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Physical Review Letters

Extreme quantum entanglement in a superposition of macroscopically distinct states

N. David Mermin

Published Oct 1, 1990• 1,482 citations
Publisher

Abstract

A Bell inequality is derived for a state of n spin-1/2 particles which superposes two macroscopically distinct states. Quantum mechanics violates this inequality by an amount that grows exponentially with n.
DOI
10.1103/physrevlett.65.1838

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Superposition in quantum states of large systems plays a crucial role in quantum theory and has important fundamental as well as technological implications, ranging from quantum measurement theory to quantum computers. The power of the potential implications of such states lies in the quantum coherence, as measured by the amount of entanglement and other quantum correlations, between its microscopic and macroscopic sectors. We show that in contrast to other macroscopic superposition states, it is possible to choose the states of the macroscopic sector in a way that the resulting state, which we term as the HC{H}_{C}HC​ state, has quantum coherence that is resistant to the twin effects of environmental noise: (a) local decoherence on all the particles and (b) loss of a finite fraction of its particles. The states of the macroscopic sector of the HC{H}_{C}HC​ state are macroscopically distinct in terms of their violation of Bell inequality.