D-WAVE SQUID. The working fluid of superconductors consists of pairs of electrons (or pairs of the holes left behind in a crystal when an electron moves somewhere else). These Cooper pairs form a coherent state with specific symmetry properties.

For example, in most low temperature superconductors, the pairs are fairly isotropic; if you imagine one electron at the origin of some coordinate system, the likelihood of finding a second electron is pretty much the same in all directions. Thus the Cooper pair is essentially spherical and the pair is said to possess "s-wave" symmetry.

In high-temperature superconductors, the symmetry is thought to resemble a four-leaf clover, referred to as a "d-wave." A fundamental consequence of the d-wave symmetry is a phase-change of pi between neighboring lobes of the clover in the quantum wave function describing the Cooper pair. All of this can be important in the design of superconducting quantum interference devices, or SQUIDs, which consist of a superconducting loop interrupted in two places by thin insulating junctions, through which the Cooper pairs must tunnel.

SQUIDs are highly sensitive to applied magnetic fields and are used in a variety of magnetometer applications (in biology, geology, new materials research, etc.). Furthermore, SQUIDs form the building blocks of superconducting electronics. A group at Augsburg University in Germany (Robert Schulz, 011-49-821-598-3650, robert.schulz@physik.uni-augsburg.de) has developed a SQUID that exploits the special nature of the d-wave symmetry of the high-Tc superconductors. Using specially prepared tetracrystalline crystals as substrates, they devised and built a SQUID in which the symmetry properties give rise to a pi phase-change over one of the two junctions (see the figure at Physics News Graphics). For this reason, the Augsburg researchers call their device a pi-SQUID.

The pi-SQUID is a realization of the recently proposed complementary Josephson electronics and its operation provides strong evidence for the d-wave symmetry in the high-Tc superconductors. Such devices present a novel approach for the fabrication of quantum computers. (Schulz et al., Applied Physics Letters, 14 Feb; Select Article.)