Quantum correlations, coherence, and macroscopicity
Istituto Nazionale di Ricerca Metrologica (INRIM)
Sala Conferenze, Palazzina M
University of Oxford
National University of Singapore
One of the best signatures of nonclassicality in a quantum system is the existence of correlations that have no classical counterpart. Different methods for quantifying the quantum and classical parts of correlations are amongst the more actively-studied topics of quantum information theory over the past two decades. Entanglement is the most prominent of these correlations, but in many cases disentangled states exhibit nonclassical behavior too. Thus distinguishing quantum correlations other than entanglement provides a better division between the quantum and classical worlds, especially when considering mixed states. In the first part of my talk, I will review different notions of classical and quantum correlations quantified by quantum discord and other related measures .
Another important and related notion  is that of macroscopicity. I will suggest that quantum macroscopicity should be quantified in terms of coherence, and propose a set of conditions that should be satisfied by any measure of macroscopic coherence [3,4,5]. I will then show that this enables a rigorous justification of a previously proposed measure of macroscopicity based on the quantum Fisher information. This might shed new light on the standard Schrödinger cat type interference experiment that is meant to demonstrate the existence of macroscopic superpositions and entanglement.
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