CPU usage /can/ drop to 0% though it's very uncommon for most people. In order for there to be 0% CPU usage, background tasks like your antivirus must be turned off.
Percent CPU usage is determined by the proportion of maximum CPU potential to current usage. The more powerful the CPU you have, the more likely you are to have 0% usage at idle. For example, I have an i7 980X, when I idle I usually get 0% CPU usage. On the other hand, doing common tasks I barely ever push 10%, with the exception of a/v encoding, CAD, etc.
If you're asking about having absolutely no activity- in other words no electrons flowing through the transistors- the computer has to be turned off.