****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.
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.
During the past three years we have studied a novel system, which might be the basis of a quantum computer. It has been shown that such a computer could solve problems that conventional computers cannot. For example, a classical computer cannot factor very large numbers into its prime factors. This fact is used to protect our bank accounts and many of the Nation’s defense secrets. However, a quantum computer could do this. No one has built such a quantum computer yet, but if it can be done, it is important for us to do it in the US first, so that new approaches to security can be found. In the computers we use now, classical computers, the information is stored in what we call bits. These can be thought of as switches, which are either on (a 1) or off (a zero) or up (1) and down (0). In a quantum computer, the bits are replace by quantum bits or qubits. The magic of quantum mechanics tells us that such qubits do not have to be either 0 or 1, but can be any superposition of 0 and 1. It is this extra flexibility that makes a quantum computer theoretically so powerful. It has proved very difficult to make qubits because the particular superposition gets disturbed by almost anything. Thermal vibrations can do this, for example. It has been proposed that a two-dimensional gas of electrons at a specific strong magnetic field might allow one to make qubits that are immune from such disturbance. When electrons are forced to move in only two dimensions inside semiconductors and are placed in a strong magnetic field, they form a kind of liquid, with special properties. The liquid acts as though it is made of new particles, instead of electrons, and these new particles may be used for qubits. Our experiments have measured some of the properties of these new particles. They have electrical charge that is ¼ that of the usual electron and they interact with each other with a strength that is different from that of usual electrons. The values of these properties are consistent with a theory that predicts that these particles will be good qubits. Graduate students and postdoctoral researchers working on this project learn semiconductor technology as well as learning the deep physics of quantum mechanics. Some have gone on to work at universities, but others have joined high-technology startup companies. This research provides excellent post-graduate education that strengthens the technological power of the Nation’s workforce.
