In a low-temperature superconductor electrons donít travel singly but
in weakly tethered pairs, Cooper pairs. In a new experiment at the Forschungszentrum
Karlsruhe in Germany, physicists have been able to send the two partners
from Cooper pairs down separate wires spaced more closely than the effective
size of the Cooper pairs themselves (see figure).
The Cooper pairs (which have the property that if one electronís spin
is up, then the spin of its partner must be down) start out in a piece
of superconducting aluminum and proceed to a frontier where they can
travel down either of two normally-conducting and magnetized iron wires.
(In general, when Cooper pairs move from a superconducting into a normally-conducting
material they can maintain their pair status for a bit into the new
material---a distance referred to as the normal-metal coherence length---before
By magnetizing the wires so as to filter out pairings of any electrons
that donít have the characteristic Cooper opposite-spin-orientation,
and by varying the distance between wires, and by measuring the resistance
across the iron wires, the experimenters can learn specific things about
the Cooper pairing mechanism (such as how large the pair is under various
This work is part of the larger study of spintronics---the exploitation
of electron spin for performing high-control electronics---and entangled
states---the quantum behavior in which two spatially separated objects
have a correlated behavior. (Beckmann
et al., Physical Review Letters, 5 November 2004; contact
Detlef Beckmann, email@example.com, 49-7247-82-6413).