Figure 1

Experimental setup for observing high intensity four-photon GHZ entanglement. Two pairs of entangled photons are produced by passing a UV laser pulse through a BBO crystal twice. The UV laser with a central wavelength of 394 nm has a pulse duration of 200 fs, a repetition rate of 76 MHz, and an average pump power of 450 mW. By optimizing the collection efficiency [14], we are able to observe about $2\times {10}^4$ entangled pairs per second for each pair behind 3.6 nm filters ($F$) of central wavelength 788 nm. Coincidences between detectors $D_1$, $D_2$, $D_3$, and $D_4$ exhibit four-photon GHZ entanglement. Polarizers (Pol) and quarter wave plates ($\lambda /4$) are used to perform the measurement of linear $H^{\prime }/V^{\prime }$ or $R/L$ polarization.Reuse & Permissions

Figure 2

Typical experimental results for polarization measurements on all four photons in the $H^{\prime }/V^{\prime }$ basis. The coincidence rates of $H^{\prime }{H}^{\prime }{H}^{\prime }{H}^{\prime }$ and $H^{\prime }{H}^{\prime }{H}^{\prime }{V}^{\prime }$ components are shown as a function of the pump delay mirror position. The high visibility obtained at zero delay implies that four photons are indeed in a coherent superposition.Reuse & Permissions

Figure 3

Experimental results observed in the first three experiments (a)–(c), and predictions of QM and of LR (normalized), and observed results for the ${\sigma }_x{\sigma }_y{\sigma }_y{\sigma }_x$ measurement (d)–(f). The visibilities in (a)–(c) are $0.820\pm 0.011$, $0.807\pm 0.011$, and $0.781\pm 0.012$, respectively. The experimental results in (f) are in agreement with the QM predictions (d) while in conflict with LR (e), with a visibility of $0.789\pm 0.012$. The integration time of each fourfold coincidence is 1000 s.Reuse & Permissions

Figure 4

Experimental results observed in 16 specific measurements to get the expectation value of $A$.Reuse & Permissions