  It is possible for two parties, Alice and Bob, to establish a secure
communication link by sharing an ensemble of entangled particles, and then
using these particles to generate a secret key. One way to establish that the
particles are indeed entangled is to verify that they violate a Bell
inequality. However, it might be the case that Bob is not trustworthy and
wishes Alice to believe that their communications are secure, when in fact they
are not. He can do this by managing to have prior knowledge of Alice's
measurement device settings and then modifying his own settings based upon this
information. In this case it is possible for shared particle states that must
satisfy a Bell inequality to appear to violate this inequality, which would
also make the system appear secure. When Bob modifies his measurement settings,
however, he produces false correlations. Here we demonstrate experimentally
that Alice can detect these false correlations, and uncover Bob's trickery, by
using loop-state-preparation-and-measurement (SPAM) tomography. More generally,
we demonstrate that loop SPAM tomography can detect false correlations
(correlated errors) in a two-qubit system without needing to know anything
about the prepared states or the measurements, other than the dimensions of the
operators that describe them.
