Spintronics is a relatively new field in which the electron's spin,
not just its charge, can be exploited in devices and circuits. The ultimate
spintronics degree of control would come from controlling a circuit
at the level of a single spin.
Physicists at the Institute for Microstructural Sciences (Ottawa) are
the first to create a prototype of a single-spin transistor, which consists
of a quantum dot connected to spin-polarized leads.
A quantum dot is an artificial atom in which electrons are confined
spatially by an electrostatic potential much in the way that a nucleus
localizes electrons in an atom. The dot can be emptied and then electrons
added one at a time to create a "hydrogen" dot (with one electron
onboard), "helium" dot (two electrons), "iron" dot,
and so forth.
The spins of the electrons in the transistor are not random but depend
on the number of electrons in the electron puddle, and on the applied
magnetic field. Most importantly, by connecting the dot to spin-polarized
reservoirs, one can insist that the electrons flowing in or out have
their spins aligned up or down, and this criterion (is the electron's
spin up or down?) can be used as a gate to allow a high or low current
to flow through the dot. Hence the spin state of the dot is encoded
in the difficulty of adding the extra electron.
In this way the group was able to "read" the spin properties
of the dot. They could also in a sense "write" (i.e.,
change the spin state of the dot controllably) by either adding an electron
or by tuning the magnetic field.
Such a unique combination of control at the single charge and single
spin may play a role in the future solid state form of quantum computing
where the unit of quantum manipulation, the qubit might consist of specially
prepared spin states. (Ciorga
et al., Physical Review Letters, 24 June 2002; contact
Pawel Hawrylak, email@example.com or Andrew Sachrajda, firstname.lastname@example.org.)