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Emmanuel Billias

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INTRODUCTION A fascinating result of quantum mechanics leads us to the idea of entangled particles. When measuring one of two entangled photons, the measurment gives us data about both particles. Measuring one particle tells us about the quantum state of the other particle instantaneously. This instantaneous transfer of information is a serious locality violation as proposed by Albert Einstein. As a result, hidden variable theory was developed. The idea of hidden variables formulated by Einstein, Podolsky, and Rosen (EPR) was meant to explain a quantum mechanical locality violation. As we will explore in greater detail, quantum entanglement seems to violate the idea of locality. In his 1905 papers, Einstein had proposed that information (or some form of physical influence) is bound to travel at or below the speed of light. In quantum entanglement if we know the original spin of the parent particle and one of the new spins, then we immediately know the spin of the second particle upon measurement of the first. It seems that we would receive information about the spin state of the second particle instantaneously! The main concept of the EPR argument was that a “hidden variable” would account for this violation and show how a quantum system does in fact obey locality. John Bell had taken this idea of hidden variables and applied it to a system of entangled photons and polarizers. He formulated an inequality under the assumption that hidden variable theory is true. Through matheatics and experimentation we will show how this inequality is violated. It appears that the idea of hidden variables must also be false and that a quantum system does indeed violate locality. Following Bell’s inequaity comes a series of experiments in which we may test the inequality. From these tests follow the ideas of “loopholes”. Loopholes in Bell test experiments provide a path in which we may question the validity of the results of these experiments. As an example, the aforementioned locality issue must be broken in order to show that the experiment is valid. For if information has been transfered in accordance with locality, then why preform the experiment at all? We will explore a recent Bell test experiment which suggests that the locality conditions are indeed violated.