  With the example of a Stern-Gerlach measurement on a spin-1/2 atom, we show
that a superposition of both paths may be observed compatibly with properties
attributed to state collapse - for example, the singleness (or mutual
exclusivity) of outcomes. This is done by inserting a quantum two-state system
(an ancilla) in each path, capable of responding to the passage of the atom,
and thus acting as a virtual detector. We then consider real measurements on
the compound system of atomic spin and two ancillae. Nondestructive
measurements of a set of compatible joint observables can be performed, one for
a superposition and others for collapse properties. A novel perspective is
given as to why, within unitary quantum theory, ordinary measurements are blind
to such superpositions. Implications for the theory of measurement are
discussed.
