Two-photon interferometry illuminates quantum measurements

TL;DR

This paper uses two-photon interferometry to experimentally explore the quantum measurement problem, demonstrating how local outcomes are definite while the global measurement state remains coherently evolving, highlighting nonlocality's role.

Contribution

It provides experimental evidence that local measurement outcomes are definite within the quantum measurement framework, emphasizing the role of nonlocality and refining the eigenstate principle.

Findings

Local measurement outcomes are definite and consistent with quantum predictions.

The global measurement state continues to evolve coherently despite local collapse.

Nonlocality underpins the observed definiteness of local states.

Abstract

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.