Now fall through the portal so you join the Cube up here and pick the Cube up again. Position it so it's directing the laser through the first power cone and through the next one directly across from you.
Drop down to the floor, and look up near where you first entered the test chamber. There's a wall panel up on the roof (to the right of the wall camera). Place a portal here and then one by your feet. You'll land on the next Displacement Cube, so pick it up and drop back down to the ground floor.
This'll cause the Cube to go flying over to the power cone you directed the beam over to a bit earlier. Leave the first portal (sitting over the jump plates) where it is and then drop a portal by your feet. You'll fall down onto the jump plates, and as you're ascending, re-fire the portal that lead to the jump plates on the wall where the Cube came out of.
As you descend again, you'll pass through the wall high up and land beside the Cube and the power cone.
Drop back down to the ground and look up to the wall to your right, now aim your portal gun at the top-left corner up here (opposite where the third and final Displacement Cube is resting). Place a portal by your feet and above the jump plates so you're once again gaining some height fire off the exit portal into the top-left corner.
Pick up the Cube and use the beam to destroy the three turrets that are blocking the exit. Finally set it down once the laser beam is lined up with the receptor panel on the left wall. Set a portal back above the jump plates and one on the ground by your feet.