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.
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.
Demonstrating that, despite loss processes, Bose-Einstein condensates can be formed in steady state is a prerequisite for obtaining a coherent beam of atoms in a continuous-wave atom laser. In this paper we propose a method for loading atoms into the thermal component of a Bose condensed cloud confined in a magnetic trap. This method is aimed at allowing steady-state dynamics to be achieved. The proposed scheme involves loading atoms into the conservative magnetic potential using the spontaneous emission of photons. We show that the probability for the reabsorption of these photons may be small.
We have realized a scheme for continuous loading of a magnetic trap (MT).
52Cr atoms are continuously captured and cooled in a magneto-optical trap (MOT). Optical pumping to a metastable state decouples atoms from the cooling light. Due to their high magnetic moment $(6 {\ensuremath{\mu}}_{B}),$ low-field seeking metastable atoms are trapped in the magnetic quadrupole field provided by the MOT. Limited by inelastic collisions between atoms in the MOT and in the MT, we load
108 metastable atoms at a rate of
108 atoms/s below $100 \ensuremath{\mu}\mathrm{K}$ into the MT. Optical repumping after the loading allows us to realize a MT of ground-state chromium atoms.
We demonstrate an efficient scheme for continuous trap loading based upon spatially selective optical pumping. We discuss the case of
1S0 calcium atoms in an optical dipole trap (ODT), however, similar strategies should be applicable to a wide range of atomic species, which do not permit efficient conventional trap loading. Our starting point is a moderately cold $(\ensuremath{\approx}300\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K})$ and dense $(\ensuremath{\approx}9\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$ reservoir of metastable
3P2 atoms in a magneto-optic trap (triplet MOT). A focused
532nm laser beam produces a strongly elongated optical potential for
1S0 atoms with up to $350\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ well depth. A weak focused laser beam at
430nm, carefully superimposed upon the ODT beam, selectively pumps the
3P2 atoms inside the capture volume to the singlet state, where they are confined by the ODT. The triplet MOT perpetually refills the capture volume with
3P2 atoms, thus providing a continuous stream of cold atoms into the ODT at a rate of ${10}^{7}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Limited by evaporation loss, in
200ms we typically load $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms with $3.5\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ peak density and axial and radial temperatures of 300 and $85\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$, respectively. After terminating the loading we observe evaporation during
50ms, typically leaving us with
105 atoms with nearly equal radial and axial temperatures close to $40\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and a peak density of $5\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. A simple model is presented, which identifies the relevant physical mechanisms of the loading and decay dynamics. We point out that a comparable scheme could be employed to load a dipole trap with
3P0 atoms.