  This paper discusses experiments with single-particle systems, some of whose
states appear to be entangled. It shows that the interpretation of the
experiments in terms of entanglement is ill-defined. Three forms of ambiguity
are discussed. The choice of state-space and its dimensions is a matter of
taste. There is not an a-priori natural partitioning of the state-space. The
observables are not necessarily experimentally accessible and only determined
by theory-laden extrapolation from experimental results. These ambiguities need
to be addressed in the formulation of any general theory of entanglement.
