Quantum entanglement makes a key distinction between the classical and the quantum world. The debate over the interpretation of this entanglement remained center stage for much of the 20th century. That an interpretation based on hidden variables could be ruled out on the basis of experimental observation is the essence of the Bell inequality, and following the seminal works of Freedman and Clauser (1) and Aspect et al. (2–4), many groups worldwide have used nonlocal correlations between pairs of photons to show the violation of this type of inequality. Notably, recent improvements in the scheme design and component performance have allowed simultaneous closing of the various loopholes present in earlier demonstrations (5–7).
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