Computing with Atoms

Now we're going to talk about small, small computing. I'm taking the point of view of 1983 rather than of 1960, and will talk about what is going to happen, or which way we should go.

Let's ask, what do we need to do to have a computer? We need numbers, and we need to manipulate the numbers and calculate an answer. So we have to be able to write the numbers.

How small can a number be? If you have N digits, you know the special way of writing them with base two numbers, that is, with ones and zeros. Now we're going to go way down to the bottomatoms! Remember that we have to obey quantum-mechanical laws, if we are talking about atoms. And each of these atoms is going to be in one of two statesactually, atoms can be in a lot of states, but let's take a simple counting scheme that has either ones or zeros. Let's say that an atom can be in a state of spin up or of spin down, or say that an ammonia molecule is either in the lowest or the next lowest state, or suppose various other kinds of two-state systems. When an atom is in the excited statea spin uplet's call it a "one"; a "zero" will correspond to spin down. Hereafter when I say a one, I mean an atom in an excited state. So to write a number takes no more atoms than there are digits, and that's really nothing!