Principles of Superconducting Quantum Computers

7 Theory of Superconductivity

When we think of a “good” conductor, we normally think of a metal like copper that is commonly used in electrical circuits. The quality that makes it “good” is a relatively low resistance to the flow of electrical current. It is natural, then to think of a “perfect” conductor as a material that has zero electrical resistance. What then is a “superconductor”? Superconductors do indeed have zero resistance to DC currents, but they have a number of other remarkable properties that distinguish them from a material that simply has zero resistance. These properties appear below a certain critical temperature. Above the critical temperature the behavior is that of an ordinary conducting material. The unusual properties that appear below the critical temperature include:

• Perfect DC conductivity
• Tendency to expel magnetic fields
• A gap for energy absorption, somewhat like a semiconductor
• An effective wave function that maintains coherence over macroscopic dimensions
• Remarkable behavior of weakly-coupled junctions between two superconductors.

A detailed study of these effects and the theory of superconductivity is beyond the scope of our discussion, but we will introduce some fundamental notions about the theory of solids with the goal of giving some insight into the differences between normal conductors—metals in particular—and superconductors, and the origins and descriptions of the properties of superconductors that are most useful for understanding superconducting ...