An odd feature of the `most classical' states of $SU(2)$ invariant quantum mechanical systems

Abstract

Complex and spinorial techniques of general relativity are used to determine all the states of the $SU(2)$ invariant quantum mechanical systems in which the equality holds in the uncertainty relations for the components of the angular momentum vector operator in two given directions. The expectation values depend on a discrete quantum number and two parameters, one of them is the angle between the two angular momentum components and the other is the quotient of the two standard deviations. Allowing the angle between the two angular momentum components to be arbitrary, \emph{a new genuine quantum mechanical phenomenon emerges}: It is shown that although the standard deviations change continuously, one of the expectation values changes \emph{discontinuously} on this parameter space. Since physically neither of the angular momentum components is distinguished over the other, this discontinuity suggests that the genuine parameter space must be a \emph{double cover} of this classical one: It must be diffeomorphic to a \emph{Riemann surface} known in connection with the complex function $\sqrt{z}$. Moreover, the angle between the angular momentum components plays the role of the parameter of an interpolation between the continuous range of the expectation values found in the special case of the orthogonal angular momentum components and the discrete point spectrum of one angular momentum component. The consequences in the \emph{simultaneous} measurements of these angular momentum components are also discussed briefly.