ABSTRACT
We have learnt in Chapter 5 how to reduce fluctuations of an
electromagnetic (EM) field below the quantum (Heisenberg) limit
imposed by the quantum nature of light. It has been shown that
the quantum limit can be beaten using non-classical squeezed light
of reduced fluctuations. Another issue of significant interest in
quantum optics is to beat the diffraction limit imposed on the
resolution of measured objects by the wave nature of light. In this
chapter, we will illustrate how one can beat the diffraction limit by
using the quantumnature of entangled light beams.We shall see that
entangled light can lead us to a new domain of quantum optics in
which detectors can resolve two closely spaced objects or spectral
lines with the minimal resolvable limit significantly reduced or even
completely suppressed. This subject is generally known as quantum
optical lithography and could be described in short as the ability to
print patterns onto certain materials using non-classical light. The
developments in this area are of fundamental interest, and they hold
promise for advances in optical interferometry and in applications
such as quantummetrology and gravitational wave studies.