  The IceCube Neutrino Observatory detects GeV-to-PeV+ neutrinos via the
Cherenkov light produced by secondary charged particles from neutrino
interactions with the South Pole ice. The detector consists of over 5000
spherical Digital Optical Modules (DOM), each deployed with a single
downward-facing photomultiplier tube (PMT) and arrayed across 86 strings over a
cubic-kilometer. IceCube has measured the astrophysical neutrino flux, searched
for their origins, and constrained neutrino oscillation parameters and cross
sections. These were made possible by an in-depth characterization of the
glacial ice, which has been refined over time, and novel approaches in
reconstructions that utilize fast approximations of Cherenkov yield
expectations.
  After over a decade of nearly continuous IceCube operation, the next
generation of neutrino telescopes at the South Pole are taking shape. The
IceCube Upgrade will add seven additional strings in a dense infill
configuration. Multi-PMT OMs will be attached to each string, along with
improved calibration devices and new sensor prototypes. Its denser OM and
string spacing will extend sensitivity to lower neutrino energies and further
constrain neutrino oscillation parameters. The calibration goals of the Upgrade
will help guide the design and construction of IceCube Gen2, which will
increase the effective volume by nearly an order of magnitude.
