  The three-box problem is a gedankenexperiment designed to elucidate some
interesting features of quantum measurement and locality. A particle is
prepared in a particular superposition of three boxes, and later found in a
different (but nonorthogonal) superposition. It was predicted that appropriate
"weak" measurements of particle position in the interval between preparation
and post-selection would find the particle in two different places, each with
certainty. We verify these predictions in an optical experiment and address the
issues of locality and of negative probability.
