Wait until the platform leading into the back of the room appears, then head up and to the right of button place your first portal. Now hit the button to release the Displacement Cube and look through the small gap and fire the second portal at the wall here.
Make your way through the first portal behind you and you'll reappear where the Cube landed. Take it with you back through the portal and over to the laser beam.
Now it's just a simple matter of lining up the Cube with the beam receiver and dropping it.
Step back through the portal or run though the displacement barrier towards the - now open - exit.