Receiver development for BICEP Array, a next-generation CMB polarimeter
  at the South Pole
  A detection of curl-type ($B$-mode) polarization of the primary CMB would be
direct evidence for the inflationary paradigm of the origin of the Universe.
The BICEP/Keck Array (BK) program targets the degree angular scales, where the
power from primordial $B$-mode polarization is expected to peak, with
ever-increasing sensitivity and has published the most stringent constraints on
inflation to date. BICEP Array (BA) is the Stage-3 instrument of the BK program
and will comprise four BICEP3-class receivers observing at 30/40, 95, 150 and
220/270 GHz with a combined 32,000+ detectors; such wide frequency coverage is
necessary for control of the Galactic foregrounds, which also produce
degree-scale $B$-mode signal. The 30/40 GHz receiver is designed to constrain
the synchrotron foreground and has begun observing at the South Pole in early
2020. By the end of a 3-year observing campaign, the full BICEP Array
instrument is projected to reach $\sigma_r$ between 0.002 and 0.004, depending
on foreground complexity and degree of removal of $B$-modes due to
gravitational lensing (delensing). This paper presents an overview of the
design, measured on-sky performance and calibration of the first BA receiver.
We also give a preview of the added complexity in the time-domain multiplexed
readout of the 7,776-detector 150 GHz receiver.
