  When implementing O-band quantum key distribution on optical fiber
transmission lines carrying C-band data traffic, noise photons that arise from
spontaneous Raman scattering or insufficient filtering of the classical data
channels could cause the quantum bit-error rate to exceed the security
threshold. In this case, a photon heralding scheme may be used to reject the
uncorrelated noise photons in order to restore the quantum bit-error rate to a
low level. However, the secure key rate would suffer unless one uses a heralded
photon source with sufficiently high heralding rate and heralding efficiency.
In this work we demonstrate a heralded photon source that has a heralding
efficiency that is as high as 74.5%. One disadvantage of a typical heralded
photon source is that the long deadtime of the heralding detector results in a
significant drop in the heralding rate. To counter this problem, we propose a
passively spatial-multiplexed configuration at the heralding arm. Using two
heralding detectors in this configuration, we obtain an increase in the
heralding rate by 37% and a corresponding increase in the heralded photon
detection rate by 16%. We transmitted the O-band photons over 10 km of noisy
optical fiber to observe the relation between quantum bit-error rate and
noise-degraded second-order correlation function of the transmitted photons.
The effects of afterpulsing when we shorten the deadtime of the heralding
detectors are also observed and discussed.
