NEW YORK: By isolating and repeatedly transporting a single, trapped electron from one point on a wire to another, two independent teams of quantum physicists have completed the first major step towards building a quantum computer.

While still in their infancy, quantum computers have the potential to be highly powerful machines, capable of solving certain complex problems much faster than classical computers.

The findings, described in separate papers today in Nature, show a very high level of control over the most fundamental aspect of electronic circuits, the movement of electrons from one place to another.

Researchers say tt could have applications for the transfer of a quantum 'bit' between processor and memory.

"This is an enabling technology for quantum computers," said co-author on the second paper Chris Ford, a quantum physicist from the University of Cambridge in the UK. "Although our experiments do not yet show that electrons 'remember' their quantum state, this is likely to be the case."

Quantum computing explained

In the classical model of a computer, the most fundamental building block, known as a bit, can only exist in one of two distinct states: 0 or 1.

In a quantum computer the rules are different. Not only can a quantum bit - usually referred to as a "qubit" - exist in the classical 0 and 1 states, it can also be in both states at once: a principle known as superposition.

So any operation performed on such a qubit effectively acts on both values at the same time. This means, the greater number of qubits available, the greater the power of the computation. Thus, it is possible to use quantum computing to solve certain problems (such as cryptography) in a fraction of the time taken by a classical computer.

An example of a qubit is an electron. Clusters of electrons have been isolated and moved through channels before, but this pair of experiments showed it is possible to control individual electrons.

"The very big point of quantum computing is that it needs to be done at the single electron level; that's what we demonstrated nicely," said Tristan Meunier, a quantum physicist at the French National Centre for Scientific Research and author of one of the Nature papers.

Quantum dots essential

To build a quantum computer, you cannot use transistors, the building blocks of traditional computers. Instead, a new technology is required to transport and allow interactions between qubits.

One possible technology physicists are working on is the quantum dot - a portion in a solid, conductive material that traps electrons. The quantum dot freezes the motion of the electron in its two states -known as spin-up and spin-down - allowing operations to be performed on it.

If the electron is like a spinning top, its two states are similar to the top spinning clockwise and anti-clockwise simultaneously. Scientists have already managed to confine electrons within a quantum dot and freeze its states, but as soon as the electron is transported out of the dot, the information of state is lost. "The electron was stuck inside the quantum dot," said Meunier.

Putting the pieces together

"[We] put all the individual components together," said Robert McNeil, a co-author and graduate student from the Cambridge team.

Both groups isolated a single electron from among a cluster of electrons, trapped it in a quantum dot and successfully transported this same electron, using sound waves, into a second quantum dot.

The completion of this experiment showed for the first time that the quantum dot technology could work correctly.

To conduct this experiment, a channel system was set up, with two quantum dots on either side. The first dot is "charged" with a single electron, using electrostatic energy. A sound wave is then sent into the dot, and it captures the electron, lifting it through the channel to the second quantum dot.

At this end, the dot is prepared with the right amount of electrostatic force, so it can catch the electron and hold on to it.

The spinning electron must be measured

In the current experiments, the electron's simultaneous or 'superposition' states were not measured during its transfer; it was just moved from one point to another. When scientists can eventually prepare an electron in its spin-up and spin-down state and transfer the electron while keeping this state alive, they will have succeeded in quantum information transfer.

The time period within which the state is alive is about 1 millisecond. Meunier's team showed that they could move the electron between the quantum dots in 1 nanosecond, well within the established lifetime of the spinning electron.

McNeil's team was able to move the single electron back and forth between the two dots more than 60 times over a cumulative distance of 250 microns - something that has never been done.

Electron movement key

Manipulating individual electrons to move over rapid time scales and large distances is important for the functioning of a quantum computer. "If you get a series of individual pairs of [electrons] to interact, they become entangled with each other," said McNeil. "And that is what allows quantum computers to be so powerful."

This entanglement, known as distant interactions between qubits, had not been previously confirmed. Now, the two papers have demonstrated that this principle can exist.

"These two pieces of work are of the highest calibre in both a technical sense and in their implications for quantum technologies," commented Elanor Huntington, head of the School of Engineering and Information Technology at Sydney's University of New South Wales.

They both demonstrate basic principles for the first time, she said, paving the way for important advances in the field of quantum computing.