  Distributing quantum entanglement between distant parties is a significant
but difficult task in quantum information science, as it can enable numerous
applications but suffers from exponential decay in the quantum channel. Quantum
repeater is one of the most promising approaches towards this goal. In a
quantum repeater protocol, it is essential that the entanglement generation
speed within each elementary link is faster than the memory decoherence rate,
to enable the scale-up of the quantum repeater by connecting neighboring
repeater segments. This stringent requirement has not been implemented over a
fiber of metropolitan scale so far. As a step towards this challenging goal, in
this work we experimentally realize multiplexing-enhanced generation of
heralded atom-photon quantum correlation over a 12km fiber. We excite the
memory modes in a multiplexed quantum memory successively to generate 280 pairs
of atom-photon quantum correlations with a train of photonic time-bin pulses
filling the long fiber. After successful detection of a heralding signal, the
excited memory mode can be identified and retrieved into idler photons on
demand with either fixed or variable storage time. With the multiplexing
enhancement, the heralding rate of atom-photon correlation can reach 1.95kHz,
and the ratio between the quantum correlation generation rate to memory
decoherence rate can be improved to 0.46 for a fiber length of 12km, which is
so far the best for long fiber length (>10km) to our knowledge. This work
therefore constitutes an important step towards the realization of a
large-scale quantum repeater network.
