Observing a Quantum Measurement

TL;DR

This paper demonstrates how superpositions in quantum measurements can be observed without contradicting the concept of collapse, using ancillae as virtual detectors in a Stern-Gerlach experiment.

Contribution

It introduces a method to observe superpositions compatibly with collapse properties using ancillae, providing new insights into quantum measurement theory.

Findings

Superpositions can be observed alongside collapse properties.

Nondestructive measurements of compatible observables are feasible.

Ordinary measurements are insensitive to superpositions within unitary quantum theory.

Abstract

With the example of a Stern-Gerlach measurement on a spin-1/2 atom, we show that a superposition of both paths may be observed compatibly with properties attributed to state collapse - for example, the singleness (or mutual exclusivity) of outcomes. This is done by inserting a quantum two-state system (an ancilla) in each path, capable of responding to the passage of the atom, and thus acting as a virtual detector. We then consider real measurements on the compound system of atomic spin and two ancillae. Nondestructive measurements of a set of compatible joint observables can be performed, one for a superposition and others for collapse properties. A novel perspective is given as to why, within unitary quantum theory, ordinary measurements are blind to such superpositions. Implications for the theory of measurement are discussed.