  We consider the possibility that the relative phase in quantum mechanics
plays a role in determining measurement outcome and could therefore serve as a
"hidden" variable. The Born rule for measurement equates the probability for a
given outcome with the absolute square of the coefficient of the basis state,
which by design removes the relative phase from the formulation. The value of
this phase at the moment of measurement naturally averages out in an ensemble,
which would prevent any dependence from being observed, and we show that
conventional frequency-spectroscopy measurements on discrete quantum systems
cannot be imposed at a specific phase due to a straightforward uncertainty
relation. We lay out general conditions for imposing measurements at a specific
value of the relative phase so that the possibility of its role as a hidden
variable can be tested, and we discuss implementation for the specific case of
an atomic two-state system with laser-induced fluorescence for measurement.
