  One of the most promising ways of detecting primordial gravitational waves
generated during inflation is to observe B-modes of polarization, generated by
Thomson scattering after reionization, in the cosmic microwave background
(CMB). Large scale foregrounds though are expected to be a major systematic
issue, so -- in the event of a tentative detection -- an independent
confirmation of large scale gravitational waves would be most welcome. Previous
authors have suggested searching for the analogous mode of cosmic shear in weak
lensing surveys but have shown that the signal to noise of this mode is
marginal at best. This argument is reconsidered here, accounting for the
cross-correlations of the polarization and lensing B-modes. A lensing survey
can potentially strengthen the argument for a detection of primordial
gravitational waves, although it is unlikely to help constrain the amplitude of
the signal.
