  We experimentally demonstrate that loop state-preparation-and-measurement
(SPAM) tomography is capable of detecting correlated errors in a two-qubit
system. We prepare photon pairs in a state that approximates a Werner state,
which may or may not be entangled. By performing measurements with multiple
different detector settings we are able to detect correlated errors between two
single-qubit measurements performed in different locations. No assumptions are
made concerning either the state preparations or the measurements, other than
that the dimensions of the states and the positive-operator-valued measures
describing the detectors are known. The only other needed information is
experimentally measured expectation values, which are analyzed for
self-consistency. This demonstrates that loop SPAM tomography is a useful
technique for detecting errors that would degrade the performance of
multiple-qubit quantum information processors.
