But how do we (out here in the much larger world) write a digital "0" or "1"?
We can do this by adding two wires that end near the left dots of a QCA cell. By charging one or the other wire, we can again exploit charge repulsion:
When the bottom wire is connected to a positive voltage, the positive charge on the lower left dot is driven to the right by “electrostatic” force (shown by the red glow). But then, with both charges side by side, the upper blue charge will run to the left. This completes the shift from a digital “1” to a digital “0.”