When fusion reaction is possible anywhere
Currently you need a fission reaction to start a fusion reaction as in H Bomb. Using the same ignition method we can start fusion process reaction anywhere. When fusion is possible, we may be able to create elements out of thin air, whichever element you desire. From subatomic we'll be able to create Hydrogen via fusion of subatomic into atomic. From Hydrogen to helium, heavier elements will be possible incrementally, sequential fusion processes. This will be possible in a device that can control radioactivity of the materials involved.