  We present observations of DES16C2nm, the first spectroscopically confirmed
hydrogen-free superluminous supernova (SLSN-I) at redshift z~2. DES16C2nm was
discovered by the Dark Energy Survey (DES) Supernova Program, with follow-up
photometric data from the Hubble Space Telescope, Gemini, and the European
Southern Observatory Very Large Telescope supplementing the DES data.
Spectroscopic observations confirm DES16C2nm to be at z=1.998, and
spectroscopically similar to Gaia16apd (a SLSN-I at z=0.102), with a peak
absolute magnitude of U=-22.26$\pm$0.06. The high redshift of DES16C2nm
provides a unique opportunity to study the ultraviolet (UV) properties of
SLSNe-I. Combining DES16C2nm with ten similar events from the literature, we
show that there exists a homogeneous class of SLSNe-I in the UV (~2500A), with
peak luminosities in the (rest-frame) U band, and increasing absorption to
shorter wavelengths. There is no evidence that the mean photometric and
spectroscopic properties of SLSNe-I differ between low (z<1) and high redshift
(z>1), but there is clear evidence of diversity in the spectrum at <2000A,
possibly caused by the variations in temperature between events. No significant
correlations are observed between spectral line velocities and photometric
luminosity. Using these data, we estimate that SLSNe-I can be discovered to
z=3.8 by DES. While SLSNe-I are typically identified from their blue observed
colors at low redshift (z<1), we highlight that at z>2 these events appear
optically red, peaking in the observer-frame z-band. Such characteristics are
critical to identify these objects with future facilities such as the Large
Synoptic Survey Telescope, Euclid, and the Wide-Field Infrared Survey
Telescope, which should detect such SLSNe-I to z=3.5, 3.7, and 6.6,
respectively.
