Place a portal directly under it and once you’re standing on the platform, fire off the exit portal at the vertical wall behind it to re-engage the beam, taking the platform upwards to the next floor.
Walk back through the portals and you’ll reappear with the Cube on the upper level of the chamber. Drop the Cube down by your side and then look back down towards the beam below you.
You’ll gain enough momentum to come out at the top right towards the exit!
Missable Achievement/TROPHY ALERT!
You need to take the Companion Cube with you to the elevator for the 'Preservation of Mass' Achievement/Trophy. To do this, once you reach the end of the chamber, shoot a portal at the wall just past the exit door, drop down and grab the Cube. Now fall into a portal on the ground below and you can carry it to the elevator from here!