Radiative Properties Of Atoms And The Quantum Thermal Effects Of Spacetime

Spontaneous emission and energy shift are the most important features of atoms and they may be attributed to vacuum fluctuations,or radiation reaction, or a combination of them.The ambiguity in physical interpretation arises from different choices of ordering of commuting operators of atom and field in the interaction Hamiltonian.Significant progress has been made by Dalibard,Dupont-Roc and Cohen-Tannoudji(DDC) in 1982,who argued that there exists a symmetric operator ordering between the operators of atom and field that renders the distinct contributions of vacuum fluctuations and radiation reaction to the atom's spontaneous emission and energy shift separately Hermitian.On the other hand, quantum field theory in curved spacetime is the theory of quantum fields propagating in a classical curved spacetime and it is not a final quantum theory of gravity for there are no quantum effects of gravity and no influence of the quantum field to the classical spacetime in this theory,but some classical works in this field have revealed intriguing effects,such as the quantum thermal effects of spacetimes including the Unruh effect,Hawking effect and the Gibbons-Hawking effect,which are of great importance for us to understand features of quantum gravity.In dissertation,we will study,using the formalism of DDC,the radiative properties of atoms and the quantum thermal effects of spacetimes in several different spacetimes,and thus reveal an interesting relationship between these two seemingly different but physically important phenomena.In flat spacetimes,we study the spontaneous excitation of a uniformly accelerated multilevel atom interacting with a vacuum electromagnetic field.Firstly, in the free Minkowski spacetime,if an atom is initially in its ground state in the vacuum,we find that on an inertial trajectory the atom is stationary,but on a uniformly accelerated trajectory the spontaneous excitation is allowed to occur, and besides a thermal correction,the acceleration of atom also gives an extra correction proportional to a~2.This extra term is not in the form of a thermal effect and makes the behavior of a uniformly accelerated atom different from that of an inertial atom immersed in a thermal bath.However,we show that the mean atomic excitation energy will reach an equilibrium value characterized by the Unruh temperature after a long time of evolution.Secondly,in the Minkowski spacetime with an infinite conducting plane,if the atom is uniformly accelerated in a parallel direction,we find that the modification of the rate of change of the mean atomic energy caused by the presence of the conducting plane boundary is also in the form of a nonthermal correction and it depends on the polarization of atom.Even if the atom is polarized isotropically,the polarizations in different directions contribute differently.Our results reveal,therefore,that the spontaneous excitation of accelerated atoms in vacuum,as an actual physical process,gives an illustration for why the accelerated particle detector in the vacuum clicks and provides a transparent physical mechanism for the Unruh effect.In addition,for both the cases of a uniformly accelerated two-level atom interacting with a quantized real massless scalar field and a uniformly accelerated multilevel atom interacting with a quantized electromagnetic field,we calculate the rate of change of the mean atomic energy from the point of view of a coaccelerated frame.Comparison with the results in the inertial frame shows that the same rate of change of the mean atomic energy can be obtained in these two different frames only when we assume a thermal bath at the Unruh temperature in the coaccelerated frame.Therefore,the vacuum defined in the inertial frame is a Rindler thermal bath at Unruh temperature T = a/2Ï€for a coaccelerated observer. This result clarifies some confusions in the past and deepens our understanding of the Unruh effect.In the curved spacetimes,we consider,in de Sitter space,both freely falling and static two-level atoms in interaction with a conformally coupled real massless scalar field in the de Sitter-invariant vacuum,and calculate the atoms' spontaneous excitation rate.We find that for the freely falling atom,the spontaneous excitations occur and the spontaneous excitation rate is equal to that of an inertial atom immersed in a thermal bath at the Gibbons-Hawking temperature in the flat spacetime.Thus we provide a new approach to the derivation of the Gibbons-Hawking effect.For the static atom,our results show that the atom also perceives a thermal bath of radiation,and the squared temperature of the thermal bath is just the sum of the squared Gibbons-Hawking temperature and the squared Unruh temperature.Finally,considering a two-level atom interacting with a quantized electromagnetic fields at a finite temperature,we study the modification of energy level shift of a static atom caused by the presence of an infinite conducting plane.When the distance z between the atom and the conducting plane varies in three different regimes,i.e.,the short distance,the intermediate distance and the long distance regime,we discuss the modification by the presence of the conducting plane of the atom's energy level shift and the induced force acting on the atom in both the low temperature limit and the high temperature limit for both the ground state and exited state atoms.Different from treating atoms as a limiting case of a macroscopic dielectric,the approach we use in this dissertation allows us to reveal the influence of different atomic polarizations on the energy level shifts and thus the force acting on the atoms.