Loopholefree Bell inequality violation using electron spins separated by 1.3 kilometres
Abstract
More than 50 years ago, John Bell proved that no theory of nature that obeys locality and realism can reproduce all the predictions of quantum theory: in any localrealist theory, the correlations between outcomes of measurements on distant particles satisfy an inequality that can be violated if the particles are entangled. Numerous Bell inequality tests have been reported; however, all experiments reported so far required additional assumptions to obtain a contradiction with local realism, resulting in `loopholes'. Here we report a Bell experiment that is free of any such additional assumption and thus directly tests the principles underlying Bell's inequality. We use an eventready scheme that enables the generation of robust entanglement between distant electron spins (estimated state fidelity of 0.92 +/ 0.03). Efficient spin readout avoids the fairsampling assumption (detection loophole), while the use of fast randombasis selection and spin readout combined with a spatial separation of 1.3 kilometres ensure the required locality conditions. We performed 245 trials that tested the CHSHBell inequality S <= 2 and found S = 2.42 +/ 0.20 (where S quantifies the correlation between measurement outcomes). A nullhypothesis test yields a probability of at most P = 0.039 that a localrealist model for spacelike separated sites could produce data with a violation at least as large as we observe, even when allowing for memory in the devices. Our data hence imply statistically significant rejection of the localrealist null hypothesis. This conclusion may be further consolidated in future experiments; for instance, reaching a value of P = 0.001 would require approximately 700 trials for an observed S = 2.4. With improvements, our experiment could be used for testing lessconventional theories, and for implementing deviceindependent quantumsecure communication and randomness certification.
 Publication:

Nature
 Pub Date:
 October 2015
 DOI:
 10.1038/nature15759
 arXiv:
 arXiv:1508.05949
 Bibcode:
 2015Natur.526..682H
 Keywords:

 Quantum Physics
 EPrint:
 Raw data will be made available after publication