  Gravitational lensing due to the large-scale distribution of matter in the
cosmos distorts the primordial Cosmic Microwave Background (CMB) and thereby
induces new, small-scale $B$-mode polarization. This signal carries detailed
information about the distribution of all the gravitating matter between the
observer and CMB last scattering surface. We report the first direct evidence
for polarization lensing based on purely CMB information, from using the
four-point correlations of even- and odd-parity $E$- and $B$-mode polarization
mapped over $\sim30$ square degrees of the sky measured by the POLARBEAR
experiment. These data were analyzed using a blind analysis framework and
checked for spurious systematic contamination using null tests and simulations.
Evidence for the signal of polarization lensing and lensing $B$-modes is found
at 4.2$\sigma$ (stat.+sys.) significance. The amplitude of matter fluctuations
is measured with a precision of $27\%$, and is found to be consistent with the
Lambda Cold Dark Matter ($\Lambda$CDM) cosmological model. This measurement
demonstrates a new technique, capable of mapping all gravitating matter in the
Universe, sensitive to the sum of neutrino masses, and essential for cleaning
the lensing $B$-mode signal in searches for primordial gravitational waves.
