  Inflation is the leading theory of the first instant of the universe.
Inflation, which postulates that the universe underwent a period of rapid
expansion an instant after its birth, provides convincing explanation for
cosmological observations. Recent advancements in detector technology have
opened opportunities to explore primordial gravitational waves generated by the
inflation through B-mode (divergent-free) polarization pattern embedded in the
Cosmic Microwave Background anisotropies. If detected, these signals would
provide strong evidence for inflation, point to the correct model for
inflation, and open a window to physics at ultra-high energies.
  LiteBIRD is a satellite mission with a goal of detecting
degree-and-larger-angular-scale B-mode polarization. LiteBIRD will observe at
the second Lagrange point with a 400 mm diameter telescope and 2,622 detectors.
It will survey the entire sky with 15 frequency bands from 40 to 400 GHz to
measure and subtract foregrounds.
  The U.S. LiteBIRD team is proposing to deliver sub-Kelvin instruments that
include detectors and readout electronics. A lenslet-coupled sinuous antenna
array will cover low-frequency bands (40 GHz to 235 GHz) with four frequency
arrangements of trichroic pixels. An orthomode-transducer-coupled corrugated
horn array will cover high-frequency bands (280 GHz to 402 GHz) with three
types of single frequency detectors. The detectors will be made with Transition
Edge Sensor (TES) bolometers cooled to a 100 milli-Kelvin base temperature by
an adiabatic demagnetization refrigerator.The TES bolometers will be read out
using digital frequency multiplexing with Superconducting QUantum Interference
Device (SQUID) amplifiers. Up to 78 bolometers will be multiplexed with a
single SQUID amplidier.
  We report on the sub-Kelvin instrument design and ongoing developments for
the LiteBIRD mission.
