  We present serial-parallel conversion for a heralded single photon source
(heralded SPS). We theoretically show that with the heralding signal, the
serial-parallel converter can route a stream of n photons to n different
spatial modes more efficiently than is the case without using a heralding
signal. We also experimentally demonstrate serial-parallel conversion for two
photons generated from a heralded SPS. We achieve a conversion efficiency of
0.533 \pm 0.003, which exceeds the maximum achievable efficiency of 0.5 for
serial-parallel conversion using unheralded photons, and is double the
efficiency (0.25) for that using beamsplitters. When the losses in the optical
converter are corrected for, the efficiency of the current setup can be
increased up to 0.996 \pm 0.006.
