Quantum discord is Bohr’s notion of non-mechanical disturbance introduced to answer EPR
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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-incompleteness-to be reached by establishing the physical reality of just a single observable q (not of both q and its conjugate observable 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.
Annals of Physics
© 2013 Elsevier Inc. This is the author-manuscript version of this paper. Reproduced in accordance with the copyright policy of the publisher. Please refer to the journal's website for access to the definitive, published version.
Quantum Information, Computation and Communication
Quantum Physics not elsewhere classified