MACROSCOPIC QUANTUM COHERENCE AS A TEST OF QUANTUM MECHANICS L. CHIATII Laboratorio di Fisica Medica, CRS IRE, Roma M. CINI Dip. di Fisica Universita "La Sapienza", Roma M.SERVA Dip. di Matematica and INFN, L'Aquila Abstract. The proposal of using Macroscopic Quantum Coherence in a SQUID as a test of the validity of Quantum Mechanics (QM) for macroscopic systems (Leggett A., 1980; Leggett A. and Garg A. 1985) is considered. We note that if only Macroscopic Realism (MR) is assumed but the requirement that the flux measurement is non invasive (NlM) 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 threshold above which QM is consistent with MR but the measurement is invasive as in QM. PACS numbers: 03.6S.Bz, 74.50.+r 1. Introduction The main property of Quantum Mechanics (QM) is the superposition principle of states: (1) There is no question about its validity for microscopic systems. One should ask however whether it is also valid for macroscopic bodies. In fact, if the system is in the state (1) one has to accept that the variable G with eigenstates "'1 and "'2 is essentially undetermined until it acquires a definite value g1 or g2 in the act of 147 C. Garola and A. Rossi (eds.), The Foundations of Quantum Mechanics, 147-153. © 1995 Kluwer Academic Publishers.