Nanospintronics: A Single-Spin Transistor

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, pawel.hawrylak@nrc.ca or Andrew Sachrajda, andy.sachrajda@nrc.ca.)