Abstract:

For more than 80 years, the counterintuitive predictions of quantum theory have stimulated debate
about the nature of reality. In his seminal work, John Bell proved that no theory of nature
that obeys locality and realism can reproduce all the predictions of quantum theory. Bell showed
that in any local realist theory the correlations between distant measurements satisfy an inequality
and, moreover, that this inequality can be violated according to quantum theory. This provided a
recipe for experimental tests of the fundamental principles underlying the laws of nature. In the
past decades, numerous ingenious Bell inequality tests have been reporter. However, because of
experimental limitations, all experiments to date required additional assumptions to obtain a contradiction
with local realism, resulting in loopholes. Here we report on a Bell experiment that is free
of any such additional assumption and thus directly tests the principles underlying Bell's inequality.
We employ an event-ready scheme that enables the generation of high- delity entanglement
between distant electron spins. Ecient spin readout avoids the fair sampling assumption (detection
loophole), while the use of fast random basis selection and readout combined with a spatial
separation of 1.3 km ensure the required locality conditions. We perform 245 trials testing the
CHSH-Bell inequality18 S  2 and nd S = 2:42  0:20. A null hypothesis test yields a probability
of p = 0:039 that a local-realist model for space-like separated sites produces data with a violation
at least as large as observed, even when allowing for memory in the devices. This result rules
out large classes of local realist theories, and paves the way for implementing device-independent
quantum-secure communication and randomness certi cation.