Purdue University physicist Albert Chang and colleagues have successfully linked two so-called quantum dots such that the tiny structures could conceivably serve as qubits¿switches for quantum computers that can be on, off or in a combination of states. So that the pairs of would-be qubits could exchange information, the scientists connected quantum dots, essentially flat pools of electrons measuring a mere 180-nanometers wide, using even finer nanowires made by way of standard electron beam lithography. But they had to do so in such a way that they could still determine which state each individual dot was in, depending on its electron spin. "Each dot can have a one [on] or a zero [off] because the spin can be up or down," Chang explains. But "without being able to isolate each spin, you cannot do quantum computation."
The trick, the team reports today in Science, lay in controlling how many electrons ended up in each quantum dot. As in atoms, the electrons fill the dot by taking up positions in successive orbitals around the center. Chang and colleagues took advantage of the fact that when there was only one electron in the dot's outermost orbital, they could analyze the flow of electricity through the dots and thereby detect spin. "Now we have proven that you can link quantum dots together," Chang says, "but the next thing will be to make them do things, to control the spins in a double-quantum dot."