  A scheme for sub-wavelength position measurements of quantum particles is
discussed, which operates with running-wave laser fields as opposed to standing
wave fields proposed in previous setups. The position is encoded in the phase
of the applied fields rather than in the spatially modulated intensity of a
standing wave. Therefore, disadvantages of standing wave schemes such as cases
where the atom remains undetected since it is at a node of the standing wave
field are avoided. Reversing the directions of parts of the driving laser
fields allows to switch between different magnification levels, and thus to
optimize the localization.
