Study Of Dissipative Force And Laser Cooling Of A Three-Level Atom In A V Configuration

This thesis presents a model to calculate dissipative force and laser cooling of a V-type three level atom in coherent fields in a squeezed vacuum.From the Hamil-tonian of the atomic system,making use of Born-Markoff approximation, drive the master equation for the system.Then we can obtain the atomic Bloch equation.Using the adiabatic approximation,we acquire the expression of the mean dissipative force.In general,the coefficients of the optical Bloch equation in a squeezed vacuum are time-dependent.In this thesis we only consider the steady state solutions of the optical Bloch equation when its coefficients are time-independent.Then we use graphical method to discuss the dependence of the attained dissipative force on parameters such as quantum interference,spontaneous decay rates,average photon number,the squeezed degree of the light field and phase difference etc.In order to study the laser cooling,we introduce a transformation for convince of mathematical operation.Then the optical Bloch equation can be changed into a different form containing only real variable.Using the quantum regression theorem,we find the diffusion coefficient of the atomic momentum fluctua-tion.The achievable equilibrium temperature is given by the Einstein relation.Then the graphical method is used to analyze the dependence of the ultimate temperature on quantum interference, average photon number,the squeezed degree of the light field and phase differences.In the second chapter,we simply introduce several cooling theories, some application and the development of laser cooling. In the third chapter,we mainly introduce some common approximation conditions.We calculate the Hamiltonian of a two level atom interacting with a coherent field by using of full quantum theory and semi-classical theory,respectively.Making use of Born-Markoff approximation, acquire the master equation for the V-type three level atom.In the fourth chapter,we study the dissipative force of a V-type three level atom in an ordinary vacuum and a squeezed vacuum.The theoretical result shows that: the phenomenon of coherent populations trapping is obtained when the two dipolar moments are parallel in an ordinary vacuum.In an ordinary vacuum,the dissipative becomes much smaller because of much bigger quantum interference when the two Rabi-frequency is lesser and unequal. The dissipative force also depends strongly on the average photon number,phase difference and the squeezed degree of the light field in a squeezed vacuum.When the phase matching condition is fulfilled,the dissipative force is diminished in a squeezed vacuum.The greater squeezed degree of the light field,the bigger of the dissipative force can be obtained when the average photon number is confirmed.In the fifth chapter we study the laser cooling of a V-type three level atom in an ordinary vacuum and a squeezed vacuum.The result shows that: The quantum interference intensively affect the ultimate temperature in an ordinary vacuum.The atom cannot be cooled under the Doppler limit when the quantum interference is 0. But the theoretical result shows that the lower temperature,less than 1/30 of the Doppler-limit can be achieved under the condition of strong quantum interference.The ultimate temperature of the atom depends strongly on the average photon number,phase dif-ference,and the squeezed degree of the light field in a squeezed vacuum: much smaller average photon number and bigger phase difference can be fit for cooling atom when the light field is in a minimum uncertain state. The greater squeezed degree of the light field,the lower temperature can be obtained when the average photon number is confirmed.