Over the course of the last two decades, neutron reflectometry has become established as an important
structural probe of thin films and multilayered composites, most notably of hydrogenous and magnetic
materials. As an introduction, the basic principles and typical applications of neutron reflectometry are
briefly reviewed. Examples of neutron reflectometry studies of thin film systems of interest in
condensed matter physics, chemical physics, and biophysics are presented. In particular, the scattering
length density (SLD) depth profile along the surface normal, averaged over in plane, can be deduced
from specular neutron reflectivity measurements (wavevector transfer Q normal to the surface). The
SLD profile, in turn, is directly related to the corresponding material composition distribution. Under
favorable conditions, specular neutron reflectometry can resolve variations in the compositional depth
profile on a length scale of the order of a nanometer for a thin film having a single unit repeat, whereas
for a periodic multilayered system, the spatial resolution can approach an Angstrom.
For specular neutron reflection, the complex reflection amplitude or phase associated with an
"unknown" segment of a composite film structure can be determined exactly, using reference segments,
and a subsequent direct inversion can be performed, thereby ensuring, in principle, a unique result .
Thus, the phasesensitive
neutron reflection / inversion process results in a realspace
fitting or any adjustable parameters. We will discuss how, because of the onetoone
between the complex reflection amplitude and the SLD, phasesensitive
NR can be viewed, in effect, as
being equivalent to a realspace
imaging process one
in which the inversion computation plays an
analogous role to that of the brain, for instance, in interpreting the optical image of an object focused on
the retina of the eye .
In performing phasesensitive
reflectivity measurements in practice, what ultimately limits the accuracy
and spatial resolution of the depth profile are the maximum range of Q attainable and the statistical
uncertainty in the measured reflected intensities. These effects can be analyzed quantitatively  and
we will consider the spatial resolution currently possible as well as what can be reasonably expected in
the future with more advanced neutron sources and instrumentation (e.g., employing polychromatic
beams at continuous sources).
Finally, we will critically examine a possible alternative approach to performing neutron reflectivity
measurements, which involves the quantum phenomenon of "Interaction Free Measurement" (IFM) of
the type first proposed by Dicke  and realized in rudimentary fashion by Kwiat et al. with visible
light . The scheme utilized by Kwiat et al. purportedly optimizes the efficiency for performing an
IFM of the reflectivity (or transmission) by application of the quantum Zeno effect (which requires
polarized photons or neutrons) within an interferometer.