In this recent TED Talk, Phil Plait outlines steps to prevent giant asteroids from demolishing the planet. This solution doesn’t involve photon cannons, but does use ion thrusters.
He mentions the B612 Foundation, which is an organization of other bona-fide scientists dedicated to this end.
Observations from NASA’s Wide-field Infrared Survey Explorer (WISE) reveal that there are fewer near-Earth midsize asteroids than originally thought (link). It seems the photon cannons are working.
WISE is also searching for the origins of the dinosaur-killing asteroid. NASA’s caption for this artist’s rendition reads:
Scientists think that a giant asteroid, which broke up long ago in the main asteroid belt between Mars and Jupiter, eventually made its way to Earth and led to the extinction of the dinosaurs. Data from NASA’s WISE mission likely rules out the leading suspect, a member of a family of asteroids called Baptistina, so the search for the origins of the dinosaur-killing asteroid goes on.
But check out these actual pictures taken by WISE. Stunning. A wide-field infrared imager is the way to photograph space.