Superconductivity is the astonishing physical phenomenon of a material conducting electrical current without any resistance, i.e. without any ohmic losses. Ever since its discovery by Kamerlingh Onnes more than 100 years ago, superconductivity has fascinated scientists, who have attempted to develop suitable explanations. The theory of Bardeen, Cooper and Schrieffer provides a basic description of conventional superconductivity as the pairing of electrons mediated by interaction with phonons, i.e. the vibrations of the lattice.