Now dash behind the turret, pick it up from behind and drop it down into the chasm behind you. Turn back around and fire your first portal off into the chequered wall panel where the transport beam connects with the wall. The second portal should be positioned on the wall panel that’s facing directly opposite the stationary Experimental Cube (look for the Cube to your right - over the pit).
Once the Cube reaches the portal, step off of the switch and fire a portal into the far left-hand corner of the wall directly opposite you, this should catch the Cube and carry it across the bottomless pit (to your immediate left).
Once the Cube approaches you, pick it up out of the beam and set it down on the switch, reversing the direction of the beam once again. Now step into the beam and it’ll carry you across the gap, and then jump out of the beam and onto the floor.
Now turn around to face the exit and place a portal on the floor so the beam reaches up to the ceiling. Now simply step into the beam and walk into the exit, leading to the elevator and the next Chamber.