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Journal of Physics Conference Series

Simple experimental setup for producing polarization-entangled photons

W Rojpattanakul, T Bongkodmalee, Kitisak Boonkham

Published Nov 1, 2019• 3 citations
Publisher

Abstract

Abstract Quantum entanglement is an important action in quantum mechanics. Basically, quantum entanglement of correlated photon pairs can be produced by spontaneous down-conversion process inside the two birefringence crystals. This work aims to create a pair of photons, i.e. signal and idler, that are entangled and to assay the relation between these photons by using polarization-entangled photon pairs to demonstrate quantum non-locality by comparing with the Malus’s law. From the experiments, the coincidence counts as a function of relative angle (α - β) between transmission axis of the polarizer and analyzer were obtained and the polarization entanglement curve was demonstrated. This result corresponding with polarization entanglement prediction term of ½ cos2 (α - β) which confirmed the entanglement of photons.

DOI
10.1088/1742-6596/1380/1/012023

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Abstract Quantum entanglement is an important action in quantum mechanics. Basically, quantum entanglement of correlated photon pairs can be produced by spontaneous down-conversion process inside the two birefringence crystals. This work aims to create a pair of photons, i.e. signal and idler, that are entangled and to assay the relation between these photons by using polarization-entangled photon pairs to demonstrate quantum non-locality by comparing with the Malus’s law. From the experiments, the coincidence counts as a function of relative angle (α - β) between transmission axis of the polarizer and analyzer were obtained and the polarization entanglement curve was demonstrated. This result corresponding with polarization entanglement prediction term of ½ cos2 (α - β) which confirmed the entanglement of photons.
Bell inequality experiments have demonstrated that quantum entanglement violates the predictions of local realism. However, all Bell experiments are based on joint measurements of two photons, and their analysis implicitly assumes that "the polarization state of a single photon is ideally linear." This paper proposes an experimental scheme to directly test this assumption: using type-II spontaneous parametric down-conversion (SPDC) to generate polarization-entangled photon pairs, employing fixed-angle measurement of the e-light channel as a coincidence gate to filter entangled photons in the o-light channel, then scanning the o-light analyzer angle with high precision to measure the coincidence count rate as a function of the analyzer angle, testing whether it strictly follows Malus' law I(\theta)=I_0\cos^2(\theta-\theta_0). If systematic deviation is observed in the curve, it would indicate the existence of asymmetric secondary polarization structures in the polarization state of entangled photons, providing an alternative explanatory pathway for Bell inequality violation distinct from nonlocality. This experiment employs a high-precision coincidence measurement scheme to ensure statistical significance and designs multiple control experiments to eliminate systematic errors. The experiment can be implemented in laboratories equipped with standard quantum optics equipment.
The purpose of this project is to set up a system for creating entangled photon pairs for use in various quantum mechanical experiments, including the testing of Bell’s inequalities which demonstrates the fundamentally indeterministic nature of quantum mechanics. The photons are created by pumping a beta barium borate crystal with 405 nm laser light then, due to a process called type 1 spontaneous parametric down conversion, a single pump photon can be converted into two 810 nm photons in an entangled state. In the same lab, a similar set up has already been achieved using a 474 nm pump laser, but by using a shorter wave length, a higher detection rate can be achieved. Two collimators are set up to collect the down converted photons and send them to two photon counting modules. The collimators need to be positioned precisely to detect any down converted photons because they emerge from the crystal in a narrow beam that is at an angle with respect to the pump beam and only a miniscule fraction of pump photons are down converted. The angle at which the down converted photons emerge is a function of how the crystal is prepared and the polarization orientation of the pump beam. It was calculated using a program by National Institute of Standards and Technology.
With the advancement of non-classical light sources such as single-photon and entangled-photon sources, innovative microscopy based on the quantum principles has been proposed over traditional microscopy. This paper introduces the experimental demonstration of a quantum polarization microscopic technique that incorporates a quantum-entangled photon source. Although the point that employs the variation of polarization angle due to reflection or transmission at the sample is similar to classical polarization microscopy, the method for constructing image contrast is significantly different. Image contrast is constructed by the coincidence count of signal and idler photons. In the case that coincidence count is recorded from both the signal and idler photons, the photon statistics resemble a thermal state, similar to the blackbody radiation, but with a significantly higher peak intensity in the second order autocorrelation function at zero delay that is derived from coincidence count. While, when the coincidence count is taken from either the signal or idler photon only, though the photon state exhibits a thermal state again, the photon statistics become more dispersive and result in a lower peak intensity of the autocorrelation function. These different thermal states can be switched by slightly changing the photon polarization, which is suddenly aroused within narrow range of analyzer angle. This polarization microscopic technique can provide a superior imaging technique compared to the classical method, opening a new frontier for research in material sciences, biology, and other fields requiring high-resolution imaging.
For the last two decades, the focus of scientific research in the field of information transmission and processing has been in such sections of quantum optics as quantum computing and quantum cryptography. These areas of optics rely on the discovery of a special type of radiation - beams with pairs of bound or entangled photons. The experimental work in Russia, devoted to the entangled photons is undeservedly little. Therefore, the proposed work is important from a methodical point of view. Moreover, the experiment is performed on domestic equipment, much cheaper than most foreign analogues. The described system can be used as a prototype for setting up educational demonstration and laboratory works. Therefore, the proposed work will be of interest to researchers and universities involved in quantum optics. As a result of the work done, the attempt to create a source of entangled photons based on a barium beta-borate crystal can be considered successful. It should be added that in a second type of crystal pairs of entangled photons arise not only with respect to directions (and frequencies), but also with respect to two orthogonal polarizations. A similar source makes it possible to set up experiments similar to the Einstein-Podolsky-Rosen thought experience in Bohm’s interpretation. Therefore, it is also suitable for testing the Bell inequality. The article considers the practical importance of the phenomenon of generation of entangled photons for the purposes of quantum cryptography, the phenomenon of spontaneous parametric scattering and the creation of a source of entangled pairs of photons based on it, the experimental system and the results of the experiment on generation of correlated photon pairs, tangled with respect to directions and frequencies, the result of the experiment on observing the correlation in the detection times of pairs of entangled photons.
We report a new method for generation of polarization-entangled photon pairs in maximally entangled polarization states. Our approach is based on quantum interference between two polarization states which are non-orthogonal. We present necessary and sufficient conditions for generation the photon pair in a maximally entangled state. The effect of dependence of entanglement on the phase between the interfering states of photons is investigated theoretically and demonstrated experimentally with quantum states produced via spontaneous parametric down-conversion in the double-crystal scheme.
This paper presents a method to convert position correlation of photon pairs into polarization entanglement. This is achieved by individually manipulating the polarization state of photons generated in different parts of a nonlinear medium and coherently superposing them. This concept is experimentally demonstrated using photon pairs produced by spontaneous parametric down-conversion. The method is used to implement a compact source producing an observed photon-pair rate of $120\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{4.pt}{0ex}}{\text{s}}^{\ensuremath{-}1}/\text{mW}$ with an entanglement fidelity of $0.991\ifmmode\pm\else\textpm\fi{}0.013$. This method can be extended to any photon-pair generation process with initial position correlation.