Josephson effect as a measure of quantum fluctuations in the cuprates

Quantitative analysis of the Josephson effect is shown to provide direct information about phase fluctuations in the superconducting banks. Applying the analysis to the cuprates, substantial quantum fluctuations between d-wave and s-wave pairing are found at low temperatures. A phenomenological model of such fluctuations is introduced and solved in a self-consistent single-site approximation. The cuprate parameters are argued to lie close to an exotic superconductor phase with condensation of electron quadruplets.