12-Photon Entanglement and Scalable Scattershot Boson Sampling with Optimal Entangled-Photon Pairs from Parametric Down-Conversion

Entangled-photon sources with simultaneously near-unity heralding efficiency and indistinguishability are the fundamental elements for scalable photonic quantum technologies. We design and realize a degenerate telecommunication wavelength entangled-photon source from an ultrafast pulsed laser pumped spontaneous parametric down-conversion (SPDC), which shows simultaneously 97% heralding efficiency and 96% indistinguishability between independent single photons without narrow-band filtering. Such a beamlike and frequency-uncorrelated SPDC source allows generation of the first 12-photon genuine entanglement with a state fidelity of $0.572\pm 0.024$. We further demonstrate a blueprint of scalable scattershot boson sampling using 12 SPDC sources and a $12\times 12$ mode interferometer for three-, four-, and five-boson sampling, which yields count rates more than 4 orders of magnitude higher than all previous SPDC experiments.

Figure 1

The design and performance of our new SPDC entangled-photon source. (a) The interferometric two-photon entanglement source. The laser beam is split into two beams by two 775-nm BDs and HWPs and focused on a BBO crystal at two different spot to generate photon pairs via type-II beamlike SPDC. The green (red) lines represents $H$ ($V$) polarization. (b) The measured joint spectrum of the photon pair, indicating the two photons are free of frequency correlations. (c) Hong-Ou-Mandel–type interference of two photons from two independent SPDC, measured without narrow-band filtering. (d) The measured heralding efficiency and photon-pair brightness as a function of the pump beam waist.

Figure 2

Experimental setup for generating the 12-photon entanglement. Six SPDC entanglement sources [as shown in Fig. 1] are pumped by laser pulses with a central wavelength of 775 nm, a bandwidth of 5.5 nm, and a repetition of 80 MHz. Dispersion of the laser pulses caused by ${\mathrm{YVO}}_4$ crystals and BBO crystals is precompensated by four prism pairs. The 24 superconducting nanowire single-photon detectors have an average efficiency of 75% at 1550 nm.

Figure 3

Experimental result for the 12-photon GHZ state. (a) The measured 12-photon counts in the $H/V$ basis. (b) Experimentally extracted expectation values of the observables $M_{\theta }^{\otimes N}={(\cos \theta {\sigma }_x+\sin \theta {\sigma }_y)}^{\otimes N}$, which is calculated from registered 12-photon coincidence events in the $(|H⟩\pm e^{i\theta }|V⟩)/\sqrt{2}$ basis. Error bars stand for one standard deviation based on binomial distribution statistics.

Figure 4

Experimental setup and results for high-efficiency scattershot boson sampling. (a) The signal photons from the same crystal are combined into one path by using the same method illustrated in Fig. 1 and guided into a 12-mode optical interferometer. Here we choose a pump beam waist of 0.8 mm and a two-photon count rate of 0.5 MHz. (b) The measured similarity and distance for the three-boson sampling. (c) Extended likelihood ratio test between the experimental data and simulated distinguishable sampler for the three-, four-, and five-photon experiments. (d) A comparison of the three-boson sampling rate with previous experiments using SPDC and quantum dots. The data points are labeled with their references.