Superposition and macroscopic observation

Nancy Delaney Cartwright

Andrew Knight

I show in this paper why the universality of quantum mechanics at all scales, which implies the possibility of Schrodinger's Cat and Wigner's Friend thought experiments, cannot be experimentally confirmed, and why macroscopic superpositions in general cannot be observed or measured, even in principle. Through the relativity of quantum superposition and the transitivity of correlation, it is shown that from the perspective of an object that is in quantum superposition relative to a macroscopic measuring device and observer, the observer is already sufficiently well correlated to the measuring device that once the object correlates to the measuring device, there is no time period in which the observer can perform an appropriate interference experiment to show that the measuring device is in a superposition.

Tomoyuki Morimae, Akira Shimizu, Guillaume Adenier +4 more

We consider a coarse‐grained quasi joint probability density Ξ of two macroscopic observables. If a quantum many‐body state has a superposition of macroscopically distinct states, the structure of the superposition can be visualized by plotting Ξ for appropriate macroscopic observables. For example, the ground state of the XY model, that of the Heisenberg antiferromagnet, and a state in Grover's quantum search algorithm, which have superpositions of macroscopically distinct states, can be visualized. Like the Wigner distribution function, Ξ can take negative values. However, if the characteristic width W of the coarse‐graining is sufficiently large, Ξ becomes nonnegative. We study a sufficient magnitude of such W. We see that W = O(N) is not sufficient, but W = O(N) would be sufficient, where N is the size of the system.

Anthony J. Leggett

Gabriel Dias Carvalho, Pedro S. Correia, Thiago R. de Oliveira

Macroscopic quantum superpositions are widely believed to be unobservable because large systems cannot be perfectly isolated from their environments. Here, we show that even under perfect isolation, intrinsic unitary dynamics in generic many-body systems, as analyzed through the eigenstate thermalization hypothesis and random matrix theory, can suppress the observable signatures of macroscopic coherence. Using the Greenberger-Horne-Zeilinger (GHZ) state as a representative example, we demonstrate that while fully correlated measurements can initially distinguish a macroscopic superposition from its corresponding classical mixture, generic many-body evolution renders them operationally indistinguishable for most times. By analyzing both distinguishability measures and established quantifiers of macroscopic quantumness, we find that equilibration not only hides coherence from accessible observables but also suppresses the corresponding signatures of macroscopic quantumness, in particular within the additive-local framework considered here. These results identify unitary thermalization, independent of environmental decoherence, as a fundamental mechanism that limits the observation of macroscopic quantum effects.