A quantum dot is a semiconductor box that is several nanometers in size and can confine electrons. When quantum dots are arranged 10 nm apart, an electron in a dot repels an electron in another dot, because of their negative electric charge. Moreover, electrons can move freely from one quantum dot to an adjacent dot due to a tunnel effect without conventional wiring.

Digital circuits using quantum dots by making use of the phenomena are being researched, and they may lead to the development of an ultra-low power microcomputer. Let’s learn how the microcomputer works.

Let’s consider a cell that consists of four or five quantum dots arranged in a square pattern as shown in the figure below. When two electrons are injected into a dot, the electrons repel each other because of their negative electric charge (Coulomb repulsion) and occupy dots in corners opposite to each other (antipodal dots). When two unit cells, each comprised of four or five quantum dots, are arranged adjacent to each other, electrons move among the dots in each unit cell and the state of each dot in the two cells is autonomously determined.

The system in which the state of all cells is determined autonomously due to interactions between cells is called a “Cellular Automaton”. A computer using this quantum-dot cellular automaton is being researched, because the quantum-dot cellular automaton makes it possible to keep recorded data in a circuit without a power supply. The computer can be developed by making logic elements of a digital IC, in which several unit cells of quantum dots are combined, and then integrating the logic elements.

Look at the figure below. Four unit cells of quantum dots are arranged. When the state of the first cell from the left is changed to either 1 or 0, the change is passed on to the fourth cell. The circuit works as a transmission channel for digital signals.

This is a three-input majority logic gate. The majority state in the orange squares is what appears in the blue square. Based on this gate, a two-input AND (logical multiplication) gate and a two-input OR (logical addition) gate can be made.

3-input Majority Logic Gate		2-input AND Logic Gate
			Truth Table Combinations of Logical States Input 1 Input 2 Output 0 0 0 1 0 0 0 1 0 1 1 1
		2-input OR Logic Gate
			Truth Table Input 1 Input 2 Output 0 0 0 1 0 1 0 1 1 1 1 1

This is a NOT gate, which is one of the essential logic gates of digital circuits. Inversion of the digital signal became possible by arranging unit cells diagonally.

NOT Logic Gate
		Truth Table Input Output 0 1 1 0

Let’s make a digital circuit with simple functions by combining all these logic gates that you have just learned. A full adder with carry input/output operating on single-digit binary numbers is constructed using quantum dot unit cells. Combining the adders makes addition of multi-digit binary numbers possible.

Full adder with carry input/output operating on single-digit binary numbers
Carry Output Input A Input B Carry Input Output By clicking one of the orange squares, the current state of one of the left cells changes, and the change is passed on to the last cell. This mark represents an overlapping cell between transmission channels. This mark is used to explain the principle in a simple way. In an actual circuit, a mechanism that prevents signal interference caused by the overlap of transmission channels is needed.		Truth Table  I A  I B C  I O C O 0 0 0 0 0 0 0 1 1 0 0 1 0 1 0 0 1 1 0 1 1 0 0 1 0 1 0 1 0 1 1 1 0 0 1 1 1 1 1 1 Digital circuit, in which four full adders are connected and that adds four-digit binary numbers

Do you understand how digital circuits using quantum dots work? Basically, a CPU in a personal computer is composed of several of the simple logic gates mentioned above.

Research on digital circuits using quantum dots has just begun. The use of digital circuits based on quantum-dot cellular automatons is tone way in which ultra-high density digital electronic circuits and ultra-low-power microcomputers are being developed. To allow digital circuits to operate, developing a system that transmits the state of each unit cell in the correct order and a method to input and output signals are needed.

(nanonet: Kazunori Komori, Hisashi Yamawaki)