Unified integration scheme using an N × N active switch for efficient generation of a multi-photon parallel state

A source to efficiently generate multiple indistinguishable single photons in different spatial modes in parallel (multi-photon parallel state) is indispensable for realizing large-scale photonic quantum circuits. "A naive scheme" may be to use a heralding single photon source with an on-off detector set at each of parallel modes and to select the cases where each mode contains one photon at the same time. However, it is also necessary to suppress the probability of generating more than two photons from a single-photon source. For this requirement, serial-parallel conversion and a multiplexed heralded single photon source (HSPS) have been proposed and demonstrated. In this paper, we propose and demonstrate a novel method to produce a multi-photon parallel state efficiently using multiple HSPSs and an N × N active optical switch. As an advantage over the simple combination of a spatial multiplexed HSPS and a serial-parallel converter, our method, called the "unified integration scheme," can generate a multi-photon parallel state with minimized optical losses in the switch. Using a 2 × 2 active optical switch and a fixed delay, we achieve an enhancement factor of 1.59 ± 0.14, compared with a naive scheme using two HSPSs, and better than the factor of 1.46 using the simple combination scheme. Furthermore, we confirm the reduction of multi-photon events to 62 % of that of the naive scheme.