  Efficient photon-photon interaction is one of the key elements for realizing
quantum information processing. The interaction, however, must often be
mediated through an atomic medium due to the bosonic nature of photons, and the
interaction time, which is critically linked to the efficiency, depends on the
properties of the atom-photon interaction. While the electromagnetically
induced transparency effect does offer the possibility of photonic quantum
memory, it does not enhance the interaction time as it fully maps the photonic
state to an atomic state. The stationary light pulse (SLP) effect, on the
contrary, traps the photonic state inside an atomic medium with zero group
velocity, opening up the possibility of the enhanced interaction time. In this
work, we report the first experimental demonstration of trapping a
free-propagating single-photon into a cold atomic ensemble via the quantum SLP
(QSLP) process. We conclusively show that the quantum properties of the
single-photon state are preserved well during the QSLP process. Our work paves
the way to new approaches for efficient photon-photon interactions, exotic
photonic states, and many-body simulations in photonic systems.
