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Los
Alamos.

Quantitative measures are introduced for the indistinguishability $U$ of two quantum
states in a given measurement and the amount of interference $I$ observable in this
measurement. It is shown that these measures obey an inequality $U\geq I$ which can be
seen as an exact formulation of Bohr's claim that one cannot distinguish between two
possible paths of a particle while maintaining an interference phenomenon. This
formulation is applied to a neutron interferometer experiment of Badurek e.a. It is
shown that the formulation is stronger than an argument based on an uncertainty relation
for phase and photon number considered by these authors.