  We study the properties of the Bose polaron, an impurity strongly interacting
with a Bose-Einstein condensate, using a field-theoretic approach and make
predictions for the spectral function and various quasiparticle properties that
can be tested in experiment. We find that most of the spectral weight is
contained in a coherent attractive and a metastable repulsive polaron branch.
We show that the qualitative behavior of the Bose polaron is well described by
a non-selfconsistent T-matrix approximation by comparing analytical results to
numerical data obtained from a fully selfconsistent T-matrix approach. The
latter takes into account an infinite number of bosons excited from the
condensate.
