Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A
  Measurement of Circular Polarization at 40 GHz
  We report circular polarization measurements from the first two years of
observation with the 40 GHz polarimeter of the Cosmology Large Angular Scale
Surveyor (CLASS). CLASS is conducting a multi-frequency survey covering 75% of
the sky from the Atacama Desert designed to measure the cosmic microwave
background (CMB) linear E and B polarization on angular scales $1^\circ
\lesssim \theta \leq 90^\circ$, corresponding to a multipole range of $2 \leq
\ell \lesssim 200$. The modulation technology enabling measurements of linear
polarization at the largest angular scales from the ground, the Variable-delay
Polarization Modulator, is uniquely designed to provide explicit sensitivity to
circular polarization (Stokes $V$). We present a first detection of circularly
polarized atmospheric emission at 40 GHz that is well described by a dipole
with an amplitude of $124\pm4\,\mathrm{\mu K}$ when observed at an elevation of
$45^\circ$, and discuss its potential impact as a foreground to CMB
experiments. Filtering the atmospheric component, CLASS places a 95% C.L. upper
limit of $0.4\,\mathrm{\mu K}^2$ to $13.5\,\mathrm{\mu K}^2$ on
$\ell(\ell+1)C_\ell^{VV}/(2\pi)$ between $1 \leq \ell \leq 120$, representing a
two-orders-of-magnitude improvement over previous limits.
