The next example of second quantization techniques we consider concerns superconductivity. Superconductivity was discovered in 1911 in Leiden by Kamerlingh Onnes, in the course of his quest to achieve ultra-low temperatures. He was applying freshly invented refrigeration techniques to study the low-temperature behavior of materials using every method available at that time. The most striking discovery he made, was that the electrical resistance of mercury samples abruptly disappeared below a critical temperature of 4.2 K – the metal underwent a transition into a novel phase.

After this first discovery, superconductivity has since been observed in a large number of metals and metallic compounds. Despite long-lasting efforts to find a material that is superconducting at room temperature, today superconductivity still remains a low-temperature phenomenon, the critical temperature varying from compound to compound. A huge and expensive quest for so-called high-temperature superconductivity took place rather recently, in the 1980s and 1990s. Although the record temperature has been increased by a factor of 5 during this time, it still remains as low as 138 K.

The most important manifestation of superconductivity is of course the absence of electrical resistance. In a superconductor, electric current can flow without any dissipation. Such a supercurrent, once excited in a superconducting ring, would in principle persist forever, at least as long as the superconductivity of the ring is preserved.