Evidence of Uncollapsed Quantum Amplitudes After Consecutive Measurements

TL;DR

This study experimentally distinguishes between collapse and unitary interpretations of quantum measurement by analyzing the coherence in consecutive measurements, supporting the unitary theory when no decoherence is present.

Contribution

The paper provides experimental evidence showing that quantum amplitudes persist after measurements, favoring the unitary interpretation over collapse models in certain conditions.

Findings

Coherence persists among consecutive measurements under unitary evolution.

Experimental results support the unitary theory of quantum measurement.

Decoherence aligns results with collapse theory predictions.

Abstract

Two of the most common interpretations of quantum measurement disagree about the fate of quantum amplitudes after measurement, yet this disagreement has not previously led to experimentally distinguishable predictions. In the standard collapse picture, commonly linked to the Copenhagen interpretation of quantum mechanics, measurements eliminate unrealized amplitudes without leaving a memory. In contrast, in the unitary theory, the measurement device registers one of the possible outcomes while remaining part of an entangled state that continues to harbor the unrealized amplitudes. This persistence arises naturally under unitary evolution, since a measurement device that is part of an entangled system cannot serve as a faithful probe of the joint quantum state. Using single-photon measurements of a tunable quantum state, we experimentally show that these two theories make different predictions when three or more consecutive measurements are performed on the same quantum system. Analysis of the joint density matrix of the three measurements reveals coherence among them and supports the unitary theory of quantum measurement. When decoherence is explicitly introduced, the joint density matrix of the quantum system of interest and the apparatus becomes consistent with what a collapse theory would predict. This work clarifies the dynamics of consecutive quantum measurements and offers new insights into the interpretation of quantum measurements.