title: Quasieigenstate coalescence in an atom-cavity quantum composite.
abstract: We report the first direct observation of an exceptional point (EP) in an open quantum composite of a single atom and a high-Q cavity mode. The atom-cavity coupling constant was made a continuous variable by utilizing the multisublevel nature of a single rubidium atom when it is optimally coupled to the cavity mode. The spectroscopic properties of quasieigenstates of the atom-cavity composite were experimentally investigated near the EP. Branch-point singularity of quasieigenenergies was observed and its 4pi symmetry was demonstrated. Consequently, the cavity transmission at the quasieigenstate was observed to exhibit a critical behavior at the EP.
authors.affiliation: School of Physics and Astronomy, Seoul National University, Seoul, 151-747, Korea.
authors.forename: Youngwoon
authors.initials: Y
authors.lastname: Choi
authors.forename: Sungsam
authors.initials: S
authors.lastname: Kang
authors.forename: Sooin
authors.initials: S
authors.lastname: Lim
authors.forename: Wookrae
authors.initials: W
authors.lastname: Kim
authors.forename: Jung-Ryul
authors.initials: JR
authors.lastname: Kim
authors.forename: Jai-Hyung
authors.initials: JH
authors.lastname: Lee
authors.forename: Kyungwon
authors.initials: K
authors.lastname: An