  Quantum - or classically correlated - light can be employed in various ways
to improve resolution and measurement sensitivity. In an "interaction-free"
measurement, a single photon can be used to reveal the presence of an object
placed within one arm of an interferometer without being absorbed by it. This
method has previously been applied to imaging. With a technique known as "ghost
imaging", entangled photon pairs are used for detecting an opaque object with
significantly improved signal-to-noise ratio while preventing
over-illumination. Here, we integrate these two methods to obtain a new imaging
technique which we term "interaction-free ghost-imaging" that possesses the
benefits of both techniques. While maintaining the image quality of
conventional ghost-imaging, this new technique is also sensitive to phase and
polarisation changes in the photons introduced by a structured object.
Furthermore, thanks to the "interaction-free" nature of this new technique, it
is possible to reduce the number of photons required to produce a clear image
of the object (which could be otherwise damaged by the photons) making this
technique superior for probing light-sensitive materials and biological
tissues.
