Moore’s law and the silicon revolution - The Computing Universe: A Journey through a Revolution - page 138

The other two ingredients that we need from quantum
theory to gain an understanding of the Periodic Table of the ele-
ments are electron spin and the Pauli Exclusion Principle . Electrons
have “spin,” somewhat like a spinning top, but unlike a clas-
sical top, an electron can only exist in one of two spin states,
called “up” and “down.” Pauli's Exclusion Principle says that
only one electron is allowed in each quantum state. This means
that in our electron potential box ( Fig. 7.22 ) we can only put
two electrons in the lowest energy state, called the ground state :
one electron with spin up and the other with spin down. If we
want to add another electron to the box, we have to give it
more energy and place it in the next energy level - called the
first excited state .
It is the exclusion principle - insisting that electrons
have to occupy distinct quantum states - that gives the stabil-
ity and volume of ordinary matter. As Richard Feynman says:
“It is the fact that electrons cannot all get on top of each other
that makes tables and everything else solid.” 17 The exclu-
sion principle applies to all “matter-like” quantum objects such as electrons, protons, and neutrons. For
“radiation-like” objects such as photons, the exclusion principle does not apply, and we can put as many
photons as we like into the same quantum state. This has led to amazing applications such as lasers and
superconductivity.
Armed with these fundamental quantum concepts, we are now able to explain the difference between
metals, semiconductors, and insulators. In a later chapter we will see how these quantum ideas are being
used to build a new type of quantum computer.

n=3

n=2

n=1

Fig. 7.22. Electrons in a box. The electrons can only
fill up the energy levels according to Pauli's Exclusion
Principle, which states that only one electron is
allowed to occupy a quantum state. Each quantum
level can therefore accommodate two electrons - one
with spin up and one with spin down. The n = 1 level
can therefore only accommodate two electrons: the
next electron must go into the more energetic, first
“excited” state, n = 2.