Seventy five years ago, three remarkable papers by
Schrödinger, Kennard and Darwin were published. They were
devoted to the evolution of Gaussian wave packets for an
oscillator, a free particle and a particle moving in uniform
constant electric and magnetic fields. From the contemporary
point of view, these packets can be considered as prototypes of
the coherent and squeezed states, which are, in a sense, the
cornerstones of modern quantum optics. Moreover, these
states are frequently used in many other areas, from solid state
physics to cosmology. This paper gives a review of
studies performed in the field of so-called `nonclassical
states' (squeezed states are their simplest representatives)
over the past seventy five years, both in quantum optics and in other
branches of quantum physics.
My starting point is to elucidate who introduced
different concepts, notions and terms, when, and what were the
initial motivations of the authors. Many new references have been found which
enlarge the `standard citation package' used by
some authors, recovering many undeservedly forgotten (or
unnoticed) papers and names. Since it is practically impossible
to cite several thousand publications, I have tried to include mainly
references to papers introducing new types of
quantum states and studying their properties, omitting many
publications devoted to applications and to the methods
of generation and experimental schemes, which can be found in
other well known reviews. I also mainly concentrate on the
initial period, which terminated approximately at the border
between the end of the 1980s and the beginning of the 1990s, when
several fundamental experiments on the generation of squeezed states
were performed and the first conferences devoted to
squeezed and `nonclassical' states commenced. The 1990s are
described in a more `squeezed' manner: I have confined myself
to references to papers where some new concepts have been
introduced, and to the most recent reviews or papers with
extensive bibliographical lists.