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We measure the nonlinearity of a telecom-wavelength superconducting nanowire single-photon detector via incoherent beam combination. At typical photon count rates and detector bias current, the observed relative deviation from a perfectly linear response is in the order of 0.1% when the flux is doubled. This arises from a balance between the counteracting nonlinearities of deadtime-induced detector saturation and of multi-photon detections. The observed behaviour is modelled empirically, which suffices for a correction of measured data. In addition, statistical simulations, taking into account the measured recovery of the detection efficiency, provide insight into possible mechanisms of multi-photon detection., Comment: 9 pages, 8 figures

We report on a multi-photon source based on active demultiplexing of single photons emitted from a resonantly excited GaAs quantum dot. Active temporal-to-spatial mode demultipexing is implemented via resonantly enhanced free-space electro-optic modulators, making it possible to route individual photons at high switching rates of 38 MHz. We demonstrate routing into four spatial modes with a high end-to-end efficiency of 79% and measure a four-photon coincidence rate of 0.17 Hz mostly limited by the single-photon source brightness and not by the efficiency of the demultiplexer itself. We use the demultiplexer to characterize the pairwise indistinguishability of consecutively emitted photons from the quantum dot with variable delay time., Comment: The supplementary material is included as ancillary file

The quest for the perfect single-photon source includes finding the optimal protocol for exciting the quantum emitter. Based on a recently proposed, so-called SUPER (swing-up of quantum emitter population) scheme, we demonstrate experimentally that two red-detuned laser pulses, neither of which could yield a significant upper-level population individually, lead to the coherent excitation of a semiconductor quantum dot. We characterize the emitted single photons and show that they have properties comparable to those achieved under resonant excitation schemes., Comment: 11 pages, 9 figures

We investigate, in a four photon interference experiment in a laser-written waveguide structure, how symmetries control the suppression of many-body output events of a $J_x$ unitary. We show that totally destructive interference does not require mutual indistinguishability between all, but only between symmetrically paired particles, in agreement with recent theoretical predictions. The outcome of the experiment is well described by a quantitative simulation which accounts for higher order emission of the photon source, imbalances in the scattering network, partial distinguishability, and photon loss., Comment: 14 pages, 7 figures