  Position measurement-induced collapse states are shown to provide a unified
quantum description of diffraction of particles passing through a single slit.
These states, which we here call `quantum location states', are represented by
the conventional rectangular wave function at the initial moment of position
measurement. We expand this state in terms of the position eigenstates, which
in turn can be represented as a linear combination of energy eigenfunctions of
the problem, using the closure property. The time-evolution of the location
states in the case of free particles is shown to have position probability
density patterns closely resembling diffraction patterns in the Fresnel region
for small times and the same in Fraunhofer region for large times. Using the
quantum trajectory representations in the de Broglie-Bohm, modified de
Broglie-Bohm and Floyd-Faraggi-Matone formalisms, we show that Fresnel and
Fraunhofer diffractions can be described using a single expression. We also
discuss how to obtain the probability density of location states for the case
of particles moving in a general potential, detected at some arbitrary point.
In the case of the harmonic oscillator potential, we find that they have
oscillatory properties similar to that of coherent states.
