Test of the linearity of quantum mechanics in optically pumpedHg201

Abstract
Measurement of the free precession of nuclear spins in optically pumped ground-state Hg201 atoms has allowed a test of Weinberg’s nonlinear generalization of quantum mechanics. Shifts among the three resolved precession frequencies in the spin-3/2 system were studied as a function of the projection of spin polarization along the precession axis. Our results set an upper limit on ε, Weinberg’s nonlinearity parameter, of ‖ε‖/2πħ<3.8μ Hz, which corresponds to 2.0×1027 of the binding energy per nucleon. This is the most stringent such limit to date.