Photon Polarization Entanglement Induced by Biexciton: Experimental Evidence for Violation of Bell’s Inequality

We have investigated the polarization entanglement between photon pairs generated from a biexciton in a CuCl single crystal via resonant hyperparametric scattering. The pulses of a high repetition pump are seen to provide improved statistical accuracy and the ability to test Bell’s inequality. Our results clearly violate the inequality and thus manifest the quantum entanglement and nonlocality of the photon pairs. We also analyzed the quantum state of our photon pairs using quantum state tomography.

Figure 1

Schematics of our experimental setup. DM: zero-dispersion double monochromator. ND: neutral density filter. OF: optical multimode fiber. M1, M2: monochromators. PA: polarization analyzers. PMT: photon counting photomultiplier.

Figure 2

RHPS spectra in CuCl single crystal. The central peak indicates the Rayleigh scattered pump light, and this energy is tuned at two-photon resonance with the biexciton (3.186 eV). Spectral bandwidth of the pump laser was set to 0.3 nm. The two side peaks around the central peak are the RHPS signals of HEP and LEP. The small peak labeled as $I_1$ is the emission of the bound exciton. The two peaks ($M_T$, $M_L$) on the lower energy side are the emission from the biexciton leaving the transverse and longitudinal excitons, respectively.

Figure 3

Time correlation histograms between the HEP and LEP for the three polarization basis ($H\mathrm{\text{−}}V$, $D\mathrm{\text{−}}\overline{D}$, $R\mathrm{\text{−}}L$). The central peak at $\tau =0$ is the correlated two-photon coincidence signal originating from one biexciton. The background signal at $\tau \ne 0$ results from the uncorrelated photons.

Figure 4

Reconstructed two-photon polarization density matrix for the photon pairs generated from RHPS.