  We present a study of the coherence properties of a variety of motional
states of a single ion confined in a Penning ion trap. We demonstrate that the
motion of the ion has a coherence time of the order of one second, using Ramsey
interferometry. We introduce a technique for preparing the ion in an incoherent
superposition of highly-excited motional states using a simple modification of
optical sideband cooling. Coherent manipulation of these states allow
measurements of optical and motional coherence to be carried out. We show that
these highly-excited motional state superpositions have long coherence times
despite the incoherent preparation of the states. Such states can be useful for
sensitive motional dephasing measurements.
