Experimental Observation of Four-Photon Entanglement from Parametric Down-Conversion

We observe polarization entanglement between four photons produced from a single down-conversion source. The nonclassical correlations between the measurement results violate a generalized Bell inequality for four qubits. The characteristic properties and its easy generation with high interferometric contrast make the observed four-photon state well suited for implementing advanced quantum communication schemes such as multiparty quantum key distribution, secret sharing, and telecloning.

Figure 1

Experimental setup. The four photons are emitted from the BBO crystal (type-II phase matching) into two spatial modes $a_0$ and $b_0$, and distributed into the four modes $a,a^{\prime },b,b^{\prime }$ by 50-50 beam splitters (BS) behind interference filters (F). To characterize the resulting four-photon state $|{\Psi }^{(4)}⟩$, polarization analysis (PA) in various bases is performed for each mode using $\lambda /4$ and $\lambda /2$ wave plates in front of polarizing beam splitters (PBS) and single photon avalanche detectors (SPAD).

Figure 2

Fourfold coincidence probabilities corresponding to a detection of one photon in each of the four polarization analyzers, oriented in (a) the $H/V$ basis, and (b) the $\pm 45°$ basis. (c) Four-photon polarization correlation with the detection basis of an observer in mode $a$ varying from $45°$ linear at ${\varphi }_a=0$ to left circular, $-45°$ linear and right circular polarization, while observers in $a^{\prime },b,b^{\prime }$ analyze in the $\pm 45°$ basis. The solid line shows a sinusoidal fit to the experimental results with a visibility of $79.3\pm 1.4\%$.

Figure 3

Fourfold coincidence probabilities (raw data) for a four-particle test of local realistic theories. For the sixteen settings of the analyzer phases ${\varphi }_a,{\varphi }_{a^{\prime }},{\varphi }_b,{\varphi }_{b^{\prime }}$, count rates not corrected for detection efficiencies are used to evaluate a generalized Bell inequality (5), leading to $S^{(4)}=1.301\pm 0.040$. This clearly exceeds the bound of 1 demanded by local realistic theories. Acquisition rates were around 150 fourfold coincidence events per hour.