In a classical computer a bit is typically stored in a silicone chip,
a metal hard drive platter, or on a magnetic tape. About 10^{5}
atoms were used in the year 2000 to represent one bit of
information. The smallest conceivable storage for a bit involves a
single elementary particle of some sort. For example a spin-1/2 of
particle such as a proton, neutron, or electron, which can be
characterized by its spin value. The spin is measured to be either
+1/2 or -1/2. We can encode 1 to be +1/2 and 0 to be -1/2, and
if we assume we can measure and manipulate the spin of such a particle
then we could theoretically use this particle to store one bit of
information. If we were to try to use this spin-1/2 particle as a
classical bit, one that is always in the 0 or 1 state, we would fail.
We would be trying to apply classical physics on a scale where it
simply is not applicable. This single spin-1/2 particle will instead
act in a quantum manner. (Williams, Clearwater)