We have developed a very robust, ultra-bright, phase-stable, and polarization-entangled state that approaches a maximally entangled Bell state, with astonishing fidelity. Using a periodically poled type-0 non-collinear KTiOPO4 (PPKTP) crystal, in a Sagnac Interferometer (SI) to achieve this remarkable feat. This entangled photon source exhibits a CHSH value of S = 2.676 ± 0.003, indicating a profound violation of the Bell-CHSH inequality by over 225 standard deviations.
Experimental setup of the fiber-coupled entangled photon source. A 405nm SLM diode laser whose polarization and phase are set by a quarter- (QWP) and a half-wave plate (HWP) is focused via a bi-convex lens (L1) into a PPKTP crystal inside a polarization Sagnac loop built up by dual-wavelength polarizing beamsplitter (DPBS) a dual-wavelength half-wave plate (HWP1) and two mirrors(M1, M2). The downconversion modes are coupled into single-mode fibers (SMF). Bandpass filters (BPF) of 10nm centered around 810 are used as interference filters. Polarizers (P1) and Quarter wave plates (QWP2) are inserted to characterize the produced photon pairs.
Quantum State Tomography. The density matrix was constructed from 16 linearly independent coincidence measurements and the real part of the density matrix is shown on the left. The resulting fidelity F to the Φ+ state was F = 0.9843 and the Concurrence C = 0.997 and the EMCCD camera images of 30-mm ppKTP sample poling period 3.425 μm pumped at wavelength 405nm at various temperature (a)T=26.5°C (b)T= 29.5°C (c)T= 32.5°C (d)T=35.5°C (e)T= 38.5°C (f)T= 41.5
The exceptional performance of our entangled state is demonstrated by the observed high brightness of up to 0.4 MHz/mW and visibilities in the H/V and D/A polarization basis ranging from 98 to 99 percent.