In the field of atomic physics, there is a growing demand for large numbers of dense, trapped atoms. The traditional method of generating trapped atoms is through laser cooling, however the field has reached saturation in terms of cold atom flux and phase-space density and is fundamentally limited to atoms that can be addressed using a two-level transition accessible with available lasers. Because of this a new, more general technique of generating dense clouds of trapped atoms is necessary. This technique will surpass laser cooling with higher cold atom flux and phase-space density, as well as be applicable to particles which cannot be put into a two-level system. This thesis explores the first step necessary for the generation of a new method of cooling which will be more general than laser cooling and will produce a higher cold atom flux in denser phase-space. This method of cooling will rely on the sympathetic cooling of vaporized particles with a pulsed supersonic beam before slowing the entrained particles to rest using magnetic fields. Because the cooling in the entrainment step relies on sympathetic cooling, there is no two-level requirement and thus it is applicable to all paramagnetic species including both atoms and molecules. The experiments outlined in this thesis focus on utilizing different methods of entraining vaporized atoms into a supersonic beam as an alternative method of generating cold atoms. A comprehensive comparison of entrainment efficiency using these different entrainment techniques is included as well as a discussion regarding future applications of this new cooling process.
In the field of atomic physics, there is a growing demand for large numbers of dense, trapped atoms. The traditional method of generating trapped atoms is through laser cooling, however the field has reached saturation in terms of cold atom flux and phase-space density and is fundamentally limited to atoms that can be addressed using a two-level transition accessible with available lasers. Because of this a new, more general technique of generating dense clouds of trapped atoms is necessary. This technique will surpass laser cooling with higher cold atom flux and phase-space density, as well as be applicable to particles which cannot be put into a two-level system. This thesis explores the first step necessary for the generation of a new method of cooling which will be more general than laser cooling and will produce a higher cold atom flux in denser phase-space. This method of cooling will rely on the sympathetic cooling of vaporized particles with a pulsed supersonic beam before slowing the entrained particles to rest using magnetic fields. Because the cooling in the entrainment step relies on sympathetic cooling, there is no two-level requirement and thus it is applicable to all paramagnetic species including both atoms and molecules. The experiments outlined in this thesis focus on utilizing different methods of entraining vaporized atoms into a supersonic beam as an alternative method of generating cold atoms. A comprehensive comparison of entrainment efficiency using these different entrainment techniques is included as well as a discussion regarding future applications of this new cooling process.
This dissertation describes a general approach of generating a high flux of cold atoms that can be confined in a magneto-optical trap. As an alternative to the state-of-the-art laser cooling method, this approach widens our ability to cool and control atoms without relying on a specific atomic transition and availability of laser. In this dissertation I will discuss the design, construction of the experiment and characterization of a pulsed cold atom source in detail. This work is based on a new paradigm: entrainment of atoms in the carrier gas of a supersonic beam, followed by the magnetic deceleration and trapping. Our methodology is based on the supersonic beam created by the expansion of a dense carrier gas from a pulsed release of gas through a small aperture. Cold noble gas emerging from this pulsed high pressure supersonic nozzle acts as a carrier, into which atoms of interest are then entrained. In our studies with the lithium atoms, up to 10¹¹ atoms per pulse could be entrained into a supersonic beam of helium at a translation temperature of below 100 mK. The supersonic valve is typically operated at a rate of below 1 Hz in the experiment. A much larger flux can be achieved at a higher repetition rate. These atoms are moving at a speed of 500 ms⁻¹ and need to be decelerated almost to a complete stop, in order to be trapped and cooled to quantum degeneracy. A 2.5-meter long moving-trap magnetic decelerator with 480 coil pairs was built and characterized. Atoms moving at a speed around 500 ms⁻¹ were trapped and decelerated to various final velocities ranging from 400 ms⁻¹ to 50 ms⁻¹, at a resulting temperature of 30 mK and a flux of 10⁸ ~ 10⁹ atoms per pulse. This whole process takes place within only 10 ms, at a repetition rate of 100 Hz 10¹⁰ ~ 10¹¹ atoms can be delivered per second. This approach is very general compared to the laser cooling, since most atoms in the periodic table have a magnetic moment in their ground state or can be optically pumped to a long-lived para-magnetic metastable state. In this dissertation, I show the working principle of each component of our experiment and characterize the atom beam at multiple stages. I describe in detail the construction and optimization of our magnetic decelerator, and demonstrate the control and monitoring system with the experiment results. Also implemented in this work is a flexible laser system that is composed of a reference diode laser and two tapered amplifiers to control and probe the internal states of atoms, as well as provide the trapping force. I also explore the optical molasses and chirped cooling techniques which help load the atoms into a magneto-optical trap. The successful demonstration of this method of creating a cold atom source leads us to believe that the magnetically decelerated supersonic atoms will play an important role in the area of cold atom physics
Supersonic beams provide a source of cold atoms where laser cooling is not applicable. Although the atoms' temperature in the co-moving frame is in the subkelvin range their velocity is on the order of several hundreds of meters per second. This thesis describes the experimental realization of a novel method to slow atoms and molecules with permanent magnetic moments using pulsed magnetic fields. The method is suitable for most elements since most atomic species are paramagnetic, and can also be applied to certain molecules, as well as electronically excited metastable states and most radicals. We show the slowing of metastable neon in a proof of principle experiment where the mean velocity is reduced from 461.0[plus or minus] 7.7 m/s to 403 [plus or minus]16 m/s in 18 stages. A second setup with 64 stages is now operating and allows us to stop metastable neon in principle. Preliminary results showing slowing from 447[plus or minus]3m/s to 136[plus or minus]5m/s with an efficiency of up to 3.9% are included here, and the slower has been shown to generate atoms as slow as 50 m/s. We find that the slowing efficiency depends strongly on the switching phase. In addition to the experimental results described above, we present simulations of a moving trap which allows trapping and decelerating at the same time.
We propose an alternative method to laser cooling. Our approach utilizes the extreme brightness of a supersonic atomic beam, and the adiabatic atomic coilgun to slow atoms in the beam or to bring them to rest. We show how internal-state optical pumping and stimulated optical transitions, combined with magnetic forces, can be used to cool the translational motion of atoms. This approach does not rely on momentum transfer from photons to atoms, as in laser cooling. We predict that our method can surpass laser cooling in terms of flux of ultracold atoms and phase-space density, with lower required laser power.
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.
Laser cooling and trapping are central to modern atomic physics. It has been roughly three decades since laser cooling techniques produced ultracold atoms, leading to rapid advances in a vast array of fields and a number of Nobel prizes. Prior to the work presented in this thesis, laser cooling had not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for a wide range of applications. The first direct laser cooling of a molecule and further results we present here provide a new route to ultracold temperatures for molecules. In particular, these methods bridge the gap between ultracold temperatures and the approximately 1 kelvin temperatures attainable with directly cooled molecules (e.g. with cryogenic buffer gas cooling or decelerated supersonic beams). Using the carefully chosen molecule strontium monofluoride (SrF), decays to unwanted vibrational states are suppressed. Driving a transition with rotational quantum number R=1 to an excited state with R'=0 eliminates decays to unwanted rotational states. The dark ground-state Zeeman sublevels present in this specific scheme are remixed via a static magnetic field. Using three lasers for this scheme, a given molecule should undergo an average of approximately 100,000 photon absorption/emission cycles before being lost via unwanted decays. This number of cycles should be sufficient to load a magneto-optical trap (MOT) of molecules. In this thesis, we demonstrate transverse cooling of an SrF beam, in both Doppler and a Sisyphus-type cooling regimes. We also realize longitudinal slowing of an SrF beam. Finally, we detail current progress towards trapping SrF in a MOT. Ultimately, this technique should enable the production of large samples of molecules at ultracold temperatures for molecules chemically distinct from competing methods.