  Strong gravitational lensing provides some of the deepest views of the
Universe, enabling studies of high-redshift galaxies only possible with
next-generation facilities without the lensing phenomenon. To date, 21 cm radio
emission from neutral hydrogen has only been detected directly out to z~0.2,
limited by the sensitivity and instantaneous bandwidth of current radio
telescopes. We discuss how current and future radio interferometers such as the
Square Kilometre Array (SKA) will detect lensed HI emission in individual
galaxies at high redshift. Our calculations rely on a semi-analytic galaxy
simulation with realistic HI disks (by size, density profile and rotation), in
a cosmological context, combined with general relativistic ray tracing.
Wide-field, blind HI surveys with the SKA are predicted to be efficient at
discovering lensed HI systems, increasingly so at z > 2. This will be enabled
by the combination of the magnification boosts, the steepness of the HI
luminosity function at the high-mass end, and the fact that the HI spectral
line is relatively isolated in frequency. These surveys will simultaneously
provide a new technique for foreground lens selection and yield the highest
redshift HI emission detections. More near term (and existing) cm-wave
facilities will push the high redshift HI envelope through targeted surveys of
known lenses.
