Quantum dots could form basis of new computers

Scientists at Purdue University have linked two quantum dots in a way that is essential for the creation of semiconductor-based quantum computers.

Professor Albert M. Chang and his colleagues have linked two quantum dots in such a way that is essential for the creation of semiconductor-based quantum computers, which could be faster and provide more memory than conventional technology.

Conventional computers work by representing information as a series binary digits called ‘bits.’ This code is relayed by transistors, which are minute switches that can either be on or off, representing a one or a zero, respectively.

Quantum computers would take advantage of a strange phenomenon described by quantum theory: Objects, such as atoms or electrons, can be in two places at the same time, or they can exist in two states at the same time.

That means computers based on quantum physics would have quantum bits, or ‘qubits,’ that exist in both the on and off states simultaneously, making it possible for them to process information much faster than conventional computers.

The switches in a quantum computer would be made of puddles of about 20 electrons called quantum dots, which are formed inside of computer circuitry. Each quantum dot, like each transistor in a conventional computer, is like a switch that defines a single qubit. For quantum computations to work, information will have to be exchanged between pairs of qubits.

Because electrons are said to have a ‘spin’ of either up or down, the direction of spin can be used instead of the on or off positions of a conventional computer circuit.

The Purdue researchers have been able to link two quantum dots, control how many electrons are in each dot and then detect the spin state in each dot.

Unlike conventional computer circuitry in which electrical current is used to carry information and perform computations, quantum-dot based quantum computers would rely on the manipulation of the electron spin.

In nature, the electrons in an atom occupy a series of levels that increase in energy with distance from the atom’s nucleus. In a similar way, the Purdueresearchers are able to control how many electrons occupy a quantum dot’s outermost level.

When quantum dots contain only one electron in their outermost level, their spins can be detected by analysing the flow of electricity through the dots, Chang said.

The researchers were able to achieve the milestone by creating extremely fine circuits using electron beam lithography. A semiconducting material called gallium arsenide was coated with a plastic.

Then extremely fine lines were cut into the plastic coating using a beam of electrons. The lines were filled with a metal and the plastic dissolved, leaving behind metal lines that are like wires only about 50 nanometers wide.

‘As far as we know, no other groups have been able to do such fine lithography,’ Chang said.

Other researchers have already built quantum computing devices based on molecules found in nature, such as fluorine. However, many experts believe it will be difficult to ‘scale up’ such devices to make large, workable computers.