SCIENCE

WEST LAFAYETTE, IN -- Scientists at Purdue University are helping researchers take a quantum leap in computer technology. They have linked two tiny structures – quantum dots – in such a way that is essential for the creation of semiconductor-based quantum computers, which could be faster and provide more memory than conventional technology. The findings will be detailed in the Friday (9/21) issue of the journal Science, in a research paper written by Albert M. Chang, a professor of physics at Purdue, doctoral physics student Heejun Jeong and Michael R. Melloch, a professor of electrical and computer engineering. Today's computers work by representing information as a series of ones and zeros, or binary digits called "bits." This code is relayed by transistors, which are minute switches that can either be on or off, representing a one or a zero, respectively. Quantum computers would take advantage of a strange phenomenon described by quantum theory: Objects, such as atoms or electrons, can be in two places at the same time, or they can exist in two states at the same time. That means computers based on quantum physics would have quantum bits, or "qubits," that exist in both the on and off states simultaneously, making it possible for them to process information much faster than conventional computers. A string of quantum bits would be able to calculate every possible on-off combination simultaneously, dramatically increasing the computer's power and memory. The switches in a quantum computer would be made of puddles of about 20 electrons called quantum dots, which are formed inside of computer circuitry. Each quantum dot, like each transistor in a conventional computer, is like a switch that defines a single qubit. The quantum dots themselves are only about 180 nanometers in diameter – about 5,000 of them could stretch across the width of a grain of sand. For quantum computations to work, information will have to be exchanged between pairs of qubits. Because electrons are said to have a "spin" of either up or down, the direction of spin can be used instead of the on or off positions of a conventional computer circuit. "Each dot can have a one or a zero, because the spin can be up or down," Chang said. The Purdue researchers have been able to link two quantum dots, control how many electrons are in each dot and then detect the spin state in each dot. They are the first scientists to be able to detect the individual spins of each of the two quantum dots linked together, information essential for quantum computing. Unlike conventional computer circuitry in which electrical current is used to carry information and perform computations, quantum-dot based quantum computers would rely on the manipulation of the electron spin. "Without being able to isolate each spin, you cannot do quantum computation," Chang said. In nature, the electrons in an atom occupy a series of levels that increase in energy with distance from the atom's nucleus. In a similar way, the Purdue researchers are able to control how many electrons occupy a quantum dot's outermost level. When quantum dots contain only one electron in their outermost level, their spins can be detected by analyzing the flow of electricity through the dots, Chang said. The researchers were able to achieve the milestone by creating extremely fine circuits using a standard process known as electron beam lithography. A semiconducting material known as gallium arsenide was coated with a plastic. Then extremely fine lines were cut into the plastic coating using a beam of electrons. The lines were filled with a metal and the plastic dissolved, leaving behind metal lines that are like wires only about 50 nanometers wide. A nanometer – or billionth of a meter – is roughly five to 10 atoms wide. "As far as we know, no other groups have been able to do such fine lithography," Chang said. Other researchers have already built quantum computing devices based on man-made molecules containing fluorine atoms. However, many experts believe it will be difficult to "scale up" such devices to make large, workable computers. "Whereas, the advantage of semiconductors is that, once you've proven the feasibility, you've got the whole semiconductor industry's expertise behind you," Chang noted. Continuing research will aim to not only detect the spins on each quantum dot, but to precisely control the spins, which is necessary for future computer applications. "Now we have proven that you can link quantum dots together, but the next thing will be to make them do things, to control the spins in a double-quantum dot," Chang said. By Emil Venere