  The Atacama Large Millimeter-submillimeter Array (ALMA) is a general purpose
telescope that performs a broad program of astrophysical observations.
Beginning in late-2016, solar observations with ALMA became available, thereby
opening a new window onto solar physics. Since then, the number of solar
observing capabilities has increased substantially but polarimetric
observations, a community priority, have not been available. Weakly circularly
polarized emission is expected from the chromosphere where magnetic fields are
strong. Hence, maps of Stokes V provide critical new constraints on the
longitudinal component of the chromospheric magnetic field. Between 2019-2022,
an ALMA solar development effort dedicated to making solar polarimetry at
millimeter wavelengths a reality was carried out. Here, we discuss the
development effort to enable solar polarimetry in the 3 mm band (ALMA Band 3)
in detail and present a number of results that emerge from the development
program. These include tests that validate polarization calibration, including
evaluation of instrumental polarization: both antenna based "leakage" terms and
off-axis effects (termed "beam squint" for Stokes V). We also present test
polarimetric observations of a magnetized source on the Sun, the following
sunspot in a solar active region, which shows a significant Stokes V signature
in line with expectations. Finally, we provide some cautions and guidance to
users contemplating the use of polarization observations with ALMA.
