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Experiments thought to prove non – locality may be artifacts Karl Otto Greulich. Fritz Lipmann Institute Beutenbergstr. 11 D Jena Entanglement, the instantaneous transfer of a quantum property through space, is at the core of numerous philosophical discussions. Usually experiments thought to prove entanglement, are described theoretically with a single atom model. However the experiments are performed in a completely different way: Such experiments are essentially counting experiments! Essentially, coincidence rates are measured. An excess of coincidences compared to some theoretical expectation is seen as proof of nonloca- lity. Mathematically it is tested, whether the Bell inequalities are violated. The present talk shows, that such an excess of coincidence rates can be caused in a very classical way.

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Experiments thought to prove non – locality may be artifacts Karl Otto Greulich. Fritz Lipmann Institute Beutenbergstr. 11 D Jena The problem is, that one has to do some dull calculations. Exactly this will in the following be done for one representative example (Gröblacher, Nature 446, (2007). It is dull, but the reward will be perhaps a clearer view on non-locality.

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The idealized version of the experiments reproduced from Aspect, A. "Bell's inequality test: more ideal than ever" Nature 398, (1999) A single atom emits a pair of photons. Polarization is analysed by a polariser on each side of the experiment. classically, measuring on the right side does not affect the spin of the photon on the left Probabilities that photons pass their polarisers are independent of each other non locally, measurement on one side affects the spin of the photon on the other side. either both polarisers simultaneously allow their photon to pass, or both stop them. higher rate of identical results on both sides (surplus coincidences)

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A real experiment Gröblacher, S., Paterek, T., Kaltenbaek, R., Brukner, C., Zukowski, M., Aspelmeyer, M. and Zeilinger, A. "An experimental test of non-local realism" Nature 446, (2007)

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Experiments thought to prove non – locality may be artifacts Karl Otto Greulich. Fritz Lipmann Institute Beutenbergstr. 11 D Jena Single photon detectors Single photon detectors are often believed to be s i n g l e p h o t o n detectors However, while they indeed can detect a single isolated photon, they usually cannot distinguish between 1, 2 or many photons. From this follows a very critical problem for measuring entanglement It cannot be safely said whether a single photon pulse really contains just one photon. This may become disastrous for measurements of entanglement.

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When a laser-pumped down converting crystal is used The geometry in a real multi - atom or molecules light source. Always billions of emitters are involved Everbody who claims to have absolute control that exactly one photon pair is emitted per time has to prove this thoroughly

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One photon pair versus two or more pairs classical (local): all outcomes possible non locality: A and B enriched at the costs of C i.e increase of coincidences situation with two photons: at least one photon has the correct orientation to pass classical increase of coincidences

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Apparent non locality caused by classical surplus coincidence rates E ik = (N ++ + N -- - N +- - N -+ ) / (N ++ + N -- + N +- + N -+ ) S NLHV = E 11 + E 23 + E 22 + E 23 If, according to the N on L ocal H idden V ariable theory, the sum S NLHV turns out to be smaller than (= 4 – (4 ) sin 9.4 o ) in the settings used here, this would indicate classical locality. At larger values one would state non- locality. If the E ik = (N ++ + N -- - N +- - N -+ ) / (N ++ + N -- + N +- + N -+ ) are too large because of the just decribed surplus coincidences, one states non – locality where the situation is local – for simply classical experimental reasons

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Experiments thought to prove non – locality may be artifacts Karl Otto Greulich. Fritz Lipmann Institute Beutenbergstr. 11 D Jena Conclusion Present experiments thought to prove non-locality can be interpreted classically. Thus, care is needed when invoking non-locality

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Experiments thought to prove non – locality may be artifacts Karl Otto Greulich. Fritz Lipmann Institute Beutenbergstr. 11 D Jena Detecting non-locality via entanglement, reflected by the instantaneous transfer of quantum properties over a large distance, requires almost ideal experimental conditions – ideal sources for entangled photon pairs and ideal detectors. Deviations from this ideal situation may critically hamper interpretations of experiments on non-locality or entanglement. Of two major known loopholes, the detector loophole is not yet unequivocally closed. Now another loophole emerges: When in such experiments downconverting crystals with lasers as primary light sources are used, exceeding threshold values in the Bell inequalities, i.e. their violation, can be caused by so far not fully recognized problems in the generation and detection of photon pairs. Taken together, nonlocality is still not yet unequivocally proven. Thus, one of the most serious experimental challenges of causality is on brittle ground. References: K.O. Greulich, Another loophole for the Bell inequalities Proc. of SPIE Vol 7421 – 08, (2009) ; K.O. Greulich Proc. of SPIE Vol , (2011); for downloads see, then klick *Physics* KEYWORDS: Non locality, Bell inequalities, single photon detectors, down conversion

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