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Zenith Grant Awardee

Dr. Paul G. Kwiat

University of Illinois


Joseph Benjamin Altepeter, <i>University of Illinois</i>

Project Title

Loophole-free Test of Bell's Inequalities

Project Summary

In 1935, Einstein, Podolsky, and Rosen (EPR) proposed one of the greatest scientific paradoxes of the 20th century: quantum mechanics, an extremely successful physical theory, appeared to violate the basic principles of locality (which states that distant objects can have no direct influence on each other) and realism (which requires that – given enough information – it be possible to predict the result of any measurement). This mystery was so complete that it took 30 years for a resolution to be proposed. In 1964, John Bell discovered that not only did the quantum mechanical phenomenon of entanglement violate local realism, an experiment could be performed which would rule out all local realistic theories. Myriad experiments have been performed which support this bizarre quantum mechanical conclusion, yet in fact no experiment to date has incontrovertibly ruled out local realism -every experiment thus far has possessed one or more loopholes arising from auxiliary experimental assumptions. Now, due to recent advances in the science of creating and detecting entanglement, it may finally be possible to perform a completely "loophole-free" test of Bell's theory. We propose to perform this test and – 70 years after it was proposed – to unambiguously resolve the EPR paradox.

Technical Abstract

One of the most anticipated results in the nascent field of quantum information is a "loophole-free" test of Bell's inequalities, which would unambiguously disprove all local realistic theories and all local hidden variable models for entanglement. This test requires simultaneously closing the detection loophole (which requires very high net detection efficiency) and the timing loophole (which requires that the choice of measurements on entangled pairs be causally separated). We propose performing this test using an extremely bright and pure source of polarization-entangled photons based on Type-I down-conversion in nonlinear birefringent crystals. The greatest technical challenge for this experiment will be closing the detection loophole by exceeding the approximately 70% limit for total photon-detection efficiency (in fact, the timing loophole has already been closed using low-efficiency photonic systems). Preliminary tests confirm that by employing cryogenically cooled single-photon detectors, custom spectral filters, and optics optimized for high down-conversion collection efficiency, a loophole-free test of Bell's inequalities appears to be within reach.

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