The role of quantum measurements in physical processes and protocols

In thismainly pedagogical article, wediscuss underwhat circumstancesmeasurements play a special role in quantumprocesses. Inparticular,wediscuss the following facts that appear to be a commonarea of confusion. (i)Froma fundamental point of view,measurements play no special rolewhatsoever: all dynamics that canbe generated bymeasurements canbe generated by unitary processes (forwhich postselection is no exception). (ii)Fromapurely physicalpoint of view,measurements are not ‘outside’of quantummechanics. (iii)The only difference between the abilities ofmeasurement-basedprotocols and unitary circuits for quantumcomputing comes frompractical (technology dependent) constraints.We emphasise the importance of distinguishingbetween differences that are (i) fundamental butwithout physical import; (ii) fundamental andpossess physical import; and (iii) are not fundamental but have practical import.We also emphasise the importance of separating theoretical and experimental elements ofmeasurement, primarilyprojection and amplification, which are physically very different.Note that sincewe are concernedwith facts regardingphysical processes, this article has little if anything to dowith interpretations of quantummechanics.