A single-spin transistor

Spintronics is a relatively new field in which an electron’s spin, not just its charge, is exploited in devices and circuits. Physicists at the Institute for Microstructural Science in Ottawa, Canada, have connected a quantum dot to spin-polarized leads in an external magnetic field. They emptied the dot of conduction electrons and then added them back one at a time. The researchers found that the total spin of the electrons depended both on the number of electrons in the dot and on the applied magnetic field. With fewer than about 20 electrons in the dot, an even number of spins paired off in singlet states with zero net spin, whereas an odd number had a net spin corresponding to the unpaired electron. Above the critical number, however, the additional electrons all had the same spin polarization. Furthermore, the single-spin, singlet, and polarized phases of the dot each allowed different currents to flow through the dot. The physicists controlled the spin state of the dot either by adding electrons or by tuning the magnetic field, and thus produced a prototype single-spin transistor. The group believes their work may play a role in future solid-state forms of quantum information. (M. Ciorga et al., Phys. Rev. Lett. 88, 256804, 2002 http://dx.doi.org/10.1103/PhysRevLett.88.256804 .)