Over the past decade there have been a lot of remarkable progress in cooling,trapping and manipulating atoms.The precise measurement of temperature and density of ultra-cold atoms becomes an important subject.In the measurement of the temperature of cold atoms,one needs to turn off the magnetic field and the light field in a very short time(less than several milliseconds).According to the requirements of the time-flying method in measuring the cold-atom temperature,one can design and make a pulse generator whose pulse-width and delay time are controllable.Then the pulses are applied to the switches of magneto-optical trap,and the magnetic and light field can be turned off in short time successfully.
Over the past decade there have been a lot of remarkable progress in cooling,trapping and manipulating atoms.The precise measurement of temperature and density of ultra-cold atoms becomes an important subject.In the measurement of the temperature of cold atoms,one needs to turn off the magnetic field and the light field in a very short time(less than several milliseconds).According to the requirements of the time-flying method in measuring the cold-atom temperature,one can design and make a pulse generator whose pulse-width and delay time are controllable.Then the pulses are applied to the switches of magneto-optical trap,and the magnetic and light field can be turned off in short time successfully.
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
The aim of this research is to pave the way for making a trap for cold neutral atoms based on the force generated by pulses of a mode-locked laser. As an onset towards such a trap we decided to use a far-off-resonance trap (FORT) loaded with cold rubidium atoms from a magneto-optical trap (MOT). Our FORT setup consists of a titanium-sapphire laser that can operate both in continuous wave mode and in mode-locked mode. Our MOT setup uses a magnetic field gradient, and two diode lasers whose frequency is locked very accurately to the atomic transition using polarisation spectroscopy. Since the measurements we do require very precise timing of the order of less than one millisecond between the various phases of the experiment, a time-sequencing program was developed. Using this sophisticated setup, several characteristics of FORTs made with continuous and pulsed lasers were analysed. For example, we have investigated the loading behaviour of the traps and found that under general circumstances there is no significant difference. However, under special conditions it is possible to load more atoms into a pulsed trap. The lifetime of the number of atoms in the trap shows no dependence on the power of the trap laser in the continuous case. For the pulsed trap, however, the increased scattering due to the high peak intensities of the pulses does limit the achievable lifetimes. Moreover, due to the presence of a large number of photoassociation lines, i.e. lines were loosely bound molecules are formed by the interaction of two atoms an a photon, the two-particle losses depend heavily on the wavelength of the trap laser. Furthermore we have done measurements of the temperature of the trapped atoms, which revealed that the temperature is a constant fraction of the trap depth. We also looked at the effects of parametric excitation of the atoms in the FORT. The anharmonicity of the trap formed by the titanium-sapphire laser results in interesting physics, including a change in the average temperature of the atoms, depending on the modulation frequency. We also investigated the trapping behaviour of the FORT at low pulse repetition rates by switching the laser intensity on and off on a kHz scale. We investigated the dependence of the number of atoms on the polarisation of the FORT laser light, both for continuous and pulsed traps. Atoms can be trapped at elliptical polarisations as well, although the number of trapped atoms is less. As the laser is detuned further to the red of the rubidium D line, this effect becomes less and less pronounced, as predicted by our model.
Cooling and trapping neural atoms with laser beams became a standard method of studying atom properties in temperatures close to absolute zero. Precise knowledge of physical parameters of the atomic sample is of prime importance in such studies, particularly the knowledge of the number of trapped atoms, their density and temperature. Diagnostics aiming at determination of these parameters has to be performed optically, mainly with the spectroscopic methods. In this paper, we present basic principles of obtaining ultra-cold atoms and methods used for their diagnostics. Some examples of quantum effects specific for low-temperatures, revealed by these methods, are also demonstrated.
We present a method for measuring the average temperature of a cloud of cold 85 Rb atoms in a magneto-optical trap using an optical nanofibre. A periodic spatial variation is applied to the magnetic fields generated by the trapping coils and this causes the trap centre to oscillate, which, in turn, causes the cloud of cold atoms to oscillate. The optical nanofibre is used to collect the fluorescence emitted by the cold atoms, and the frequency response between the motion of the centre of the oscillating trap and the cloud of atoms is determined. This allows us to make measurements of cloud temperature both above and below the Doppler limit, thereby paving the way for nanofibres to be integrated with ultracold atoms for hybrid quantum devices.
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.
Physicists know how to produce gases at a few billionths of a degree above absolute zero. The cooling methods apply not only to atoms but also to ions and molecules. This field of research has three times been awarded the Nobel Prize. The field experienced remarkable growth when experimentalists learned how to vary at will the interactions between particles, trapping them with optical tweezers or in optical gratings with adjustable geometry. Artificial crystals made of atoms or molecules can be built to simulate the structure of matter and elucidate some of its magnetic properties, hopefully contributing to the understanding of high-temperature superconductivity. The phenomenon of quantum entanglement is the basis for new devices for the storage and transmission of quantum information. Spectacular progress is constantly being made in metrology. For example, ultra-cold atom or ion clocks measure time to better than one second over the lifetime of the Universe. New types of industrial gravimeters and gyroscopes are improving the sensitivity of seismology and navigation in space. In addition, the extreme precision of the measurements allows tests of the fundamental laws of physics, such as quantum electrodynamics, Lorentz invariance or possible variations of the fundamental constants. The field of ultra-cold particles has now reached the stage where it provides insights in the fields of condensed matter, chemistry and even cosmology.