In the last decades, experimental access to the quantum regime has been gained in a large variety of systems, including dilute ultracold gases, trapped ions, micromechanical oscillators, superconducting circuits, and many more.These systems allow for such exciting applications as quantum computing and communication, quantum metrology and quantum simulation.A physical system's quantum mechanical features can only be revealed by cooling it until the thermal excitations are negligible.To date, the coldest temperatures are reached with ultracold gases by a combination of laser cooling and evaporation, inherently producing individual ensembles.However, many applications can greatly benefit from a continuous source of ultracold atoms, for example sympathetic cooling of molecules or nanoscopic objects.Ideally, this cooling source operates without light, as most molecules have a rich internal structure and can absorb photons of many wavelengths.Another promising application of continuous sources of ultracold matter is atom interferometry.Such interferometers perform measurements of inertial forces, time and gravity.Currently, each cold sample is destroyed by its evaluation and the measurement is interrupted by the preparation of a new samples.Continuous sources can help to eliminate this restriction.So far, no dark and continuous sources exist in the microkelvin regime.This thesis presents the first preparation of such an ultracold atom cloud which is continuously replenished and protected from light.The continuously refilled reservoir is prepared by loading a magnetically guided atom beam into an elongated magnetic trapping potential.This static potential has an entrance barrier which allows atoms from a guided beam to enter the trap.Once in the trap, the atoms undergo collisions and redistribute their kinetic energy.Those gaining energy can escape again, but those losing energy are trapped.A steady state with 3.8 × 10 7 atoms atoms at a temperature of 102 µK is reached and maintained.The setup of the continuous loading scheme is described in this thesis.A key component is a mesoscopic atom chip which produces all magnetic fields for the continuous loading scheme.The initial atom source is a high-flux double magneto-optical trap system.An atomic beam is created out of the precooled atoms and guided to the static trap through a stray light protection allowing for a dark retrapping region.The cooling scheme does not rely on any specific atomic properties of the atoms and can thus be applied to a wide range of particles.The results offer exciting perspectives for an application as a continuous source of ultracold atoms, either as a coolant for sympathetic cooling or as a resource for metrology.
In the last decades, experimental access to the quantum regime has been gained in a large variety of systems, including dilute ultracold gases, trapped ions, micromechanical oscillators, superconducting circuits, and many more.These systems allow for such exciting applications as quantum computing and communication, quantum metrology and quantum simulation.A physical system's quantum mechanical features can only be revealed by cooling it until the thermal excitations are negligible.To date, the coldest temperatures are reached with ultracold gases by a combination of laser cooling and evaporation, inherently producing individual ensembles.However, many applications can greatly benefit from a continuous source of ultracold atoms, for example sympathetic cooling of molecules or nanoscopic objects.Ideally, this cooling source operates without light, as most molecules have a rich internal structure and can absorb photons of many wavelengths.Another promising application of continuous sources of ultracold matter is atom interferometry.Such interferometers perform measurements of inertial forces, time and gravity.Currently, each cold sample is destroyed by its evaluation and the measurement is interrupted by the preparation of a new samples.Continuous sources can help to eliminate this restriction.So far, no dark and continuous sources exist in the microkelvin regime.This thesis presents the first preparation of such an ultracold atom cloud which is continuously replenished and protected from light.The continuously refilled reservoir is prepared by loading a magnetically guided atom beam into an elongated magnetic trapping potential.This static potential has an entrance barrier which allows atoms from a guided beam to enter the trap.Once in the trap, the atoms undergo collisions and redistribute their kinetic energy.Those gaining energy can escape again, but those losing energy are trapped.A steady state with 3.8 × 10 7 atoms atoms at a temperature of 102 µK is reached and maintained.The setup of the continuous loading scheme is described in this thesis.A key component is a mesoscopic atom chip which produces all magnetic fields for the continuous loading scheme.The initial atom source is a high-flux double magneto-optical trap system.An atomic beam is created out of the precooled atoms and guided to the static trap through a stray light protection allowing for a dark retrapping region.The cooling scheme does not rely on any specific atomic properties of the atoms and can thus be applied to a wide range of particles.The results offer exciting perspectives for an application as a continuous source of ultracold atoms, either as a coolant for sympathetic cooling or as a resource for metrology.
Abstract Typical sources of ultracold atoms operate with a considerable delay between the delivery of ensembles due to sequential trapping and cooling schemes. Therefore, alternative schemes for the continuous generation of ultracold atoms are highly desirable. Here, we demonstrate the continuous loading of a magnetic trap from a quasi-continuous atom beam. We achieve a steady state with magnetically trapped atoms and a temperature of K. The ensemble is protected from laser light sources, a requirement for its application in metrological tasks or sympathetic cooling. The continuous scheme is robust and applicable to a wide range of particles and trapping potentials.
Ultracold matter offers a unique capability to reach the best high-precision spectroscopy measurements and leads to exciting perspectives in many different areas of science and technology. Molecules, thanks to their complex spectra, couple with a broader range of photons compared to atoms, so they are extremely attractive for fundamental science or to design new quantum technologies. However, to date, only a few molecular species have been brought to temperatures of the order of the microkelvin. This was done by laser cooling, a process that has been demonstrated only for species featuring an unpaired electron that does not participate to the chemical bond. \n \nA different approach, which is indifferent to the molecule electronic structure and thus potentially universal, is sympathetic cooling, where neutral molecules are cooled in a bath of ultracold atoms. However, inelastic collisions between molecules and the coolant is a major obstacle that has hindered the advances of this method thus far. Trapping the molecules in their absolute ground state would circumvent this problem or at least greatly simplify it. In this thesis we simulate the feasibility of an experiment in which metastable CO molecules are first slowed down to a few m/s with a microstructured Stark decelerator, then they are stopped by a strong DC electrical barrier and finally they are transferred irreversibly to their absolute ground state and captured in an optical trap. \nUnfortunately, the results of the simulations indicate that the total number of molecules that can be accumulated in the optical trap is rather low, far behind the observation threshold. Therefore, we concluded that other approaches to produce ultracold molecules must to be searched.
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.
Summary form only given.Laser cooling, where excess kinetic energy is extracted by a photon, is ideally suited to loading atoms into a trap; however, quantization and reabsorption of light, limit the maximum phase-space density attainable. A sequence of laser cooling followed by evaporative cooling, where the excess energy is extracted by escaping atoms, may be used to overcome the laser cooling limit; however, such a scheme dramatically reduces the number of trapped atoms. A remaining challenge is to achieve continuous loading without the phase-space density limit imposed by near-resonant light. We consider continuous evaporative loading of a magnetic trap using a magnetically insensitive buffer gas. The key is to deliver sufficient cold atoms to the trapping region such that the steady-state trap population is large. To achieve this we employ an atomic fountain guided by a far-off resonant laser beam. At the apex of the fountain, the density can be a factor of 30-40 higher than in a magneto-optical trapping region, and importantly, the atoms are spatially separated from the region of resonant laser light. We propose to use this high-density region as a reservoir for continuous evaporative loading of a trap.
This thesis examines two complimentary methods for the laser cooling of a magnetically guided ultra-cold atom beam. If combined, these methods could serve as a starting point for high-through put and possibly even continuous production of Bose-Einstein condensates. First, a mechanism is outlined to harvest ultra cold atoms from a magnetically guided atom beam into an optical dipole trap. A continuous loading scheme is described that dissipates the directed kinetic energy of a captured atom via deceleration by a magnetic potential barrier followed by optical pumping to the energetically lowest Zeeman sublevel. The application of this scheme to the transfer of ultra cold chromium atoms from a magnetically guided atom beam into a deep optical dipole trap is investigated via numerical simulations of the loading process. Based on the results of the theoretical studies the feasibility and the efficiency of our loading scheme, including the realisation of a suitable magnetic field configuration, are analysed. Second, experiments were conducted on the transverse laser cooling of a magnetically guided beam of ultra cold chromium atoms. Radial compression by a tapering of the guide is employed to adiabatically heat the beam. Inside the tapered section heat is extracted from the atom beam by a two-dimensional optical molasses perpendicular to it, resulting in a significant increase of atomic phase space density. A magnetic offset field is applied to prevent optical pumping to untrapped states. Our results demonstrate that by a suitable choice of the magnetic offset field, the cooling beam intensity and detuning, atom losses and longitudinal heating can be avoided. Final temperatures below 65 µK have been achieved, corresponding to an increase of phase space density in the guided beam by more than a factor of 30.
From studies of fundamental physics to quantum technologies the production of ultracold molecules will have a huge impact across a range of applications. For many years laser cooling, which became an invaluable tool in cold atomic physics, was deemed to be too impractical for application to molecules. Nevertheless, laser cooling has now been demonstrated for a few molecular species. Using a frequency-chirped laser slowing technique, the velocity distribution of a pulse of CaF molecules is compressed and slowed from 180 m/s to about 10 m/s. These slow molecules are then captured in a magneto-optical trap. I present measurements that show how the number of molecules, the photon scattering rate, the oscillation frequency, damping constant, temperature, cloud size and lifetime depend on the key parameters of the magneto-optical trap, especially the intensity and detuning of the main cooling laser. The trap contains up to 2*10^4 molecules, the maximum photon scattering rate is 2.5*10^6 s-1 per molecule, the maximum oscillation frequency is 100 Hz, the maximum damping constant is 500 s^-1, and the minimum rms radius of the trapped cloud is 1.5 mm. A minimum temperature of 730 microkelvin is obtained by ramping down the laser intensity to lower values. To reach lower temperatures, the cloud is loaded into a blue-detuned optical molasses, which cools the molecules to 55 microkelvin, well below the Doppler-limiting temperature. I characterise the cooling process and suggest the sub-Doppler mechanisms responsible. These ultracold molecules are the optically pumped into a single quantum state, and coherently transferred between selected hyperfine components of the ground and first-excited rotational states. Finally, the ultracold, state-selected molecules are loaded into a magnetic trap that has a lifetime of about 1 s.