The Superradiant Phase Transition In ~6Li Fermi Gases Coupled With An Optical Cavity

The interaction of light and matter is one of the most basic problems in physics.A well-known example is the Jaynes-Cummings model,which describes the strong coupling interaction between a quantized light field in an optical cavity and a single two-level atom.This model leads to important physics problems such as vacuum Rabi splitting,high-efficiency single-photon source,and quantum network.In recent years,with the development of precision control techniques of cold atoms,the strong coupling between an optical cavity and ultra-cold quantum gases has opened a new direction for studying quantum phase transition and non-equilibrium dynamics in many-body physics.In the 1970s,it was predicted that there will be a steady-state superradiant Dicke phase transition in such system.Theoretically,it is believed that the phase transition threshold needs to reach the energy scale of THz.Therefore the superradiant phase transition in the cold atom was considered impossible to achieve.Until recently,in the ultra-cold Bose gases,the Dicke quantum phase transition was observed using the Raman process between the momentum states of the Bose gases mediated by the cavity.Coupling fermion with cavity-QED has attracted wide attention for a long time,but experimental research is extremely challenging.Fermi gases coupled with an optical cavity can trigger many novel physical phenomena due to the existence of long-range interactions,such as glass state,topological phase transition,etc,which still need to be confirmed experimentally.This thesis focuses on the experimental work of Fermi gases in an optical cavity.In this work,we prepare a quantum degenerate ultra-cold~6Li Fermi gases by all-optical means and realize the strong coupling between atoms and the optical cavity.Furthermore,we realize the steady-state superradiant phase transition by increasing a lateral pump lattice potential.It has been observed experimentally that the spontaneous establishment of the optical field inside cavity is accompanied by the self-organization of the atomic cloud,which marks the occurrence of superradiant phase transition.By changing the pump-cavity detuning,we obtain the steady-state phase diagram.Especially,we confirmed for the first time that Fermi statistics play an important role in the superradiant phase transition by precisely manipulating the temperature of atoms and the number of atoms in the optical cavity.Due to the Pauli principle of inclusion,the threshold of superradiant phase transition is inversely proportional to the square root of atom number at low temperature,which is significantly different from the bosons whose threshold is inversely proportional to the atom number.We also discovered a novel slow dynamic behavior:when the pump field is gradually turned off at different speeds,atoms of low momentum states are slower than that of high momentum states when returning to the initial state.This behavior,similar to the band-mapping process,reflects the differences between fermions and bosons.We have also studied the phase transition threshold dependence on temperature,density,and other conventional experimental parameters.The fermionic superradiant phase transition achieved by this work will open up the new research direction of many-body physics such as many-body localization and quantum prethermalization etc.In this thesis,Chapter 1 introduces the development of the research field;Chapter 2 introduces the theoretical background of Fermi gas;Chapter 3 intro-duces the experimental system of an optical cavity and ultra-cold Fermi gas in detail:including ultra-high vacuum system,laser system,magneto-optical trap,and high-power far detuned dipole trap,the optical cavity used for the science experiment,as well as self-made reference cavity for two-color light stabilization;Chapter 4 introduces the imaging technology;Chapter 5 introduces the theory of Fermi superradiance and the experimental results about quantum statistics in detail,and also introduces the results of the dependence of quantum phase transitions on experimental parameters are introduced;the last is a summary of this thesis and prospects for follow-up research.