Depending on the strength of the electric field created by the top nanowire, an electron could be found in one of three states. At low electric fields, the electron remained bound to the arsenic atom. At high electric fields, the electron was pulled away from the atom. But when the electric field was at just the right level, the electron would be in both places at once.

In order for a quantum computer to work, its qubits--the quantum equivalent of a classical computer's bits--need to be "entangled": their quantum states have to be coupled with each other. Pulling an electron away from its atom "might be an interesting way to couple" adjacent qubits, says Schenkel.

"While this result is an important one, the real challenge to making future single-dopant devices is in figuring out how to position the [arsenic atoms] into the silicon host with the required precision," says Bruce Kane, a research scientist at the University of Maryland. The researchers found their six devices by chance; to produce working circuits, they would need to be able to position atoms of arsenic--or some other material--in the transistors more reliably.

While the researchers eventually hope to be able to control the position of the atoms in the transistor, "our next step is to add a second electron and see what happens to the configuration of the electron state," says Gabri Lansbergen, another Delft researcher. "In the far future," Rogge adds, "we would like to experiment with several [materials] and see how they interact."