  Electromagnetically induced transparency (EIT) and Autler-Townes splitting
(ATS) are similar,but different quantum optical phenomena: EIT results from a
Fano interference, whereas ATS is described by the AC-Stark effect. Likewise,
despite their close resemblance, light-storage techniques based on the EIT
memory protocol and the recently-proposed ATS memory protocol (E. Saglamyurek
et al. Nature Photonics 12, 2018) are distinct: the EIT protocol relies on
adiabatic elimination of absorption, whereas the ATS protocol is based on
absorption. In this article, we elaborate on the distinction between EIT and
ATS memory protocols through numerical analysis and experimental demonstrations
in a cold rubidium ensemble. We find that their storage characteristics
manifest opposite limits of the light-matter interaction due to their inherent
adiabatic vs. non-adiabatic nature. Furthermore, we determine optimal memory
conditions for each protocol and analyze ambiguous regimes in the case of
broadband storage, where non-optimal memory implementations can possess
characteristics of both EIT and ATS protocols. We anticipate that this
investigation will lead to deeper understanding and improved technical
development of quantum memories, while clarifying distinctions between the EIT
and ATS protocols.
