All the early quantum computing work was done with light. Light is very easy to manipulate: a few mirrors, crystals, and light detectors and you to can have your very own quantum computer. Over the last two decades, though, that’s changed. Almost all the major developments have used things like ions, rings of superconducting current, or defects in crystals.

This was, in some sense, a reasonable progression. To perform logical operations, you have to modify one quantum state based on the state of another. Light beams, however, tend to pass right through each other without even waving, let alone stopping to chat. Contrast that with ions. Two ions, being charged, cannot avoid talking to each other. That means the quantum state of one ion can strongly influence the state of the other. This makes logical operations much easier.