  Advanced ACTPol is a polarization-sensitive upgrade for the 6 m aperture
Atacama Cosmology Telescope (ACT), adding new frequencies and increasing
sensitivity over the previous ACTPol receiver. In 2016, Advanced ACTPol will
begin to map approximately half the sky in five frequency bands (28-230 GHz).
Its maps of primary and secondary cosmic microwave background (CMB)
anisotropies -- imaged in intensity and polarization at few arcminute-scale
resolution -- will enable precision cosmological constraints and also a wide
array of cross-correlation science that probes the expansion history of the
universe and the growth of structure via gravitational collapse. To accomplish
these scientific goals, the Advanced ACTPol receiver will be a significant
upgrade to the ACTPol receiver, including four new multichroic arrays of
cryogenic, feedhorn-coupled AlMn transition edge sensor (TES) polarimeters
(fabricated on 150 mm diameter wafers); a system of continuously rotating
meta-material silicon half-wave plates; and a new multiplexing readout
architecture which uses superconducting quantum interference devices (SQUIDs)
and time division to achieve a 64-row multiplexing factor. Here we present the
status and scientific goals of the Advanced ACTPol instrument, emphasizing the
design and implementation of the Advanced ACTPol cryogenic detector arrays.
