  On-demand generation of indistinguishable single- and multi-photon states is
a key technology for scaling up optical quantum information and communication
applications. Nonlinear parametric photon-pair sources and heralded
single-photon sources (HSPSs) had been the most standard resource of quantum
information applications for decades. However, the intrinsic uncertainty of the
produced number of photon pairs in such sources is a critical drawback that
prevents on-demand photon-pair and heralded single-photon generation. Here we
demonstrate large-scale time multiplexing of indistinguishable heralded single
photons, employing a low-loss HSPS and adjustable delay line. We observed 66.7%
presence probability of single-photon states collected into a single-mode
optical fiber by multiplexing 40 periodic time bins of heralded single photons.
To our knowledge, this is the highest fiber-coupled single-photon probability
achieved to date. A high indistinguishability (~90%) of our time-multiplexed
photons has also been confirmed. We also experimentally investigate trade-off
relations of single-photon probability and unwanted multi-photon contribution
by using different pump powers for a HSPS. Our results demonstrate that
low-loss, large-scale multiplexing can realize highly efficient single-photon
generation as well as highly scalable multi-photon generation from inefficient
HSPSs. We predict that our large-scale time multiplexing will pave the way
toward generation of > 30 coincident photons with unprecedented efficiencies,
enabling a new frontier in optical quantum information processing.
