How often is the coordinate of a harmonic oscillator positive?

Abstract

The coordinate of a harmonic oscillator is measured at a time chosen at random among three equiprobable instants: now, after one third of the period, or after two thirds. The (total) probability that the outcome is positive depends on the state of the oscillator. In the classical case the probability varies between 1/3 and 2/3, but in the quantum case -- between 0.29 and 0.71.