Electron pairs power quantum plan

By Eric Smalley, Technology Research News

The shortest route to practical quantum computers, which promise to be phenomenally powerful, may be through proven manufacturing processes, namely the semiconductor technology of today's computer chips. It wouldn't hurt if the machines also used aspects of quantum physics that are relatively easy to control.

Researchers from Hewlett-Packard Laboratories and Qinetiq plc in England have mapped out a way to manipulate a pair of very cold electrons that could eventually lead to practical quantum computers made from quantum dots, or tiny specks of the type of semiconductor material used in electronics.

The researchers showed that at low temperatures, a pair of trapped electrons operate relatively simply and can be manipulated using electric and magnetic fields. "For... two electrons in a square-shaped quantum dot, there are just two states," John Jefferson, a senior fellow at Qinetiq.

The electrons repel each other to diagonally-opposite corners of the quantum dot, leaving the two electrons in one of two possible configurations: upper right corner and lower left corner, or upper left corner and lower right corner.

These two states can represent the 1s and 0s of digital information; the quantum dots, or qubits, that contain them are the quantum computing equivalent of today's computer transistors, which use the presence or absence of electricity to represent 1s and 0s.

Quantum computers have the potential to solve very large problems fantastically fast. The weird rules that quantum particles like atoms and electrons follow allow them to be in some mix of states at once, so a qubit can be a mix of both 1 and 0. This means that a single string of qubits can represent every possible answer to a problem at once.

This allows a quantum computer to use one set of operations to check every potential answer to a problem. Today's electronic computers are much slower, in contrast, because they must check answers one at a time.

Key to the researchers method is the square shape of the microscopic quantum dot -- a speck of the semiconductor gallium arsenide measuring 800 nanometers a side -- that they used to trap the electrons. A nanometer is one millionth of a millimeter. "Two electrons in a square quantum dot repel each other [to the corners] due to the usual Coulomb repulsion force between them," said Jefferson.

The Coulomb force kicks in when particles carry a charge. Particles of the same charge, like electrons, which are negatively charged, repel each other.

Due to the weird nature of quantum particles, however, the electron pair may also jump, or tunnel, from one position, or state, to the other, said Jefferson. "This happens periodically... and the system can also be in a strange superposition state where it is partly in one state and partly in the other," he said. "This is the basis of our two-electron semiconductor quantum-dot qubit."

The researchers showed that they could use voltage pulses and magnetic fields to take this type of qubit through all the necessary operations needed to compute, said Jefferson.

This was tricky because it is not possible to turn the Coulomb force on and off, said Jefferson. "A severe potential problem with the Coulomb interaction is that it is always there," he said. The researchers showed, however, that it is possible to control the effects of the force, and thus harness it to do computing.

The researchers scheme differs from many other quantum dot quantum computing designs because it uses the positions of two electrons rather than their spin, which is a quality that can be likened to a top spinning clockwise or counterclockwise. The electrons' positions determine the charge states of the quantum dot, meaning if an electron is in one corner of the quantum dot that corner has a charge. "It is often easier to manipulate charge states compared to spin states," said Jefferson. In addition, "it is... certainly easier to measure charge states compared to spin states," he said.

To turn this building block into a practical computing device, however, the qubits must be stable. This requires "some means of preparing the qubits in a specific state, after which they have to [be affected only] according to the basic laws of quantum mechanics," said Jefferson. This includes isolating them from other interactions, he said.

Practical quantum computers would require hundreds or thousands of connected qubits. "It should be possible to add more qubits," said Jefferson. There must also be a way to measure the final results when the computation has taken place, he said.

The researchers showed that these requirements can theoretically be satisfied using the two-electron qubits, said Jefferson. "In principle, these criteria may be met, though to do so in a practical device would be technologically very challenging," he said.

Researchers generally agree that practical quantum computing of any type is one to two decades away. "Ten to 20 years is more realistic than 2 to 5," for a practical application of the two-electronic quantum dots, said Jefferson.

Rather than using semiconductor quantum dots, the researchers' basic method could possibly be achieved more quickly and effectively using a series of individual molecules, said Jefferson. "The energy and temperature scales [for molecules] are higher and thus less prone to random errors," he added.

This could address one of the main hurdles to using qubits practically, Jefferson said. "One of the main challenges is to reduce the interaction of a quantum system with its environment -- the so-called decoherence problem," he said.

The other main technical challenge to using the system practically would be to produce quantum dots containing precisely two electrons, and to coax the electrons to switch states with acceptable error rates, he said.

Jefferson's research colleagues were M. Fearn and D. L. J. Tipton of Qinetiq and Timothy P. Spiller of Hewlett-Packard Laboratories. They published the research in the October 30, 2002 issue of the journal Physical Review A. The research was funded by the British Ministry of Defense, the European Union, Hewlett-Packard and Qinetiq.

Timeline:   10-20 years
Funding:   Corporate, Government
TRN Categories:  Physics; Quantum Computing and Communications
Story Type:   News
Related Elements:  Technical paper, "Two-Electron Quantum Dots as Scalable Qubits," Physical Review A, October 30, 2002.


January 1/8, 2003

Page One

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Electron pairs power quantum plan

Aligned fields could speed storage


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