A 25mm × 25mm chip (science chip) forms the third and final layer with structures of 50 μm width. It is covered with a dielectric transfer coating to reflect two of the four MOT beams, creating a mirror MOT configuration. The base and science chip feature a set of four and five parallel wires, respectively, in each case intersecting with one central orthogonal wire. They both offer an abundance of possible U-, Z- and H-shaped trap configurations including dimple traps.
Performance of QUANTUS-2
With this compact setup we are able to produce BECs with 4 × 105 atoms within 1.6 seconds (see Fig.6). To do so, we load the atoms provided by the 2D+-MOT in our 3D-MOT for about 500ms (1). After this, the atomic ensamble is transfered closer to the chip's surface and compressed in a c-MOT stage (2). A brief optical molasses cools the atoms to approximately 20µK. Subsequently the atoms are transfered into the magnetic trappable sub-state by optical pumping and trapped in a purely magnetic Ioffe-Prichard-type chip trap (3). Shortly after this transfer, the atomic ensemble gets strongly compressed and heats up to 180µK (4). In this strong confinement the scattering rate of the atoms is high enough to quickly perform evaporative cooling within 1s to form a BEC (5).
While our setup produces the highest atom number overall among the fastest BEC machines, its flux of condensed atoms is also on par with the best lab-based devices.
Results of microgravity campaigns
The first drop of QUANTUS-2 has been performed at 18.07.2014. During this first try, already two subsequently produced BECs could be observed. In figure 7 one can clearly see, that in comparison to our experiments on ground (Fig. 7 - left) the BEC is not dropping any more downwards (Fig. 7 - middle) due to the microgravitational environment. It is even possible for us to produce and image a next BEC right after the 45g impact after the drop with almost the same performance than before (Fig. 7 - right).
Characterization of QUANTUS-2 in microgravity
The first 48 drops were used to characterize the behavior of the apparatus and the produced BECs. Figure 8 depicts a measurement of the trap frequency recorded within a campaign of two weeks. The center of the BEC after 100 ms of free propagation after release has been measured in dependence of the holding time of the BEC inside the magnetic chip trap. Despite the measured trap frequency of 60 Hz, these 43 data points show a remarkable phase stability, although the experiment has been thrown more then 20 times from a height of 110 m to record this data, proofing the high stability of the whole apparatus.
Using the catapult mode of the Droptower Bremen
After successfully having proven the capabilities of QUANTUS-2 to acquire data using the drop mode of the Drop Tower, on the 20th October 2014 we decided to test its catapult mode (see also Tom Scott at the Drop Tower Bremen). In this mode the capsule is accelerated with 30 g at the beginning of the flight and by this the duration of microgravity can be extended to 9.3 seconds. The very first flight failed due to a problem with the shutter of our detection system, but right the next attempt at the next day was successfull. During this flight we were able to produce and image four BECs after different free expansion times of 25 ms, 50 ms, 75 ms and 100 ms (see Fig. 9). By this and further catapult launches the QUANTUS-2 apparatus qualified for the usage of the catapult mode of the Drop Tower Bremen.
Transport of the BEC away from the chip
To be able to observe the atomic ensemble after a time of free propagation in the range of seconds, the expansion rate of the cloud needs to be significantly reduced than shown in figure 9, where the BEC simply got released from the magnetic atom chip trap. Therefore we investigate the atom-optical technique of matter-wave lensing. To apply specific magnetic lenses, two prominent positions have been identified (see Fig.10 b & c).
To transport the BEC to position (b) with minimising residual excitation of a center of mass oscillation in the trap at this position, a sigmoid-shaped trajectory of the magnetic trap position im time was used. This transport was accomplished within 250 ms and led to an in-trap residual oscillation amplitude of 0.6 µm (see Fig. 8).
Position (c) offers more favourable magnetic lens properties, than position (b), but is roughly twice the distance away from the chips surface. This makes a transport without excitations challenging, since magnetic traps built-up by atom chips loose in strengh with distance. To accomplish this transport, different approaches have been tried. The best results were obtained by using a reverse-engeneering method. By this, the residual in-trap oscillation amplitude could be minimised to 4.6 µm (see Fig. 11), although the transport time was reduced to 150 ms.
Magnetic lensing of BECs
In analogy to a lens for light in optics, the trajectories of freely propagating atoms can be re-directed by the application of a force for a certain time. With our atom chip setup, we can exert a force onto each of the atoms, which (more or less) exactly acts in opposite direction of the momentum of the atom. By choosing the right timing, we can re-focus or even stop all the atoms.
In our first attempts (Flight 57 - 131) we tested different lensing strategies at position (b) (see Fig. 10) and were able to reduce the expansion rate of the atomic ensemble below 150 µm/s in all directions (see Fig. 12 upper part). This led to the possibility to image the BEC even after 2000 ms of free propagation. This observed expantion rate corresponds to a thermal atomic cloud beeing cooled down to less than 75 pK.
In the first lensing campaign we observed and understood different effects leading to lens abberations and identified a more suited lens. This new lens is applied at location (c) (see Fig. 10). Within the last flight campaign (Flight 178 - 211) we investigated its performance (see Fig. 12 lower part). The peak density during free propagation drops significantly slower than in the previous lens measurements and the shape of the atomic ensemble is less distorted, verifying the application of a magnetic lens with less abberations and better collimation properties. Evaluating this data leads to the conclusion, that we unintendedly focussed our emsemble around 350 ms. But still, after the focus the ensemble shows expansion rates of only up to 100 µm/s in all directions, beeing comparable to a thermal expansion with only 50 pK temperature.
Up to now we performed 217 flights with QUANTUS-2 in the drop tower. Recent findings lead to the resonable suspicion, that, although we minimised the center of mass oscillation at position c, we excited shape oscillations of our ensemble in the final trap. These shape oscillation could explain our measured low expansion rate in one of the directions and is object of acual research.
Within the next drop campaigns, we will investigate the shape oscillations and correct for the focus around 350 ms. By this we aim to demonstrate expansion rates comaprable to a thermal ensamble of a temperature well below 20 pK.
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