Quantum computers promise to enable immense computing power in systems based on units of information called qubits, which are similar to the binary bits in today's classical computers but which also can simultaneously represent both a 0 and a 1. Accordingly, investigators are looking for the tools to fabricate logic gates in systems governed by quantum mechanics. A research team at the University of Michigan in Ann Arbor, the Naval Research Laboratory in Washington, Michigan State University in East Lansing and the University of California, San Diego, has demonstrated a logic gate in an optically excited semiconductor quantum dot that approximates the behavior of a standard controlled-NOT gate."Quantum dots are usually defined as nanostructures of semiconductors where the dimensions are small, relative to the characteristic quantum scale of the electronic wave function," explained Duncan G. Steel of the University of Michigan. In their experiment, he said, the researchers employed GaAs structures in which the appropriate scale length is the exciton Bohr diameter on the order of 20 nm. Such structures are potentially important for quantum computing because they offer a localized spatial domain that can be used to contain a quantum bit.The researchers used one laser pulse to drive a quantum dot from the ground state to an exciton state. They used a second laser pulse to further drive the quantum dot into an entangled state involving the biexciton state, and they used a third pulse to read out the state of the system. The fidelity of the controlled-NOT gate was 0.7.Although the setup is not scalable to work with more qubits, the experiment does demonstrate that semiconductors can perform quantum-logic operations, Steel said. The team next will investigate the use of the more robust quantum spin of electrons as a unit of information, with hopes of stringing together multiple quantum dots into a linear array for multiqubit operations.