How to make this jump across in Testchamber 16 in Chapter 8

I actually came up with a different solution myself. I stood by the portal and noticed that despite looking like the bridge is keeping you from using it, you still have half a usable portal. So I walked through and picked up the turret and tossed it back through the portal.

Here's what I did for the chamber it sounds like you're describing:

1. Obtain a cube
   • Place a hard light wall over the turret pit so when you call for a cube it's not immediately emancipated
   • Hit the button and go catch it (or if you're really lucky, it will bounce back enough to stay up)
2. Go up near the exit with the cube
   • Use the light wall to prevent overshooting the catwalk
3. Once there, place a portal to block off the turrets
   • this is before walking around to the ledge above the button
4. Go put the cube on the button
5. Portal out, using the one you blocked the turrets with (orange in above picture would go on the surface to the left on this side of the emancipation field)
6. Go back up to the exit

If you get it down (or don't mind getting more cubes), you can just throw the cube off the ledge onto the button and skip 3-6 entirely.