Nonlocal realism tests and quantum state tomography in Sagnac-based type-II polarization-entanglement SPDC-source

Abstract

We have experimentally created a robust, ultrabright and phase-stable polarization-entangled state close to maximally entangled Bell-state with %98-fidelity using the type-II spontaneous parametric down-conversion (SPDC) process in periodically-poled KTiOPO 4 (PPKTP) collinear crystal inside a Sagnac interferometer (SI). Bell inequality measurement, Freedman's test, as the different versions of CHSH inequality, and also visibility test which all can be seen as the nonlocal realism tests, imply that our created entangled state shows a strong violation from the classical physics or any hidden-variable theory. We have obtained very reliable and very strong Bell violation as = 2.78 ± 0.01 with high brightness  HV = %(99.969 ± 0.003) and  DA = %(96.751 ± 0.002) and very strong violation due to Freedman test as F = 0.01715 ± 0.00001. Furthermore, using the tomographic reconstruction of quantum states together a maximum-likelihood-technique (MLT) as the numerical optimization, we obtain the physical non-negative definite density operator which shows the nonseparability and entanglement of our prepared state. By having the maximum likelihood density operator, we calculate some important entanglement-measures and entanglement entropies. The Sagnac configuration provides bidirectional crystal pumping yields to high-rate entanglement source which is very applicable in quantum communication, sensing and metrology as well as quantum information protocols, and has potential to be used in quantum illuminationbased LIDAR and free-space quantum key distribution (QKD).

Preparing the deployment of quantum key distribution over a classical network infrastructure in Prague

“…The density matrix was obtained from the coincidence measurements on the state with different bases using the maximum likelihood approach. 10 From the density matrix, we computed the purity, tangle, and entanglement entropy 11 of the estimated state and obtained the values 0.9171, 0.9924, and 0.6925, respectively. These metrics quantify the quality of the entangled state.…”

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“…Here, it should be noted that such an entangled state can be easily generated in the optical domain using the well-known nonlinear optical process of spontaneous parametric downconversion (SPDC) [52], [53]. In contrast to the other sources such as superconductors [16], [28] or electro-optomechanical systems [15] that are used for entanglement generation in microwave domain, SPDC sources are more inexpensive, user-friendly, and notably work at room temperature (i.e., no need for ultra-low temperature cooling), and finally could be implemented on-chip via integrated photonics techniques and interestingly can be commercialized.…”