Measures and applications of quantum correlations

TL;DR

This paper reviews various measures of quantum correlations beyond entanglement, exploring their physical foundations, operational significance, and potential applications in quantum information processing and technology.

Contribution

It provides a comprehensive overview of different approaches to define and quantify general quantum correlations and discusses their relevance for quantum technological tasks.

Findings

Quantum correlations extend beyond entanglement and are resilient in many states.

Different measures of quantum correlations are linked to physical principles like disturbance and coherence.

Quantum correlations can enhance performance in noisy quantum protocols.

Abstract

Quantum information theory is built upon the realisation that quantum resources like coherence and entanglement can be exploited for novel or enhanced ways of transmitting and manipulating information, such as quantum cryptography, teleportation, and quantum computing. We now know that there is potentially much more than entanglement behind the power of quantum information processing. There exist more general forms of non-classical correlations, stemming from fundamental principles such as the necessary disturbance induced by a local measurement, or the persistence of quantum coherence in all possible local bases. These signatures can be identified and are resilient in almost all quantum states, and have been linked to the enhanced performance of certain quantum protocols over classical ones in noisy conditions. Their presence represents, among other things, one of the most essential manifestations of quantumness in cooperative systems, from the subatomic to the macroscopic domain. In this work we give an overview of the current quest for a proper understanding and characterisation of the frontier between classical and quantum correlations in composite states. We focus on various approaches to define and quantify general quantum correlations, based on different yet interlinked physical perspectives, and comment on the operational significance of the ensuing measures for quantum technology tasks such as information encoding, distribution, discrimination and metrology. We then provide a broader outlook of a few applications in which quantumness beyond entanglement looks fit to play a key role.