Quantum discord is Bohr's notion of non-mechanical disturbance introduced in his answer to EPR

TL;DR

This paper formalizes the EPR argument and Bohr's reply, revealing that Bohr's concept of disturbance aligns with quantum discord, which explains measurement effects beyond entanglement and impacts views on quantum nonlocality.

Contribution

It establishes a rigorous connection between Bohr's notion of non-mechanical disturbance and quantum discord, clarifying their roles in quantum measurement and nonlocality.

Findings

Bohr's disturbance criterion prevents EPR's conclusion of incompleteness.

Quantum discord characterizes when measurements disturb a system in Bohr's sense.

Discord can exist without entanglement, affecting interpretations of nonlocality.

Abstract

By rigorously formalizing the Einstein-Podolsky-Rosen (EPR) argument, and Bohr's reply, one can appreciate that both arguments were technically correct. Their opposed conclusions about the completeness of quantum mechanics hinged upon an explicit difference in their criteria for when a measurement on Alice's system can be regarded as not disturbing Bob's system. The EPR criteria allow their conclusion (incompletness) to be reached by establishing the physical reality of just a single observable $q$ (not a conjugate pair $q$ and $p$), but I show that Bohr's definition of disturbance prevents the EPR chain of reasoning from establishing even this. Moreover, I show that Bohr's definition is intimately related to the asymmetric concept of quantum discord from quantum information theory: if and only if the joint state has no Alice-discord, she can measure any observable without disturbing (in Bohr's sense) Bob's system. Discord can be present even when systems are unentangled, and this has implications for our understanding of the historical development of notions of quantum nonlocality.