Simple model for the detection of a particle by a point detector

A model for a one-dimensional quantum particle interacting with a point detector is developed and analyzed. The analysis leads to any simple, intuitively pleasing expression for the final density operator of the particle in terms of the freely propagating particle density operator, a projection operator representing the detector volume, and a probability density for the time at which the detector fires. This probability density is found explicitly to be the presence-time density given by ${\mid {\psi }_f(0,t)\mid }^2∕\int {\mid {\psi }_f(0,t)\mid }^{2}dt$ where ${\psi }_f(0,t)$ is the free particle wave function evaluated at the detector position $(x=0)$. Moreover, it is shown that the resultant density operator transformation is a nonlinear transformation, not describable as a positive operator valued measure.