Loophole-free 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 local-realist 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 event-ready scheme that enables the generation of robust entanglement between distant electron spins (estimated state fidelity of 0.92 ± 0.03). Efficient spin read-out avoids the fair-sampling assumption (detection loophole), while the use of fast random-basis selection and spin read-out combined with a spatial separation of 1.3 kilometres ensure the required locality conditions. We performed 245 trials that tested the CHSH-Bell inequality S ≤ 2 and found S = 2.42 ± 0.20 (where S quantifies the correlation between measurement outcomes). A null-hypothesis test yields a probability of at most P = 0.039 that a local-realist model for space-like 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 local-realist 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 less-conventional theories, and for implementing device-independent quantum-secure communication and randomness certification.

Ground-to-satellite quantum teleportation

“…Teleportation of a subsystem of an entangled pair translates itself into the protocol of entanglement swapping [22][23][24][25] where two remote particles can become entangled without direct interactions. Further, teleportation of quantum logic gates, a key element in distributed quantum computing schemes, is also possible assisted by shared multiparticle entangled states 5,6 .…”

Correlation, Entanglement and Locality of Quantum Theory

“…Actually in the recent experiment of Hensen et al [17] as well as in earlier series of similar experiments just this correlated noise was measured at the macroscopic distances. One should emphasize that the so-called nonlocality of this noise is caused by the local common cause in the past discussed above and not by an imaginary momentary action at a distance frequently mentioned in the literature (see, e.g.…”

“…In this case there will be equal average number of positive and negative units fixed by the detector. Now let us see how the Bell inequality is violated by the quantum exchange correlation described above for the case (17). We choose two directions a and a of the detector …”

“…[8,9,10,11,12,13,14,15,16]). Recently the most careful experiment of this type and its interpretation as the final proof of the non-locality of nature was published [17,18] and was intensively discussed. Nevertheless, there is no agreement now among the physical community on the meaning and physical origin of the observed correlation.…”

Quantum entanglement: facts and fiction – how wrong was Einstein after all?

“…1 Bohm & Aharonov (1957) (Aspect et al 1982a, b;Clauser et al 1969;Clauser & Horne, 1974;Clauser & Shimony, 1978;Freedman & Clauser, 1972;Giustina et al 2015;Guerreiro et al 2012;Hensen et al 2015;Horne & Zeilinger, 1985;Kimble et al 1977;Weihs et al 1998;Zbinden et al 2001).…”