Testing Bell's Inequality with Astrophysical Observations

This INSPIRE project is jointly funded by the Atomic, Molecular, and Optical Physics--Experiment Program in the Physics (PHY) Division in the Directorate for Mathematics and Physical Sciences (MPS), and the Atomic, Molecular, and Optical Physics--Theory Program in PHY/MPS, and the Particle Astrophysics and Cosmology--Theory Program in PHY/MPS, and the Extragalactic Astronomy & Cosmology Program in the Astronomy (AST) Division of MPS, and the Science, Technology, and Society Program in the Division of Social and Economic Sciences (SES) in the Directorate for Social, Behavioral, and Economic Sciences (SBE), and the Division of Graduate Education (DGE) in the Directorate for Education & Human Resources (EHR),and the Office of Integrative Activities and the Office of International Science and Engineering. For nearly a century, physicists have used quantum mechanics to understand many properties of the physical world, from the behavior of atoms and molecules to the nuclear forces that govern sub-atomic particles. Predictions from the theory have matched experimental observations to impeccable accuracy. Conceptually, however, the theory includes some strikingly strange features. Among the most curious is known as "quantum entanglement." According to quantum mechanics, particles that have been prepared in a special way can retain a connection, even after they have moved arbitrarily far apart from each other--a property which Albert Einstein dubbed "spooky actions at a distance." Nowadays entanglement is at the heart of many cutting-edge technologies, including quantum encryption and quantum computing. Yet every experimental test of quantum entanglement to date has been subject to various loopholes: alternative explanations, different than quantum theory, that might account for the long-distance correlations in the particles' behavior. In this project, the principal investigators aim to address the most stubborn, and least studied, of these loopholes, known as the "setting independence loophole." To shield against any unintended coordination between the particles and the measurement apparatus -- coordination that could mimic the predictions of quantum mechanics--the selection of which properties of the particles to be measured will be determined by real-time observation of some of the oldest light in our universe: light that was emitted from astronomical objects so far away from Earth and from each other that neither object would have been able to receive any signals from each other prior to the moment they emitted the light that is observed on Earth today. The new series of experiments will thus test entanglement on an entirely new scale. If, as expected, the results match the predictions from quantum mechanics, then any alternatives will be ruled out or severely constrained, and new technologies such as quantum encryption will be placed on the strongest possible footing. If, on the other hand, the experiment finds novel departures from predictions, that could point toward profoundly new physics. This project also has an informal education component that will take place through exhibits and programs at the MIT Museum; these will connect the public to the experiment as it evolves in real time, and will be evaluated and widely disseminated.

Experimental tests of Bell's inequality have been subject to several loopholes which hold out the possibility, however slim, that individual particles could possess simultaneously sharp values for noncommuting variables. Such behavior would be at odds with quantum mechanics, and would subject quantum-encryption protocols to new vulnerabilities. The most subtle loophole is known as "setting independence." In any test of entanglement, one must select detector settings on each side of the experimental apparatus, choosing to measure, for example, a particle's spin along the x-axis, the y-axis, or some intermediate angle. The usual assumption is that no third party, acting in the shared causal past of the entangled particles and the measurement apparatus, has affected the joint probability distribution for detector settings. Yet even a tiny coordination among detector settings and the entangled particles could mimic the predictions of quantum mechanics. In this series of experiments, the principal investigators aim to address the setting-independence loophole using real-time observations of distant astronomical sources, such as quasars--sources that were causally isolated from each other and from the worldline of the Earth at the time they emitted the light that is observed on Earth today. Any non-quantum-mechanical coordination among elements of the experiment would thereby be pushed back billions of years, in some scenarios back to the big bang itself, an improvement of 20 orders of magnitude over current constraints.