Taking charge of artificial molecules.

Although computers using molecule-size components are still only a distant possibility, numerous research groups are laying the groundwork for such machines. One team now reports demonstrating for the first time control of electron motion within an artificial molecule composed of linked microscopic electron cages, called quantum dots. Such control--which was achieved with a beam of pulsed microwaves--may prove vital for synchronizing components of a future molecule-based computer, they say.

"We can force the coupled dots to keep up with our excitations," says Daniel W. van der Weide of the University of Delaware in Newark. He and his German coauthors describe their work in the July 20 Physical Review Letters.

Quantum dots are areas bound by electric fields in the surfaces of semiconductors. Electrons trapped within them exhibit energy levels and quantum-mechanical behavior similar to electrons in atoms. When placed in tandem, quantum dots can serve as an artificial molecule woven together by shared electrons that shuttle from dot to dot.

The experimenters built such a pseudomolecule from two adjacent dots and subjected it to pulses of microwave energy. In response, they detected a "sloshing" of electrons between the dots in phase with the pulses. "It's like two buckets connected with pipe. If you use a plunger in one to push the level down, the level in the other goes up. Then there's an oscillation if you let go," van der Weide says.

The sloshing appears to be the artificial-molecule equivalent of what are called Rabi oscillations in excited, real molecules, the researchers say. However, Leo P. Kouwenhoven, a physicist at Delft University of Technology in the Netherlands who is doing similar work, is unconvinced. He would be swayed more, he says, if the experimenters had varied the ability of electrons to hop back and forth between the dots. Adjusting a gate voltage that controls the electrons' passage would have helped clarify the role, if any, of interdot electron traffic in the signal detected, he says.

Van der Weide concedes that the evidence is not unambiguous, but he notes that the team calculated the frequency at which Rabi-like oscillations would be expected for the quantum-dot pair. The calculated value equaled the frequency at which the observed oscillations were most pronounced. Detection of such oscillations suggests that artificial molecules resemble real ones more than anyone knew, he says.