Delensing Cosmic Microwave Background B-modes with the Square Kilometre
  Array Radio Continuum Survey
  We explore the potential use of the Radio Continuum (RC) survey conducted by
the Square Kilometre Array (SKA) to remove (delens) the lensing-induced B-mode
polarization and thus enhance future cosmic microwave background (CMB) searches
for inflationary gravitational waves. Measurements of large-scale B-modes of
the CMB are considered to be the best method for probing gravitational waves
from the cosmic inflation. Future CMB experiments will, however, suffer from
contamination by non-primordial B-modes, one source of which is the lensing
B-modes. Delensing, therefore, will be required for further improvement of the
detection sensitivity for gravitational waves. Analyzing the use of the
two-dimensional map of galaxy distribution provided by the SKA RC survey as a
lensing mass tracer, we find that joint delensing using near future CMB
experiments and the SKA phase 1 will improve the constraints on the
tensor-to-scalar ratio by more than a factor of $\sim 2$ compared to those
without the delensing analysis. Compared to the use of CMB data alone, the
inclusion of the SKA phase 1 data will increase the significance of the
constraints on the tensor-to-scalar ratio by a factor $1.2$-$1.6$. For LiteBIRD
combined with a ground-based experiment such as Simons Array and Advanced ACT,
the constraint on the tensor-to-scalar ratio when adding SKA phase 2 data is
improved by a factor of $2.3$-$2.7$, whereas delensing with CMB data alone
improves the constraints by only a factor $1.3$-$1.7$. We conclude that the use
of SKA data is a promising method for delensing upcoming CMB experiments such
as LiteBIRD.
