All-Optical Bose-Einstein Condensates in Microgravity
G. Condon, Martin Rabault, B. Barrett, Laure Chichet, Romain Arguel, H. Eneriz-Imaz, D. Naik, Andréa D Bertoldi, Baptiste Battelier, Philippe Bouyer, Arnaud Landragin
We report on the all-optical production of Bose-Einstein condensates in microgravity using a combination of grey molasses cooling, light-shift engineering and optical trapping in a painted potential. Forced evaporative cooling in a 3-m high Einstein elevator results in 4×10^{4} condensed atoms every 13.5 s, with a temperature as low as 35 nK. In this system, the atomic cloud can expand in weightlessness for up to 400 ms, paving the way for atom interferometry experiments with extended interrogation times and studies of ultracold matter physics at low energies on ground or in Space.
We report on the all-optical production of Bose-Einstein condensates in microgravity using a combination of grey molasses cooling, light-shift engineering and optical trapping in a painted potential. Forced evaporative cooling in a 3-m high Einstein elevator results in 4×10^{4} condensed atoms every 13.5 s, with a temperature as low as 35 nK. In this system, the atomic cloud can expand in weightlessness for up to 400 ms, paving the way for atom interferometry experiments with extended interrogation times and studies of ultracold matter physics at low energies on ground or in Space.
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.
In microgravity environments, the evaporative cooling time required to prepare a Bose–Einstein condensate (BEC) in a crossed optical dipole trap is significantly longer than that observed in ground-based experiments using identical apparatus. This slowdown arises from the simultaneous decrease in both trap depth and trap frequency of the trap under microgravity conditions, which results in an extended duration necessary for achieving thermal equilibrium. To address this issue, we propose incorporating a saddle-shaped blue-detuning light field to create a composite optical trap. This approach effectively mitigates the decline of trap frequency while simultaneously reducing the depth of the composite trap, thereby facilitating rapid evaporative cooling. Numerical simulations indicate that this strategy allows the atomic phase-space density to approach the Bose–Einstein condensation threshold in less than 4 s under microgravity. Such performance represents an order-of-magnitude improvement in the evaporative cooling rate of optical dipole traps operating in microgravity. This advancement not only facilitates rapid BEC preparation under orbital conditions but also opens new avenues for analogous experiments in ground-based drop towers.
In 1925, Einstein predicted the condensation of bosons into the ground state of the system for low (but finite) temperatures. Several phenomena, including superfluidity and superconductivity have been associated with Bose-Einstein condensation, but these systems interact strongly with their environment and pure Bose-Einstein condensation could not be established. It took 70 years, in which time the laser was discovered, and laser cooling techniques to manipulate atoms in a dilute atomic gas, before Bose-Einstein condensation in dilute atomic gases could be demonstrated in 1995. In the first condensation experiments, BECs were created in a magnetic trap. Since in a magnetic trap not all mF states of the atom can be trapped simultaneously, thereby limiting the number of experiments that can be done, other ways of trapping and generating BECs were sought and found. In 2001, the first all-optical BEC was made, where the dipole force was used to trap atoms in the crossing of two far red detuned laser beams. In an optical dipole trap not only atoms in different internal states can be trapped, but also different atomic species simultaneously. In this thesis, the formation of an all-optical Bose-Einstein condensate with rubidium atoms is presented. Conventional all-optical BECs are usually created in high power CO2 laser dipole traps, or have complicated laser cooling schemes and complex dipole trap setups. In our simple and straightforward setup, we load rubidium atoms from a magneto-optical trap into a crossed optical dipole trap created by a single frequency Yb:YAG laser with a wavelength at 1030 nm. The small wavelength allows for a small diffraction limit, and permits us to use standard optical materials, thus making the experimental setup cost effective. Other attempts to achieve Bose-Einstein condensation in a multi-mode (frequency) fiber laser at 1064 nm failed, because the atom loss was quite high. It is assumed that the multi-mode character of the fiber laser induces Raman transitions in rubidium atoms, thereby heating them. We can trap about ∼ 5E7 atoms in a single beam dipole trap out of ∼ 5E9 atoms trapped in theMOT, and ∼ 350,000 atoms can be trapped in a crossed beam dipole trap due to the smaller trap volume. 70% of the atoms in the dipole trap is optically pumped into one mF state. Quantum degeneracy is reached by evaporatively cooling the atoms in the crossed dipole trap by ramping down the laser power with three linear ramps. We can independently change the power of each beam by an AOM. This allows us to use one beam as an atom waveguide for future experiments. After evaporation, we typically have about 10,000 atoms at a temperature below the critical temperature. We have proved Bose-Einstein condensation by using the anisotropic expansion of a quantum degenerate gas trapped in an anisotropic potential. The aspect ratio of our atom cloud changed during a time of flight from 0.7 to 1.2 in 10 ms, thus proving that we have reached quantum degeneracy. We have about 5,000 condensed atoms in our optical dipole trap at a temperature less than 100 nK. The remaining atoms are thermal. Bose-Einstein condensation is obtained within 8 s, and we can repeat the experiment every 30 s. It should be mentioned that the Bose-Einstein experiment was moved from the ”Leibniz Universität Hannover” to the ”Technische Universität Darmstadt”, and had to be completely rebuilt. All-optical Bose-Einstein condensation was reached within one year after the move. Our Bose-Einstein condensation setup presents an ideal starting point for using our condensates in combination with miniaturized atom optical setups based on our novel microfabricated optical elements. With our microlenses, we can create a number of possible dipole trap configurations, such as the dipole trap array or the cylindrical microlens array. Using microlenses in miniaturized atom optical setups opens a completely new field of coherent atom optics. Also because the tight confinement of the microtraps allows us to load a 3D BEC, a 1D BEC, or a Tonks-Girardeau gas in the micropotentials depending on the density.
Optical interferometers are well known for their high precession in several measurement situations. They are based on a readout of phase differences between optical path length. In contrast, atom interferometers rely on the De Broglie wavelength of atoms and their corresponding phase. A cloud of atoms is split and recombined with light pulses, while in between, the atoms are moving freely. These precise quantum tools will enhance a broad variety of measurements, ranging from large scale phenomena like gravitational wave detection to short scale Casimir-Polder forces and everything in between. The sensitivity of atom interferometers scales with the time squared between light pulses, which is generally limited by gravity and temperature. One technical challenging sollution is the realization of atom interferometers in microgravity. While ultra cold atoms can be prepared in optical and magnetic traps, under weightlessness their preparation is limited to magnetic traps until now. These experiments are based on atom chips with highly asymmetric trapping potentials. The improved symmetry of optical traps can improve the most advanced cooling technique, called delta kick collimation, leading to atomic clouds in the yet unreached femtokelvin range. This thesis is about the first realization of an optical trapping potential in microgravity. It describes the experimental setup, identifies a molasseses technique as the optimal process for dipole trap loading and investigates the further cooling process under the influence and in the absence of gravity. In optical and magnetic traps, the preparation of ultracold atoms is based on evaporation. However, the underlying physics to drive this process are fundamentally different and for optical traps, evaporation is driven by gravity. It is demonstrated that evaporation from an optical potential performs approximately equal with and without gravity, due to a strong mixing of the trapping potentials in all three spatial dimensions. These findings are confirmed with computational simulations, based on the Direct Simulation Monte Carlo method. Atomic ensembles of rubidium-87 with temperatures as low as a 300 nK were generated in the microgravity environment of the drop tower in Bremen with an evaporation time as short as 0.5 s. The final confining trap was too shallow to be reproduced in a laboratory environment. The findings of this thesis will possibly guide the way to improved atom interferometers, with countless applications in precise sensing. Furthermore, optical potentials in microgravity on their own will open up a broad field of fundamental physical experiments. One example is the applicability of magnetic Feshbach resonances. Allowing almost arbitrary tuning of the interatomic scattering length offers new insights in scattering processes or miscibility scenarios.
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.
We use D1 gray molasses to achieve Bose-Einstein condensation of a large number of 41K atoms in an optical dipole trap. By combining a specific configuration of a compressed magneto-optical trap with D1 gray molasses, we obtain a cold sample of $2.4\ifmmode\times\else\texttimes\fi{}{10}^{9}$ atoms with a temperature as low as 42 $\ensuremath{\mu}\mathrm{K}$. After magnetically transferring the atoms into the final glass cell, we perform a two-stage evaporative cooling. A condensate with up to $1.2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms in the lowest Zeeman state $|F=1,{m}_{F}=1\ensuremath{\rangle}$ is achieved in the optical dipole trap. Furthermore, we observe two narrow Feshbach resonances in the lowest hyperfine channel, which are in good agreement with theoretical predictions.