MADISON, Wis. — Researchers at the University of Wisconsin in Madison claim to have created the world's first successful simulation of a quantum-computer architecture that uses existing silicon fabrication techniques. By harnessing both vertical and horizontal tunneling through dual top and bottom gates, the architecture lays out interacting, 50-nanometer-square, single-electron quantum dots across a chip.
"Our precise modeling elucidates the specific requirements for scalable quantum computing — for the first time we have translated the requirements for fault-tolerant quantum computing into the specific requirements for gate voltage control electronics in quantum dots," said professor Mark Eriksson of the university's Department of Physics.
The group of researchers has concluded that existing silicon fabrication equipment can be used to create quantum computers, albeit at only megahertz speeds today due to the stringent requirements of its pulse generators. To achieve gigahertz operation, the group has pinpointed the device features that need to be enhanced to prevent leakage errors, and has already begun work on fabricating a prototype.
"We believe that quantum computers are possible today with the component technologies we already have in place for silicon," Eriksson said. The team composed their quantum "bits" out of electron spin: up for "1," down for "0." Encoding bits in spins allows a single electron to represent either binary value, and because of the indeterminacy of quantum spins, they can represent both values during calculations to effectively create a parallel process.
"Our technique may enable quantum computers to actually begin performing calculations that can't be performed any other way," Eriksson said. Others have demonstrated a few quantum dots interacting to perform calculations but Eriksson estimates that a million quantum bits (qubits) will be needed to create quantum computers that perform useful real-world applications. For that, silicon fabrication equipment offers the best solution, according to Eriksson.
Eriksson's team matched silicon germanium fabrication capabilities to quantum-dot requirements. The result is an array of quantum dots, each of which houses a single electron, with electrostatic gates controlling qubit interactions. The team then optimized and exhaustively simulated the model, which it declared to be a successful design.
The design constraints included reducing the population of electrons in quantum dots to one, while permitting tunable coupling between neighboring dots. The team met those conditions by employing both vertical and horizontal tunneling to first confine and then slightly alter the location of individual electrons.
A back gate serving as the chip substrate acts as an electron reservoir from which quantum dots can draw their single electrons using vertical tunneling into the quantum-well layer. That layer acts as the vertical confinement barrier, with an insulator above and below it, enabling the vertical size of the quantum dots to be just big enough for one. A grid of top gates then provides the horizontal separation between dots by supplying electrostatic repulsion from above.
The semiconductor layers were formed from strain-relaxed SiGe, except for the quantum-well layer, which was pure, strained silicon. The bottom gate was formed from a thick n-doped layer with a 10-nm, undoped tunneling barrier separating it from the 6-nm-thick quantum-well layer. Another 20-nm-thick tunnel barrier above the quantum-well layer separated it from the metallic top gates, the team reported.
Researchers load the electrons into the quantum dots from below by adjusting the potentials on the top gates to induce an electron from the bottom gate to tunnel vertically up into the quantum-well layer. Once loaded, the electron stays in place because of the electrostatic force from the top gates. When the team weakens the force between selected quantum dots by adjusting the top gates between them, the adjacent dots are permitted to interact, thus enabling calculations to be made.
The normal errors encountered during quantum calculations could mostly be corrected, according to Eriksson's simulations. Careful consideration of the simulations led the researchers to predict that leakage could be tuned out sufficiently by low temperatures combined with a modified heterostructure that allowed larger electrical fields.
With existing fabrication techniques, the team estimates that a million-quantum-dot computer (1,024 x 1,024 array) could be built today and operated in the megahertz range.
