The role of quantum measurements in physical processes and protocols

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.

Room-temperature photonic logical qubits via second-order nonlinearities

“…The logic required to determine the error syndrome for this code requires 16 CNOT gates and 4 auxiliary qubits 27 . The auxiliary qubits can either be measured, in which case the error can be determined using a classical computer, or a unitary circuit could process the auxiliary qubits and perform the correction [48][49][50][51][52] . For each of the 16 different values of the four-bit syndrome, a unitary correction circuit would need to perform a different correction operation.…”

Continuous measurements for control of superconducting quantum circuits

“…The second is engineering the system such that the measurement back-action is evidently deterministic, driving the system to the desired state in an autonomous fashion, commonly referred as coherent feedback and represented by (c) in Figure 4. It is now understood that the difference between these processes is physical and not fundamental, and which one can be applied is determined by the technological ability [8,9]. We conclude by expanding on the role of autonomous feedback in quantum error correction.…”