Evidence for the Cross-correlation between Cosmic Microwave Background
  Polarization Lensing from POLARBEAR and Cosmic Shear from Subaru Hyper
  Suprime-Cam
  We present the first measurement of cross-correlation between the lensing
potential, reconstructed from cosmic microwave background (CMB) {\it
polarization} data, and the cosmic shear field from galaxy shapes. This
measurement is made using data from the POLARBEAR CMB experiment and the Subaru
Hyper Suprime-Cam (HSC) survey. By analyzing an 11~deg$^2$ overlapping region,
we reject the null hypothesis at 3.5$\sigma$\ and constrain the amplitude of
the {\bf cross-spectrum} to $\widehat{A}_{\rm lens}=1.70\pm 0.48$, where
$\widehat{A}_{\rm lens}$ is the amplitude normalized with respect to the
Planck~2018{} prediction, based on the flat $\Lambda$ cold dark matter
cosmology. The first measurement of this {\bf cross-spectrum} without relying
on CMB temperature measurements is possible due to the deep POLARBEAR map with
a noise level of ${\sim}$6\,$\mu$K-arcmin, as well as the deep HSC data with a
high galaxy number density of $n_g=23\,{\rm arcmin^{-2}}$. We present a
detailed study of the systematics budget to show that residual systematics in
our results are negligibly small, which demonstrates the future potential of
this cross-correlation technique.
