Verification experiment of compressed entanglement witnesses using entangled photon pairs in bipartite qubit system

Abstract

Quantum entanglement is an unique and special quantum correlation of system which follows the quantum mechanics. Unlike classical correlations, quantum entanglement is known to be nonlocal and the correlation between two entangled quantum states is known to be stronger than the classical correlation. Using this property, quantum entanglement has been used as a core technology in various technologies such as quantum key distribution, quantum teleportation, and quantum computing. In quantum infromation engineering field, researches to detect the quantum entanglement have been continuously conducted, and their importance has also emerged. This seemingly simple problem is NP-hard problem, many researchers have been published the way to confirm the existence of quantum entanglement, but the general technique is still unknown. Entanglement witness(EW) has a simple idea that a positive operaotr satisfying certain conditions for only separable states. Since this positive operator can be composed of a local Pauli measurement operator, it can be easily confirmed experimentally and also has the advantage of cheaper than state tomography. Compressed entanglement witness(CEW) can determine the quantum entanglement of many quantum states efficiently, based on the fact that an EW is always paired and does not require additional experimental setups. However, unlike EW, CEW has not been verified by actual experiments, so it is necessary to verify whether this method is sufficiently efficient in the actual experimental settings. In this paper, entangled photon pairs were created through spontaneous parametric down-conversion(SPDC) using nonlinear birefringent BBO crystals, and CEW verification experiements were performed in the quantum optical setup. Injection of a high power photon to nonlinear birefingent crystal generates entangled photon pairs satisfying the law of energy conservation and momentum conservation. We conducted this study using entangled photon pairs in the 1550nm wavelength based on the fact that infrared photons in the 1550nm wavelength are used in many optical experiments and techniques. Since it is very difficult to produce entangled photon pairs using nonlinear birefringent crystals, the entangled photon pairs were generated in free space, and CEW experiments were performed using optical fibers.