  Incoherent noise is manifest in measurements of expectation values when the
underlying ensemble evolves under a classical distribution of unitary
processes. While many incoherent processes appear decoherent, there are
important differences. The distribution functions underlying incoherent
processes are either static or slowly varying with respect to control
operations and so the errors introduced by these distributions are refocusable.
The observation and control of incoherence in small Hilbert spaces is well
known. Here we explore incoherence during an entangling operation, such as is
relevant in quantum information processing. As expected, it is more difficult
to separate incoherence and decoherence over such processes. However, by
studying the fidelity decay under a cyclic entangling map we are able to
identify distinctive experimental signatures of incoherence. This result is
demonstrated both through numerical simulations and experimentally in a three
qubit nuclear magnetic resonance implementation.
