Cooper pairs unpaired

In a low-temperature superconductor, electrons don’t travel singly but in weakly tethered pairs called Cooper pairs. When an electron is sent from a normal metal into such a superconductor, a process called Andreev reflection occurs: A hole of opposite spin is retroreflected at the interface back into the metal and a Cooper pair appears in the superconductor. In a new experiment at the Karlsruhe Research Center in Germany, physicists attached tiny iron wires to a superconducting aluminum bar. The wires were magnetized to pick out pairs with the characteristic opposite-spin orientation of Cooper pairs. If the wires were spaced more closely than the coherence length of the Cooper pairs themselves, the physicists found that the Andreev reflection was “crossed”—the electron went in one wire but the hole was reflected into another. Looked at differently, the Cooper pair split up into the two wires. Although the partners are in different wires, they are quantum-entangled, which might prove useful in quantum information processing. (D. Beckmann et al., Phys. Rev. Lett. 93, 197003, 2004 http://dx.doi.org/10.1103/PhysRevLett.93.197003 .