SOUTH BEND, Ind. In a step toward developing quantum computers, researchers at Notre Dame University have demonstrated logic gates that use quantum-dot cellular automata (QCA), and which switched using only two electrons. Future versions of the QCA promise to squeeze 1 trillion gates into a scant 1-cm square.
"We demonstrated inverters, AND gates and OR gates that used just four quantum dots connected in a ring by tunnel junctions, and which occupied just 5 nanometers square," said Greg Snider, the team's leader. Other members include researchers Gary Bernstein and Craig Lent.
Quantum dots are tiny structures made from aluminum islands surrounded by a thin oxide. Individual electrons can tunnel through the oxide under the control of a capacitively coupled gate and remain trapped in a potential well under the dot. The interaction of the electronic-wave function with the boundary of such structures causes the energy levels to be quantized. When arranged in a cell, quantum dots can interact via the electrostatic Colomb force, switching single electrons with no current flowing between cells and thus consuming almost infinitesimal amounts of power. Quantum dots can be arranged in groups to form gates, or can be lined up end to end to form binary "wires."
"We believe that the limits of densities in semiconductor fabrication techniques will be the amount of power they generate. The power-density product of conventional semiconductors is about 10-15 joules, but for quantum cellular automata the power delay product is just 10-24 joules, or nine orders of magnitude smaller," said Snider.
The basic quantum-dot cellular automata used four quantum dots arranged in a square. Two extra free electrons are trapped among the four cells, and since the electrostatic charges on the electrons repel each other, they always settle at the diagonally opposite corners of the square. The two diagonal polarizations are energetically equivalent ground states of the cell and can thus be encoded into equally likely states called by convention "0" and "1." When one of the electrons can be coaxed into switching corners, it induces the other electron to likewise switch corners so as to remain diagonally across from its counterpart, thus forming a transistorless gate.
The fundamental QCA logic device is a three-input majority gate, the polarization of which becomes that of the majority of its three inputs. A single output polarization follows the state of the cell, allowing the gate's internal state to be read out nondestructively. If one of the inputs is held at "0" the remaining two inputs act as an AND gate. Or if one of the inputs is held at "1," then the remaining two inputs perform the logical OR operation.
Arrays of these devices can be "wired" only to nearest neighbors, a limitation that fits with a processor architecture called a cellular automata. Such processors have been emulated in software and also built with conventional electronics. A large problem set, mostly related to physical simulation, has been programmed on them.
The Notre Dame implementation constructed the four quantum dots from small aluminum islands with a thin surrounding oxide permitting tunneling of electrons from one dot to adjacent dots. Each dot was capacitively coupled to a gate that influenced the charge state of its respective dot. A separate quantum dot read out the state of the cell with a capacitive coupling that acted as a noninvasive electrometer. Electron-beam lithography was used to create the island dots, followed by oxidation and a subsequent shadow evaporation process.
One key advantage of this transistorless approach, the researchers said, was the discovery that the smaller dots were found to improve signal-to-noise ratios, making the technique a natural for scaling to smaller sizes. And the devices are fast: for dots separated by 35 nanometers, switching speeds as fast as 200 picoseconds were observed. One downside, however, is that the current implementation had to be cooled to 0.1Kelvin to make the QCA operate properly.
Problem on signal path?
Another problem those building quantum-dot systems will face is endemic to any attempt to eliminate heat dissipation: reversible signal paths. In conventional computers, heat dissipation, while creating problems, does play an important role in giving a physical direction to the computation. In the ideal quantum world, a sequence of switches that do not dissipate heat could spontaneously reverse themselves so that there is no guarantee that a computation will inexorably move toward the solution.
"Our next goal is to build a quantum-dot cellular automata at a higher temperature, perhaps as high as the temperature of liquid nitrogen [77K]," said Snider. "But we have a long ways to go before we can demonstrate room-temperature operation."