  In these notes, based on lectures given as part of the Les Houches summer
school on Quantum Optics and Nanophotonics in August, 2013, I have tried to
give a brief survey of some important approaches and modern tendencies in
quantum measurement. I wish it to be clear from the outset that I shy
explicitly away from the "quantum measurement problem," and that the present
treatment aims to elucidate the theory and practice of various ways in which
measurements can, in light of quantum mechanics, be carried out; and various
formalisms for describing them. While the treatment is by necessity largely
theoretical, the emphasis is meant to be on an experimental "perspective" on
measurement -- that is, to place the priority on the possibility of gaining
information through some process, and then attempting to model that process
mathematically and consider its ramifications, rather than stressing a
particular mathematical definition as the {\it sine qua non} of measurement.
The textbook definition of measurement as being a particular set of
mathematical operations carried out on particular sorts of operators has been
so well drilled into us that many have the unfortunate tendency of saying "that
experiment can't be described by projections onto the eigenstates of a
Hermitian operator, so it is not really a measurement," when of course any
practitioner of an experimental science such as physics should instead say
"that experiment allowed us to measure something, and if the standard theory of
measurement does not describe it, the standard theory of measurement is
incomplete." Idealisations are important, but when the real world breaks the
approximations made in the theory, it is the theory which must be fixed, and
not the real world.
