  Spectroscopy is an important tool for providing insights into the structure
of core-collapse supernova explosions. We use the Monte Carlo radiative
transfer code ARTIS to compute synthetic spectra and light curves based on a
two-dimensional explosion model of an ultra-stripped supernova. These
calculations are designed both to identify observable fingerprints of
ultra-stripped supernovae and as a proof-of-principle for using synthetic
spectroscopy to constrain the nature of stripped-envelope supernovae more
broadly. We predict very characteristic spectral and photometric features for
our ultra-stripped explosion model, but find that these do not match observed
ultra-stripped supernova candidates like SN 2005ek. With a peak bolometric
luminosity of $6.8\times10^{41}\,\mathrm{erg}\,\mathrm{s}^{-1}$, a peak
magnitude of $-15.9\,\mathrm{mag}$ in R-band, and $\Delta
m_{15,\mathrm{R}}=3.50$, the model is even fainter and evolves even faster than
SN 2005ek as the closest possible analogue in photometric properties. The
predicted spectra are extremely unusual. The most prominent features are Mg II
lines at 2,800 Angstrom and 4,500 Angstrom and the infrared Ca triplet at late
times. The Mg lines are sensitive to the multi-dimensional structure of the
model and are viewing-angle dependent. They disappear due to line blanketing by
Fe group elements in a spherically averaged model with additional microscopic
mixing. In future studies, multi-D radiative transfer calculations need to be
applied to a broader range of models to elucidate the nature of observed Type
Ib/c supernovae.
