[CL] Testing Bell's Inequality with Cosmic Photons: Closing the Settings-Independence Loophole


We propose a practical scheme to use photons from causally disconnected cosmic sources to set the detectors in an experimental test of Bell’s inequality. In current experiments, detector settings are determined by local quantum random number generators. In such experiments, only a small amount of correlation between detector settings and some local hidden variables, established less than a millisecond before each experimental run, would suffice to mimic the predictions of quantum mechanics. By setting the detectors using cosmic sources instead, observed violations of Bell’s inequality in our proposed “Cosmic Bell” experiment would require any such coordination to have been in place for billions of years rather than milliseconds — an improvement of 20 orders of magnitude. Quasar pairs can be used as real-time triggers to establish detector settings using existing technology. For quasars on opposite sides of the sky with redshifts z > 3.65, there is no event after the hot big bang 13.8 billion years ago (following any period of inflation) that lies in the past light cones of both quasars. Alternatively, detector settings could be set by observing patches of the Cosmic Microwave Background (CMB, z ~ 1089). For CMB patches with angular separations > 2.3 degrees, the events determining the detector settings would have no shared causal past since the hot big bang. In the case of the CMB, noise from the receiver and atmosphere make it difficult to rule out local influences on causal grounds alone, but a specially designed balloon-based system using state-of-the-art detectors is a realistic near-term possibility.

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Date added: Tue, 15 Oct 13