title: Optical precursor of a single photon.
abstract: We report the direct observation of optical precursors of heralded single photons with step- and square-modulated wave packets passing through cold atoms. Using electromagnetically induced transparency and the slow-light effect, we separate the single-photon precursor, which always travels at the speed of light in vacuum, from its delayed main wave packet. In the two-level superluminal medium, our result suggests that the causality holds for a single photon.
authors.affiliation: Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.
authors.forename: Shanchao
authors.initials: S
authors.lastname: Zhang
authors.forename: J F
authors.initials: JF
authors.lastname: Chen
authors.forename: Chang
authors.initials: C
authors.lastname: Liu
authors.forename: M M T
authors.initials: MM
authors.lastname: Loy
authors.forename: G K L
authors.initials: GK
authors.lastname: Wong
authors.forename: Shengwang
authors.initials: S
authors.lastname: Du