This thesis covers the construction and characterisation of a magneto-optical trap (MOT) \nfor 85Rb from the very beginning. It details both the optical and mechanical aspects\nfrom laser diode assembly, tuning and stabilisation to the preparation and assembly of\nthe vacuum system.\nThe MOT construction forms the first goal of the project, the second was to develop\na laser system capable of producing custom programmable, amplitude modulated and\nfrequency chirped pulses (on the tens of microseconds timescale) from a continuous wave\nsource. This involved developing software and computer control for several arbitrary\nsignal generators linked to drivers for acousto-optic and electro-optic modulators. This\nchirped, pulsed laser system will be used to perform state manipulations on the 85Rb\nMOT cloud using two-photon Raman transitions in an adiabatic rapid passage regime.\nThe chirped pulse system was initially tested with a rubidium vapour cell in an attempt\nto perform atomic interferometry that would produce spatial interference fringes along\nthe length of the cell. However, due to the beam power requirements, the beam diameter\ntogether with the large Doppler shift at room temperature meant these fringes were not\nseen and so the vapour cell was replaced with the cold atom cloud in the MOT.\nTwo-photon experiments were attempted with the MOT cloud using the chirped pulse\nsystem, however despite greatly improved laser power and detection eficiency, the signal\nindicating ground state population transfer via a two-photon interaction was not seen.\nThe results indicate that the hyperfine-ground state splitting frequency has been shifted\ndue to the proximity of the ion pump magnet to the vacuum system. Presently, efforts\nare being directed towards searching for the correct frequency.
This thesis covers the construction and characterisation of a magneto-optical trap (MOT) \nfor 85Rb from the very beginning. It details both the optical and mechanical aspects\nfrom laser diode assembly, tuning and stabilisation to the preparation and assembly of\nthe vacuum system.\nThe MOT construction forms the first goal of the project, the second was to develop\na laser system capable of producing custom programmable, amplitude modulated and\nfrequency chirped pulses (on the tens of microseconds timescale) from a continuous wave\nsource. This involved developing software and computer control for several arbitrary\nsignal generators linked to drivers for acousto-optic and electro-optic modulators. This\nchirped, pulsed laser system will be used to perform state manipulations on the 85Rb\nMOT cloud using two-photon Raman transitions in an adiabatic rapid passage regime.\nThe chirped pulse system was initially tested with a rubidium vapour cell in an attempt\nto perform atomic interferometry that would produce spatial interference fringes along\nthe length of the cell. However, due to the beam power requirements, the beam diameter\ntogether with the large Doppler shift at room temperature meant these fringes were not\nseen and so the vapour cell was replaced with the cold atom cloud in the MOT.\nTwo-photon experiments were attempted with the MOT cloud using the chirped pulse\nsystem, however despite greatly improved laser power and detection eficiency, the signal\nindicating ground state population transfer via a two-photon interaction was not seen.\nThe results indicate that the hyperfine-ground state splitting frequency has been shifted\ndue to the proximity of the ion pump magnet to the vacuum system. Presently, efforts\nare being directed towards searching for the correct frequency.
ENGLISH ABSTRACT: A Magneto Optical Trap (MOT) is a configuration formed by three orthogonal pairs of counterpropagating circularly polarized laser beams and a magnetic field gradient. A MOT is used to cool, capture and trap large numbers of atoms in vacuum at very low temperature in K range. In this thesis the development of an experimental setup for realising a MOT of 87Rb atoms is presented. The atomic structure of Rb and the theoretical background of laser cooling and magneto optical trapping was reviewed. The influence of rubidium background pressure in the vacuum system, the laser beam size and the power and frequency on the number of the trapped atoms were studied in literature. The trapping and repumping lasers were characterised experimentally. Six circularly polarised trapping beams with equal power were formed and properly aligned to intersect at the center of the trapping cell. Two optical setups were designed and exploited to investigate and optimise the trapping beam polarisation. The repumping laser beam was successfully aligned and colinearly combined into all the trapping beams. Three different experimental setups for saturated absorption spectroscopy were developed. Saturated absorption spectra showing the hyperfine structure of both 85Rb and 87Rb isotopes were measured and are discussed. Using two saturated absorption spectroscopy setups, the frequencies of both lasers were successfully locked to the trapping and repumping transitions of 87Rb respectively. A rectangular trapping cell was designed and attached to the vacuum system. A pressure of about 10¯7 mbar was achieved. The magnetic field coils were characterised and affixed on both sides of the cell in an anti-Helmholtz configuration. Setups for imaging and quantification of the 87Rb atoms in the MOT were designed. Finally, the procedures for demonstrating a MOT are presented. In conclusion, the current status of the project is reported, with recommendations for the future work.
This thesis reports on the design, setup, and operation of a new atom trap apparatus, which is developed to confine few rubidium atoms in ultrahigh vacuum by means of taylored and dynamically variable light potentials (dipole traps), thus making the atoms available for spectroscopy and controlled manipulations. To maintain low background pressure, atoms of a vapour cell are transferred into a cold atomic beam by laser cooling techniques, and accumulated by a magneto-optic trap (MOT) in a separate part of the vacuum system. The laser cooled atoms are then transferred into dipole traps made of focused far-off-resonant laser fields in single or crossed-beam geometry, which are superimposed with the center of the MOT. Gaussian as well as hollow Laguerre-Gaussian (LG01) beam profiles are used with red-detuned and blue-detuned light. Microfabricated dielectric phase objects allow the efficient and robust mode conversion of Gaussian into Laguerre-Gaussian laser beams. Trap geometries can easily be changed due to the highly flexible experimental setup. The dipole trap laser beams have powers up to several hundred milliwatts and are focused to around 10 m. At a detuning of several tens of nanometers from the atomic resonance, the resulting traps are a few millikelvin deep, and the trapped atoms exhibit motional frequencies near 30 kHz, trap light scattering rates of a few hundred photons per second per atom, and storage times of several seconds. The number of dipole-trapped atoms ranges from more than ten thousand to below ten. The dipole-trapped atoms are detected either by a photon counting system with efficient straylight discrimination, or by recapture into the MOT, which is imaged onto a sensitive photodiode and a CCD-camera. Due to the strong AC-Stark shift imposed by the high-intensity trapping light, energy-selective resonant excitation and detection of the stored atoms is possible. The measured energy distribution is consistent with a harmonic potential shape and allows for the determination of temperatures and heating rates. From initial measurements, the thermal energy is found to be about 10 % of the trap depth. Using a crossed-beam geometry with red-detuned laser light, efficient transfer of atoms between the beams is observed. Optimum transfer occurs when the two beams cross at a radial offset, which can qualitatively be understood when the particle energy and geometrical properties of the two-beam trapping potential are considered. Numerical simulations reproduce the general features of the measured transfer efficiency vs. radial beam offset. This experiment is a first step towards dynamically variable potentials. Atoms have been radially confined in a blue-detuned hollow beam, and their decay time has been determined. This configuration is currently extended to a three-dimensionally confining blue-detuned dipole trap. For advanced laser cooling, state manipulation and spectroscopy of trapped atoms, a system of two diode lasers has been set up which are phase-coherently locked with a difference frequency near 6.834 GHz, to drive Raman transitions between the hyperfine-split ground states of 87Rb atoms. Resonances of such a transition have been recorded in a buffer gas loaded rubidium vapour cell with linewidths below 100 Hz.
We have laser cooled an atomic beam of rubidium with diode lasers and have used these slow atoms to load a magneto-optical trap. The cooling is performed with the frequency-chirp method,1,2 in which the laser's frequency is ramped linearly (by the injection currant) to maintain resonance with the decelerating atoms. Two synchronously chirped lasers are required to prevent optical hyperfine pumping of the ground state. The chirp typically covers 500 MHz in 10 ms. At the end of each ramp, the resulting velocity distribution is measured with a pulsed probe laser. By varying the end point of the chirp, we are able to obtain slow atoms with average velocities ranging from 200 to −50 m/s. The widths of the velocity distributions are measured to be approximately 40 m/s. This relatively large width results from the inherent linewidth (~30 MHz) of the free-running diode lasers used for both cooling and probing.
The range of atoms which can be cooled by lasers is limited to those which have a closed two level structure. Several schemes have been proposed which aim to extend this range by using coherent control of the particle momenta, but none have yet been demonstrated. We hope to implement these and other coherent manipulation schemes, and we begin with a system which is well understood and over which we can exert precise control. This thesis covers the design and construction of an experiment to demonstrate coherent manipulation of cold rubidium atoms collected in a magneto-optical trap. The lower hyperfine levels of these cold atoms very closely mimic the ideal two-level atom, and we use carefully crafted laser pulses to prepare, manipulate, and read their quantum state. The hyperfine levels are coupled using two fields whose frequency difference is equal to the hyperfine splitting. The way in which these Raman coupled levels can be used to emulate a two-level atom is explored, and the experimental apparatus used to create and control the driving fields is described in detail. The amplitude, frequency and phase of these fields is programmable, and complex manipulation schemes can be implemented merely by programming a computer. We have observed Raman transitions in the cold rubidium atoms, and the experimental methods used to detect these features amidst large experimental noise are discussed. Although we have not yet seen Rabi oscillations, we are confident that we can now have sufficient control to begin to implement simple interferometric sequences. However, there remain significant challenges if we are to coherently manipulate the momentum, and the prospects for such manipulation are discussed.
The long term aim of the work is to study non-equilibrium and symmetry-breaking phenomena for a Bose-Einstein condensate in time-dependent optical traps. The work presented in this thesis is mostly devoted to the design, construction and testing of the apparatus, in which condensation was successfully achieved. The first part of this thesis describes the design and construction of a hybrid setup for the production of a Bose-Einstein condensate (BEC) of 87 Rb in the |F = 2, m F = +2 > state. The vacuum system is composed of a first chamber, where a low-velocity intense source (LVIS) of atoms is produced to provide a pre-cooled atomic beam to the science chamber, where the magneto-optical trap (MOT) is produced, trapping more than 3 x 10 8 atoms at a temperature of roughly 400 μK. Forced radio- frequency evaporation is performed in a quadrupole magnetic trap and the atoms are transferred to a hybrid far-detuned optical trap in a crossed-beams configuration; evaporation in the optical potential leads to reliable production of a BEC every 50 s, composed of 10^5 atoms at a temperature of 10-30 nK. To ensure fine control on the optical potential depth and high intensity and pointing stability, a stabilisation system has been developed and characterised. The second part of this thesis describes the realisation of a 1D optical lattice, produced by two phase-locked laser beams emitting at 780 nm, with a red detuning of approximately 3.5 GHz, in counter-propagating configuration. Oscillating forces obtained by phase-modulating the optical potential are applied.
We describe an experiment which combines cold87Rb atoms from a grating magneto-optical trap (GMOT) with Lin±Lin coherent population trapping (CPT) and pulsed Ramsey interrogation. The bichromatic fields required for Lin±Lin are generated by combining light from a single external cavity diode laser (ECDL) with an electro-optic modulator (EOM) and an acousto-optic modulator (AOM). With this laser system and the GMOT, we are able to produce Raman-Ramsey fringes using either the F' = 1 or the F' = 2 excited states of the87Rb Di line. As a step towards realising a frequency standard based on the GMOT, we measure the Ramsey fringe amplitude as a function of the magnetic bias field and the excited state. We observe dark state interference with F'= 1 and show that this interference is suppressed with F' = 2, as expected from prior work on CPT with87Rb in thermal vapour cells.