This thesis presents a study on the realisation of a usable rubidium atomic beam inside a sealed vapour cell. A velocity selective optical pumping scheme was implemented to enable the detection of atoms of a particular velocity class travelling between two separated laser beams. This represents a new spectroscopic technique, the usefulness of which has been demonstrated through its applications. In the present work, the vapour cell atomic beam has been used to measure Larmor precession of polarised atoms in a magnetic field and one-dimensional laser cooling.
This thesis presents a study on the realisation of a usable rubidium atomic beam inside a sealed vapour cell. A velocity selective optical pumping scheme was implemented to enable the detection of atoms of a particular velocity class travelling between two separated laser beams. This represents a new spectroscopic technique, the usefulness of which has been demonstrated through its applications. In the present work, the vapour cell atomic beam has been used to measure Larmor precession of polarised atoms in a magnetic field and one-dimensional laser cooling.
We describe a simple experiment that demonstrates one-dimensional laser cooling in a sealed vapor cell. A velocity selective optical pumping scheme was first used to define a collimated beam of atoms within the cell. A particular velocity group of atoms was labeled by optical pumping with one laser and detected by absorption with a second laser. Transverse cooling of this velocity group, which formed an atomic beam between the two laser beams, was then observed by applying a third laser beam, in analogy with transverse cooling of a conventional atomic beam.
Quantum optical sensing is a maturing field that offers the capability to achieve extraor- dinary sensitivity. Atomic magnetometers, for example, now have the capability to detect brain signals and to precisely measure Earth’s gravity field. In optical atomic magnetome- ters, the electronic spins of the atoms in vapor form are aligned in the same direction by “pumping” the ensemble using laser light at a select frequency. Any magnetic field alters the spin states, which changes the optical properties of the vapor, such as the transmission of the laser light, providing a way to measure magnetic fields. After reviewing the theory of atomic structure and atom-light interactions relevant to the research presented here, I describe in the first part of this thesis a vector magnetometry method using microwave-assisted optical pumping, in which a microwave field resonant to the ground state hyperfine splitting of 87Rb pumps an ensemble of warm rubidium vapor, in addition to a strong optical pump beam, and a weaker optical probe beam. I present theoretical details as well as preliminary data demonstrating magnetic field measurements of a DC magnetic field. The measurements were taken when the DC field was aligned at various angles relative to the microwave magnetic field alignment. I compare theoretical and experimental differences in the measurements. With new methods for quantum devices comes the need for portability. In the second part of this thesis, we demonstrate the fabrication of millimeter-sized rubidium vapor cells using silicon micro-machining techniques. This method is relatively inexpensive compared to glass-blowing methods and allows rapid, large-scale production. I describe our process flow and demonstrate the presence of rubidium atoms inside our vapor cells. To conclude, suggestions for improvement are discussed.
The development of an experimental apparatus to produce Bose-Einstein condensates (BECs) of ⁸⁷Rb atoms and their application to magnetometry are discussed. Optical detection of atomic Larmor precession is a widely explored method for high-sensitivity measurements of magnetic fields. In this context, short laser/atom interaction time, atomic thermal diffusion and decoherence effects are among the main limitations. In this thesis, we overcome such problems by using spin-polarised ⁸⁷Rb ultra-cold atoms as the sensing element. After the atoms are polarised, a resonant pulse of radio-frequency excites Larmor precession, which is sensitive to external magnetic fields. By measuring the perturbations of the radio-frequency induced spin precession, information on the magnetic fields of interest. This is achieved by monitoring the polarisation plane’s rotation of a linearly polarised resonant laser probe. In the first part of this thesis, the building and optimisation of a laser-cooling set up to obtain a BEC in a hybrid trap is reported. In order to achieve the Phase Space Density (PSD) required for BEC, several different stages of trapping and cooling are necessary. Each phase has been implemented and optimised. The first step consists in the magneto-optical trap (MOT). Here a velocity dependent damping force and a spatially dependent confining force give the largest changes in PSD. Then atoms are loaded into a hybrid trap obtained by overlapping a quadrupole magnetic potential and a far detuned optical crossed dipole trap. The final stage for the condensation consists of forced evaporative cooling, both via magnetic and optical evaporation. In the second part of the thesis, a general overview of the principles of optical atomic magnetometry is provided and the advantages of using ultra-cold atoms with respect to conventional thermal vapours are discussed. The implementation, operation and a preliminary characterisation of the ultra-cold atom magnetometer are described along with the preliminary results collected. Finally, a plan for future improvements of its sensitivity is presented.
A rubidium vapour magnetometer has been made which measures the total geomagnetic field and records its variations continuously over the frequency band 0-1 c/s with sensitivity of 5 × 10 -6 Oe. Optical pumping technique is used to observe the energy separation between the Zeeman sub-levels in the ground state of 85 Rb atom, which is the measure of the ambient magnetic field and is equal to 220 kc/s for the local field of 472 × 10 -3 Oe, with the line width of 200 c/s. Resonance is detected when an applied r.f. field at the Larmor frequency causes redistribution of the population of the energy levels thus increasing absorption in the gas cell of the pumping radiation at 7948 Å. A voltage controlled oscillator supplying the r.f. field is locked on to the Larmor frequency. Resonance radiation is obtained from a rubidium lamp excited from a 150 Mc/s, 2 w power supply. The gas cell contains natural rubidium. This is kept at a temperature of 35-45° C thus maintaining vapour pressure of about 10 -6 mmHg. The design of the lamp and the gas cell is given. Further work to improve the light source and the gas cell should improve the sensitivity of the instrument. Redesign of the electronics to make the system oscillate at the Larmor frequency may also improve sensitivity and increase the frequency response of the instrument. The instrument described was demonstrated at The Institute of Physics and The Physical Society Exhibition in January 1961.
This thesis set out to investigate light storage based on dynamic electromagnetically induced transparency (EIT) in a room-temperature atomic ensemble of rubidium as a means to provide a quantum memory for single-photons created by a single rubidium atom coupled to a high-finesse optical resonator. Setting up the light storage medium presented a new addition to the research group's portfolio of experimental techniques and led to investigations of EIT, slow light and stored light in warm rubidium-87 vapour. Lambda level schemes connecting Zeeman or hyperfine substates on the D1 and D2 lines were addressed in rubidium vapour cells containing different buffer gases and different isotopic fractions of rubidium-87 and rubidium-85. Single beam spectroscopy with a weak probe was used to characterise the vapour cells. A numerical method to fit the D line spectrum to a theoretical model to include isotopic fractions and collisional broadening of a buffer gas has been implemented. Temperature and isotopic fractions could be reliably extracted from the fit parameters. For an offset-stabilisation of two lasers to address a lambda level scheme connecting the two different hyperfine groundstates in rubidium a phase locked loop including a frequency divider has been designed and implemented. Light storage and retrieval has been demonstrated using a Zeeman scheme on the D1 line. Two microsecond long classical light pulses containing one million photons on average were stored and retrieved with an efficiency of 15% after a delay of one microsecond. Several methods of attenuating the strong co-propagating control laser beam to allow for lowering the signal pulse intensity in future experiments are discussed.
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.