  We discuss spin-$\frac12$ Heisenberg antiferromagnet on simple square lattice
in magnetic field $H$ using recently proposed bond-operator technique. It is
well known that magnetically ordered phases of quantum magnets are well
described at least qualitatively by the conventional spin-wave theory that only
introduces quantum corrections into the classical solution of the problem. We
observe that quantum fluctuations change drastically dynamical properties of
the considered model at $H$ close to its saturation value: the dynamical
structure factor shows anomalies corresponding to Green's function poles which
have no counterparts in the spin-wave theory. That is, quantum fluctuations
produce multiple short-wavelength magnon modes not changing qualitatively the
long-wavelength spin dynamics. Our results are in agreement with previous
quantum Monte-Carlo simulations and exact diagonalization of finite clusters.
