Understanding the mechanism responsible for superconductivity in the high-temperature cuprates has been one of the major goals of condensed-matter physicists since the discovery of these exciting materials in 1986. Experimental evidence suggests that the pairing state may be unconventional, featuring an anisotropic order parameter for which a wide range of theoretical models for the superconducting pairing have been proposed. Recently, a new class of experiments has been presented that are sensitive to the phase of the superconducting order parameter, allowing an unambiguous determination of the symmetry of the pairing state. These experiments, based on the interference of the quantum-mechanical phases in Josephson tunnel junctions and dc SQUID devices, give strong evidence for pairing in a channel with $d$--wave symmetry in the most widely studied cuprate, $Y{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$. Confirmation of this pairing state will focus efforts to develop a microscopic theory for the high-temperature superconductors and to apply them in power transmission and electronic device technologies.

DOI:https://doi.org/10.1103/RevModPhys.67.515