* 30 single-photon interferometers in total.
* The outputs are detected by 48 single-photon detectors
Scheme and experimental setup for creating and verifying 18-qubit GHZ state consisting of six photons and three degrees of freedom. a. The generation of six-photon polarization-entangled GHZ state. An ultrafast laser with a central wavelength of 788 nm, a pulse duration of 120 fs and a repetition rate of 76 MHz is focused on a lithium triborate (LBO) and up-converted to 394 nm. The ultraviolet laser is focused on three custom-designed sandwich-like nonlinear crystals, each consists of two 2-mm thick β-barium borates (BBOs) and one half-wave plate (HWP), to produce three pairs of entangled photons. In each output, two pieces of YVO4 crystals with different thickness and orientation are used for spatial and temporal compensation for the birefringence effects. The three pairs of entangled photons are combined on two polarizing beam splitters (PBSs) to generate a six-photon polarization-entangled GHZ state. b.