The role of quantum measurements in physical processes and protocols

TL;DR

This paper clarifies the fundamental and practical roles of quantum measurements in physical processes, emphasizing their equivalence to unitary dynamics and the importance of distinguishing theoretical and experimental aspects.

Contribution

It provides a pedagogical analysis of when measurements are physically special in quantum processes, highlighting distinctions between fundamental, physical, and practical differences.

Findings

Measurements can be simulated by unitary processes.

Measurements are not outside quantum mechanics.

Practical constraints influence measurement-based quantum computing.

Abstract

In this mainly pedagogical article, we discuss under what circumstances measurements play a special role in quantum processes. In particular we discuss the following facts which appear to be a common area of confusion: i) from a fundamental point of view measurements play no special role whatsoever: all dynamics that can be generated by measurements can be generated by unitary processes (for which post-selection is no exception), ii) from a purely physical point of view, measurements are not "outside" of quantum mechanics, iii) the only difference between the abilities of measurement-based protocols and unitary circuits for quantum computing comes from practical (technology dependent) constraints. We emphasize the importance of distinguishing between differences that are i) fundamental but without physical import, ii) fundamental and possess physical import, iii) are not fundamental but have practical import. We also emphasize the importance of separating theoretical and experimental elements of measurement, primarily projection and amplification, which are physically very different. Note that since we are concerned with facts regarding physical processes, this article has little if anything to do with interpretations of quantum mechanics.