Is macroscopic quantum coherence incompatible with macroscopic realism?

Summary

The proposal by Leggett and Leggettet al. (Suppl. Prog. Theor. Phys.,69 (1980) 80;Directions in Condensed Matter Physics, edited byG. Grinstein andG. Mazenko (World Scientific, Singapore) 1986, p. 189;Jpn. J. Appl. Phys.,26 (1987) 1986,Suppl. 26–3;Phys. Rev. Lett.,54 (1985) 857) of using Macroscopic Quantum Coherence (MQC) in a SQUID as a test of the validity of Quantum Mechanics (QM) for macroscopic systems is considered. We note that if only Macroscopic Realism (MR) is assumed but the requirement that the flux measurement is non-invasive (NIM) is dropped, only the measurement of the charge would discriminate between QM and MR. This discrimination, however, depends critically on the experimental parameters. There is a region in which QM is consistent with MR, but the measurement is invasive as in QM. In this region the system is not fully macroscopic because the uncertainty relation between the flux and the conjugated charge is of the order ofh.