Physics of Electron Spins in Quantum Dots

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

****NON-TECHNICAL ABSTRACT**** Advances in semiconductor fabrication technology have made it possible to control how electrons behave when confined to very small, nanometer-size, regions. It has now become possible to control the number of electrons confined in a small region of semiconductor, called a quantum dot. This individual investigator award supports a project with an overall goal of creating a quantum dot with just one electron and controlling the magnetic property know as the "spin" or magnetic moment of that electron. The spin of an electron may be thought of as a small bar magnet attached to the electron. Like a bar magnet, the spin can point in different directions. These different directions represent different "quantum mechanical states." It has recently been proposed that the electron spin in a quantum dot can be used as the bit in a quantum computer; a quantum computer could solve problems a conventional classical computer cannot. Crucial to this application is that the spin remains in an undisturbed quantum mechanical state for a long enough time to carry out a calculation. The goal of the project is to better characterize and control the mechanisms that disturb the spin quantum state. It is known that the spin in a quantum dot made of the semiconductor Gallium Arsenide changes state too quickly for computation. Dots will be made in Silicon Germanium, in which the spin should remain in its state much longer. Experiments will then be done to measure how long the spin remains in a single quantum state. Students will be trained in state-of-the-art fabrication and characterization techniques. ****TECHNICAL ABSTRACT**** Various groups have shown that one can confine a single electron in a surface-gated lateral quantum dot, and one can use a nearby conducting channel to measure the charge on the dot and its time dependence. Because the tunneling rate in a magnetic field can be made to depend on the orientation of the electron spin, one can use these techniques to measure the spin state of an electron in a quantum dot. This individual investigator award supports a project to measure the relaxation rate of a single electron spin from its excited state to its ground state as a function of magnetic field strength and direction to test current theories concerning spin relaxation mechanisms. Experiments will be performed in both GaAs/AlGaAs and strained SiGe heterostructures, the latter are expected to have a lower dephasing rate. Possible applications to nanoelectronics include higher functionality of semiconductor devices, lower power consumption and, the possibility of entirely new functions such as associative memory and quantum computing. Research on semiconductor nanostructures has proven to be an outstanding training ground for young physicists.