6.4 Quantum Chromodynamics
Strictly speaking, one could study quantum computing without much knowledge of quantum chromo-dynamics (QCD). However, this underpins the very structure of matter; therefore, one should have a basic idea of the topic. QCD is the study of the strong interaction between quarks and gluons. Quarks are the particles that make up protons and neutrons (also called hadrons). At one time, it was believed that protons and neutrons were fundamental particles; however, it was discovered that they are in turn made up of quarks. The names for the quarks are frankly whimsical, and not too much attention should be paid to the meanings of the names. Quarks have properties such as electric charge, mass, spin, etc. Combining three quarks can product a proton or neutron. There are six types of quarks. The whimsical nature of nomenclature will become clear here. The types are referred to as “flavors,” and these flavors are up, down, strange, charm, bottom, and top. Figure 6.1 illustrates the families of quarks.
FIGURE 6-1 Quarks
Evidence for the existence of quarks was first found in 1968 at the Stanford Linear Accelerator Center. Since that time, experiments have confirmed all six flavors of quarks. Therefore, these are not simply hypothetical constructs, but the actual building blocks of hadrons, and have been confirmed by multiple experiments over several decades. As one example, a proton is composed of two up quarks and one down quark. The gluons mediate the forces between the quarks, thus binding them together.
The next somewhat whimsical nomenclature comes with the concept of color charge. This has no relation at all to the frequency of light generating visible colors. The term color, along with the specific labels of red, green, and blue, is being used to identify the charge of a quark. However, this term has had far-reaching impact. That is why the study of the interaction between quarks and gluons is referred to as chromodynamics.
There are two main properties in QCD. The first is color confinement. This is a result of the force between two color charges as they are separated. Separating the quarks in a hadron will require more energy the further you separate them. If you do indeed have enough energy to completely separate the quarks, they actually spontaneously produce a quark-antiquark pair, and the original hadron becomes two hadrons.
The second property is a bit more complex. It is called asymptotic freedom. In simple terms, it means that the strength of the interactions between quarks and gluons reduces as the distance decreases. That might seem a bit counterintuitive. And as I stated, it is complex. The discoverers of this aspect of QCD—David Gross, Frank Wilczek, and David Politzer—received the 2004 Nobel Prize in Physics for their work.