Advanced Tests and Applications of Quantum Nonlocality

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Nonlocal entanglement has long been of great fundamental interest in quantum mechanics, and is now seen as a cornerstone in the nascent field of quantum information. However, although nearly half a century has passed since John Bell first showed that it was possible to perform a true, unambiguous test of the nonlocal, non-realistic aspects of quantum mechanics, such an unambiguous test has never been performed. Experimental tests of this quintessential quantum phenomenon have thus far been incomplete, all of them plagued by one or more "loopholes". We are addressing three main topics in advanced tests of quantum nonlocality. First, we employ new very efficient, fast detectors to realize an unambiguous test of nonlocality, a major quantum physics milestone. In the first phase, the "detection loophole" can be closed, which has never been done in optics; in the second phase, the "timing loophole" will also be closed, leading to the first truly loophole-free test of the nonlocal nature of reality. Realizing a system that can perform such a test is relevant for "verified" quantum random number generation and "device-independent" ultra-secure quantum cryptography systems, advantageous over traditional implementations because they do not rely on extra assumptions about the equipment being used. The second activity capitalizes on our recent NSF-funded successes in producing and controlling hyperentanglement -- photons that are simultaneously entangled in multiple degrees of freedom -- to study novel features of quantum nonlocality in larger Hilbert spaces. Specifically, using polarization, spatial-mode and time-bin entanglement, we explore states that reside in an unprecedented ~37,000,000-dimensional Hilbert space. In collaboration with University of Illinois colleague Prof. Anthony Leggett, we study the nonlocal character and robustness of the complex hyperentanglement in the face of decoherence, and as the dimensionality of the Hilbert space increases. Finally, using techniques developed here, we address two applications in the realm of advanced quantum information processing: realization of a completely verifiable source of quantum random numbers, and first implementation of a "device independent" quantum cryptography protocol. This project should lead to a number of other interesting investigations at the boundary between foundations of quantum mechanics and quantum information processing.One can hardly discuss the nature of reality without the notion of quantum entanglement -- ultrastrong-correlations between separated quantum systems -- in fact, entanglement is now seen as a cornerstone in the nascent field of quantum information. Therefore, testing it thoroughly is essential, if challenging. Our experiments will, once and for all, exclude all models of nature that rely on the intuitive principles of locality (there are no actions at a distance) and realism (things we measure have values before we measure them), as well as testing for the presence of correlations beyond those predicted by quantum mechanics. Should a "positive result" (i.e., an apparent breakdown of QM predictions) be found and confirmed, the eventual implications for our view of the physical world itself would be truly revolutionary. Moreover, the ability to study nonlocal correlations in much more complex quantum systems than has heretofore been possible -- using hyperentanglement -- may reveal new features in the robustness/fragility of entanglement phenomena. Finally, as part of the project, nonlocality experiments for undergraduate laboratories are being developed in collaboration with colleagues at undergraduate institutions, bringing the realities of quantum information phenomena to students at an earlier age and in broader venues.