Quantum discord cannot be shared

TL;DR

This paper confirms Bohr's idea that nonclassical measurement devices lead to information loss in communication, quantified by quantum discord, and explores the limitations of quantum communication in sharing measurement results.

Contribution

The work uses modern quantum information tools to validate Bohr's postulate and reveals how quantum discord relates to information loss during measurement communication.

Findings

Quantum discord quantifies information loss in classical communication of measurement results.

Quantum communication offers no advantage for pure system-apparatus states when sharing with multiple recipients.

Quantum communication can outperform classical methods in mixed states, distinct from quantum cloning effects.

Abstract

Niels Bohr proposed that the outcome of the measurement becomes objective and real, and, hence, classical, when its results can be communicated by classical means. In this work we revisit Bohr's postulate using modern tools from the quantum information theory. We find a full confirmation of Bohr's idea: if a measurement device is in a nonclassical state, the measurement results cannot be communicated perfectly by classical means. In this case some part of information in the measurement apparatus is lost in the process of communication: the amount of this lost information turns out to be the quantum discord. The information loss occurs even when the apparatus is not entangled with the system of interest. The tools presented in this work allow to generalize Bohr's postulate: we show that for pure system-apparatus states quantum communication does not provide any advantage when measurement results are communicated to more than one recipient. We further demonstrate the superiority of quantum communication to two recipients on a mixed system-apparatus state and show that this effect is fundamentally different from quantum state cloning.