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Quantum Leap

Researchers have controlled the position of a single electron in a silicon circuit.

An international team of researchers has shown that it can control the quantum state of a single electron in a silicon transistor–even putting the electron in two places at once. Their discovery could help pave the way toward a practical quantum computer.

Fence sitter: A computer model shows an arsenic atom with an electron that’s in two distinct quantum states at the same time. An international team of researchers experimentally confirmed the model’s predictions.

Quantum computers take advantage of the strange properties of subatomic particles to perform certain types of calculations much faster than classical computers can. Researchers are exploring a host of different approaches to quantum computing, and some have even built primitive quantum circuits that can perform calculations. But practical quantum computing would require the ability to manufacture devices with millions of quantum circuits–rather than the 12 or 16 achievable now–that can be integrated with more-conventional electronics.

One theoretical approach to practical quantum electronics is to use conventional electronics–tiny semiconductor transistors–to control the state of a quantum system. Researchers led by Sven Rogge, a researcher at Delft University of Technology, in the Netherlands, performed the first practical experiments to verify the approach’s theoretical predictions. The team–which also included researchers from Purdue University; the University of Melbourne, in Australia; and IMEC, in Belgium–found that it could control the quantum state of a single electron simply by altering the voltage applied to a transistor. “This represents a nice step towards future devices where performance is determined by manipulation of quantum states of single atoms,” says Thomas Schenkel, a scientist at Lawrence Berkeley National Laboratory.

The researchers used prefabricated transistors built for nanotech research, each of which consisted of two crossed silicon nanowires. One of the nanowires–the bottom one–was connected to electrodes that contained arsenic. When that wire was charged, it would sometimes draw arsenic atoms into the transistor. After applying a voltage across about 100 transistors, the team found six that appeared to have individual arsenic atoms embedded in the nanowire. They then found that varying the voltage across the top wire would control the quantum state of one of the atom’s electrons. Using an imaging technique called scanning tunneling spectroscopy, they were able to distinguish three states of the atoms in all six devices. One of those states corresponded with the electron’s being in two places at the same time–a property necessary for quantum computing.

“But to understand the behavior of one atom, you have to model millions,” says Rogge. So he and his colleagues used an off-the-shelf program called NEMO 3D to characterize a large-scale system containing 1.4 million atoms. They found that the measurements of their model system were in good agreement with the spectroscopy results. “It is really cool to see how well their simulations can describe randomly doped transistors, which are all different in detail,” Schenkel says.

Altered states: As the electric field induced by a silicon nanowire (gray) increases, an electron in an arsenic atom moves from its ground state (left) to an excited state (right). During this transition, the electron enters a hybridized state (middle) in which it is in both of the other states simultaneously. In theory, such an electron could serve as a “qubit” in a quantum computer.

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.”

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