Laser cooling and trapping of strontium atoms for experiments towards precision measurements and frequency metrology

Abstract

Measurement of physical parameters is the foundation of physical science. The improvement in precision of measurements has led to the discovery and validation of fundamental laws of physics. Development of measurement techniques in the field of atomic molecular and optical physics has opened the path to various discoveries such as the constancy of the speed of light, the early verification of quantum electrodynamics by performing the microwave spectroscopy for the fine structure of hydrogen atoms and the measurement of magnetic moment anomaly of the electron. Among all the base units, ‘time’ is the most important and the most accurately measured quantity. It forms the basis for the definition of other base units in the SI system. Precision measurement of time is thus one of the most crucial fields of research in science, called the frequency metrology. Currently, the unit of time, the ‘SI second’, is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of 133^Cs atom. This transition freuqency lies in the microwave regime of the electromagnetic spectrum. It has been shown that by switching to an optical transition with high Q value, one can increase the precision of measurement. These clocks with optical frequencies have been shown to be operated at the fractional uncertainty of the range of 10^−18. With such precision and accuracy, it holds the promise to revolutionize global timekeeping and tests of fundamental theories through the measurement of stability of fundamental constants of nature. This thesis describes the design and construction of an experimental setup for the generation of cold strontium (Sr) atoms in a magneto-optical trap (MOT) with the ultimate aim of making an optical atomic clock operating on the singlet to triplet intercombination line. Special efforts have been put to simplify and make the entire system robust. The whole setup is built on a 1.5 m X 3 m optical table. This table consists of both the optical setup for lasers as well as the vacuum assembly for generation, slowing, and trapping of the most abundant even isotope of Sr atoms. The MOT is loaded from an intense beam of Sr atoms generated by an oven and slowed by a zero-field crossing Zeeman slower. The oven is designed in such a way that it removes the complications of thermal isolation of the vacuum chamber form the high-temperature oven region. The laser for the first stage of cooling is stabilized to the atomic transition by performing an atomic beam spectroscopy employing a novel design of the spectroscopy cell with an inbuilt titanium sublimation pump (TSP). The experimental setup is capable of producing an ensemble of cold 88Sr with the atom number of 10^7 at a temperature of approximately mK. The atomic transition used as a reference for the clock freqeuncy is affected by several external factors. The total uncertainty budget of the clock transition is an addition of all the uncertainties caused by various factors e.g., AC stark shift, Zeeman shift, black body radiation shift, background gas collision induced shift, etc. Systematic evaluation of these sources of uncertainties is one of the vital steps towards realizing the full potential of the ultra-narrow clock transition. Among the factors mentioned above, the collision induced shift and broadening of the clock transition is of the order of 10^−19 (e.g., for H2 it is 6 X 10^−19). For the current generation of clocks, this shift is not yet the limiting factor; however for next-generation clocks, it is necessary to consider the effects of this small yet important effect. In this thesis, we also focus on the study of loss induced by the background N2 molecules and use this to measure the collision cross section between 88^Sr−N_2 in MOT. We employed the measured collision cross section for the determination of C6 of the ground state of Sr atoms. This quantity along with C6 of the excited state is useful in determination of collision-induced shift and broadening. One of the focus of the present work is the study of the dynamics of atoms inside the MOT. We present a model for considering the various loss mechanism in MOT and evaluate their contributions. We experimentally determine the losses due to the decay of atoms in the long-lived state 3^P_0 and due to the escape of atoms out of the MOT capture region. It occurs due to the branching of atoms into the state, which are not responsive to the first stage cooling laser. We further verify the experimentally measured loss rates with the calculated one in the presence of two different repumping schemes. We show that the contribution due to the latter can be the dominant loss channel. This loss channel is proportional to the atomic cloud temperature.