Quantum Interference Effects Induced By Excitation Phase

The relation between phase and interference has always attracted lots of attentions.The effects of phase on the interference phenomena are usually due to the phasematching between two beams or two photons. However，in this paper we investigatequantum interference under single photon condition. This kinds of quantuminterference is inspired by Harris’s question:“In what sense and how exactly doesphase matching hold in the single photon limit?”Scully and his colleagues found akind of phase which is induced in the process of atom excitation can answer theHarris’s question and this phase is called excitation phase. The most specialcharacteristic of this excitation phase in quantum interference is that it can be realizedby only one photon, i.e. phase matching for two atoms is caused by one photon. Inthis paper we investigate the quantum interference effects induced by excitation phaseunder different conditions in a system which contains two two-level atoms.This paper contains three parts: firstly, we investigate the effects of excitationphase on each atom of the two-atom system. Secondly, we investigate the effect ofexcitation phase in a cavity QED system. And finally, we consider the effects of theexcitation phase and observation phase (introduced by different observation positions)on the incoherent emission spectrum. The details are as follows:In the first part, we present the dynamical behavior of each atom in the two-leveltwo atom system. It is found that the dynamical behavior of each atom maydemonstrate differently with the existence of excitation phase. While the dynamicalbehaviors of two atoms are identical if the excitation phases are not considered. Thedifferent dynamical behaviors between two atoms are caused by the combining effectof the dipole-dipole interaction and excitation phase, and therefore differentdipole-dipole interaction can lead to different phenomena. Because the dipole-dipoleinteraction can lead to the excitation (photon) transfer between two atoms and theexcitation phase brought by photon is also transferred in this process. A relative phase is introduced in this process, which finally lead to a new quantum interference effectin the system. The physical essence of this kind of quantum interference is that thetransfer of excitation phases between two atoms can induce coherence term. We alsoinvestigate the entanglement between each atom and environment with the existenceof excitation phase, which can be used to explain the effect of excitation phase onentanglement between two atoms.In the second part, we discuss quantum coherence caused by the excitation phase ina cavity QED system under single photon transfer. Because the coupling betweenatom and vacuum modes is very weak, so the interference term induced by excitationphase transfer is also very weak and the quantum interference in vacuum is a weakeffect. However, when two atoms are located at a cavity, the coupling between atomand cavity mode can be very strong, in this situation the excitation phase transfer ismainly by cavity mode instead of the vacuum mode. Quantum interference inducedby excitation phase in a cavity QED system can be displayed evidently due to thestrong coupling between atoms and cavity mode.In the third part, we investigate the effects of both excitation phase and observationphase which is introduced by the different observation positions on the incoherentemission spectrum in the two-level two atom system. We find that the spectrum mayshow asymmetric forms with the existence of excitation phase. Though theasymmetric spectrum is induced by the excitation phase, the spectrum can be furthermodified by the observation phase, so the total spectrum exhibits the effects of bothexcitation phase and observation phase. We also notice that the incoherent emissionspectrum show symmetric forms with the existence of the excitation phase in thelimitation conditions that the intensity of driving field is very weak or very strong.Thus we should consider both the impact of the vacuum field and the intensity ofdriving field on the effects of quantum interference caused by excitation phase. Thesystem contains two kind of timed Dicke states with the existence of both the drivingfield and dipole-dipole interaction. We further give the dynamical evolution of coherence term asas function of driving field intensity and incident angle, we findthat the coherence term is affected by excitation phase as well as the intensity ofdriving field, which explain the characteristics of incoherent emission spectrum.