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10.2.3 Ion and Photon
In the attempt to close both loopholes, D.L. Moehring published an experiment
in 2004 that used an atom and a photon as the entangled pair [Moehring
et al., 2004]. The high detection efficiency of the measurement of the atom’s state
combined with the theoretical possibility to remove the photon far from the atom
could eventually allow this approach to implement a loophole-free Bell inequality
test. But due to limitations of the experiment, the group was not able to close
the locality loophole in this version of the experiment. The group reported an
S-value of S = 2.218 ± 0.028.
10.3 The Bell Inequality versus Phase Qubits?
Given the fact that the overwhelming evidence [Weihs et al., 1998, Roos et al.,
2004] has effectively settled the question of whether a local hidden variable theory
should replace quantum mechanics, one might ask why another implementation
of a test of Bell inequality in superconducting qubits is desirable, especially since
such an experiment would most likely also be susceptible to the locality loophole.
The argument in favor of the experiment is two-fold:
• On the one hand, it would be the first implementation of the test using
a macroscopic quantum state. All experiments to date have relied on mi-
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