An Electrical Pump for Nuclear Spins

Dynamic Nuclear Polarization by Electrical Spin Injection in Ferromagnet-Semiconductor Heterostructures. The processing functions of a computer are carried out at the lowest level by the flow of electrons in semiconductors. Although the ability of electrons to transmit information in the form of a current or voltage is well known, they also possess a property known as spin, which is analogous to the rotation of a child's top. The direction of rotation, which can be either "clockwise" or "counterclockwise," can be used to store information, and entirely new types of computing based on the quantum mechanical properties of spin have been proposed.

One drawback of using electron spins for computing is that they lose track of their orientation in only a billionth of a second or so, which is far too short for carrying out computations. There is, however, a way to get around this problem: the spin of an electron can be transferred to an atomic nucleus, where it can last for many minutes. This process is usually carried out with the aid of large magnets, lasers, or microwaves. Work by MRSEC IRG3 co-workers demonstrates that it requires only a battery and an electrode made of iron, which is deposited on the surface of a semiconductor. The electrons flow from the iron, which acts as a source of spin, into the semiconductor, where they transfer their spin to many nuclei. In effect, the device acts like a pump for nuclear spins, and the rate at which nuclear spins are created is controlled by the magnitude of the electrical current. An additional feature of the experiment is that the nuclear spins are also detected using electrons, and so it thus incorporates a "read-out" as well as a "write" mechanism. These would be two essential components of a quantum computer that uses nuclear spins to do calculations. . [J. Strand, B. D. Schultz, A. F. Isakovic, C. J. Palmstrom, and P. A. Crowell, Phys. Rev. Lett. 91, 036602 (2003).]

Fig. 1: Principle of the nuclear spin pump. The circular arrows indicate the direction of the spin. An electron travels from iron, in which it acquires a spin, into the semiconductor (GaAs). The spin then gets transferred to nuclei. The total nuclear spin increases as the electrical current increases. The nuclear spin can last for several minutes, even after the current is turned off. The device can then be turned back on to "read" the value of the stored spin.