I saw a lot of good answers here, but wanted to point out that there are some very common devices in personal computer architecture that use multiple voltage levels to represent more data - MLC solid state devices, one example of which is the Triple Level Cell SSD from Samsung. These use 8 voltage levels to represent 8 bits of information per cell (as far as I remember). The details have gotten a little hazy, but this^] article explains it pretty well. In other words, 0 is only represented by the accepted range of the voltage present, as any other value would be.