Phase transition induced by a laser field in a system of two-level atoms

Chih-Chun Chang, Lee Lin

In a quantum electrodynamics (QED) system of arrayed two-level atoms interacting with light, because the energy of the photon is around the spacing between two atomic energy levels, the photon will be absorbed and is not in the propagating mode but the attenuated mode. Our theoretical study shows that coherent phasing of atomic states occurs when the photons are in the attenuated mode. The existence of this ordered state is a result of a quantum phase transition induced by the mediation of the attenuated photonic field. By tuning the intensity of the light, the ordered state can be manipulated so that two-level atoms can be in an arbitrary uniform linear combination of the single-atom ground state and the excited state. Potential applications of this phenomenon are quantum computation, lasing physics, optical lattices and related subjects in nanotechnology. Similar phase transitions induced by the mediation of transmuted intermediate boson-like phonons, magnons or other fields may also occur in condensed-matter systems.

S. V. Lawande

Kozo Fujii

Jiepeng Zhang, Yifu Zhu

Atomic coherence and interference manifested by electromagentically induced transparency (EIT) and coherent population trapping (CPT) plays an important role in the current studies of atom-photon interactions and has found numerous applications in optical physics. EIT is created in a three-level atomic system by a coupling field and results in destructive interference between two excitation paths of a weak probe laser interacting with the atomic medium. This leads to suppressed linear absorption and rapidly varying atomic dispersion for the probe laser near the atomic resonance, which provides the platform for a variety of applications such as nonlinear optics at low light levels, slow light manipulation, and quantum state engineering for photons and atoms. Here we extend the simple three-level EIT system to more complicated and versatile configurations in a multi-level atomic system coupled by multiple laser fields. We show that with multiple excitation paths provided by different laser fields, phase-dependent quantum interference is induced: either constructive or destructive interference can be realized by varying the relative phases among the laser fields. Two specific examples are discussed. One is a three-level system coupled by bichromatic coupling and probe fields, in which the phase dependent interference between the resonant two-photon Raman transitions can be initiated and controlled. Another is a four-level system coupled by two coupling fields and two probe fields, in which a double-EIT configuration is created by the phase-dependent interference between three-photon and one-photon excitation processes. We analyze the coherently coupled multi-level atomic system and discuss the control parameters for the onset of constructive or destructive quantum interference. We describe two experiments performed with cold Rb atoms that can be approximately treated as the coherently coupled three-level and four-level atomic systems respectively. The experimental results show the phase-dependent quantum coherence and interference in the multi-level Rb atomic system, and agree with the theoretical calculations based on the coherently coupled three-level or four-level model system.

Seyed M. Sadeghi, Jochen Meyer

We study the generation of an ultranarrow gain spectrum in an atomic transition when the upper transition level is coupled to the lower transition level by one laser and to a metastable state by another laser. It is shown that this spectrum switches abruptly into an ultranarrow absorption line with an increase in the first laser intensity. These results are explained by investigating the electromagnetically induced transparency (EIT) in a system where the upper transition level is coupled to two metastable states by two laser fields (K system). We discuss various features of such a transparency and show how it can be described as a structuring process, similar to the laser-induced continuum structure. Utilizing such a description of EIT, we explain the ultranarrow gain generation as a stimulated Rayleigh scattering process.

E. Krüger