  Quantum coherence in quantum optics is an essential part of optical
information processing and light manipulation. Alkali metal vapors, despite the
numerous shortcomings, are traditionally used in quantum optics as a working
medium due to convenient near-infrared excitation, strong dipole transitions
and long-lived coherence. Here, we proposed and experimentally demonstrated
photon retention and subsequent re-emittance with the quantum coherence in a
system of coherently excited molecular nitrogen ions (N2+) which are produced
using a strong 800 nm femtosecond laser pulse. Such photon retention,
facilitated by quantum coherence, keeps releasing directly-unmeasurable
coherent photons for tens of picoseconds, but is able to be read-out by a
time-delayed femtosecond pulse centered at 1580 nm via two-photon resonant
absorption, resulting in a strong radiation at 329.3 nm. We reveal a pivotal
role of the excited-state population to transmit such extremely weak re-emitted
photons in this system. This new finding unveils the nature of the coherent
quantum control in N2+ for the potential platform for optical information
storage in the remote atmosphere, and facilitates further exploration of
fundamental interactions in the quantum optical platform with strong-field
ionized molecules.
