The quantum measurement problem still finds no consensus. Nonlocal
interferometry provides an unprecedented experimental probe by entangling two
photons in the "measurement state" (MS). The experiments show that each photon
"measures" the other; the resulting entanglement decoheres both photons;
decoherence collapses both photons to unpredictable but definite outcomes; and
the two-photon MS continues evolving coherently. Thus, contrary to common
opinion, when a two-part system is in the MS, the outcomes actually observed at
both subsystems are definite. Although standard quantum physics postulates
definite outcomes, two-photon interferometry verifies them to be not only
consistent with, but actually a prediction of, the other principles.
Nonlocality is the key to understanding this. As a consequence of nonlocality,
the states we actually observe are the local states. These actually-observed
local states collapse, while the global MS, which can be "observed" only after
the fact by collecting coincidence data from both subsystems, continues its
unitary evolution. This conclusion implies a refined understanding of the
eigenstate principle: Following a measurement, the actually-observed local
state instantly jumps into the observed eigenstate. Various experts' objections
are rebutted.Comment: 1 figure. arXiv admin note: substantial text overlap with
arXiv:1206.518