Fundamental Distinction of Electromagnetically Induced Transparency and Autler-Townes Splitting in Breaking the Time-Reversal Symmetry

Despite the essential difference in underlying physics, electromagnetically induced transparency (EIT) and Autler-Townes splitting (ATS) are difficult to be discriminated because they cause resemble absorption and dispersion to a probe electromagnetic field. They are mainly discerned in the sense of absorption profile fitting. Here, the breakdown of the time-reversal symmetry (TRS), namely optical nonreciprocity in the EIT and ATS configurations are experimentally observed by using warm rubidium atoms. The microscopic Doppler effect due to atomic thermal motion causes strong optical nonreciprocity to the probe field in the EIT configuration. In stark contrast, the propagation of the probe field is primarily reciprocal in the ATS configuration. The experimental observations in this study objectively distinguish the EIT and ATS effects in the fundamental physics of breaking the TRS. This experiment proves a concept of using the TRS as a testbed for discerning fundamental physical effects.