A single electron surfing on a sound wave is transfered between two distant quantum dots.
Credit: Laurent Revellin, Service Communication CNRS-Alpes
Illustration of the potential-energy landscape seen by an electron, and the potential wave produced by a sound pulse coming from bottom right and moving past the first dot, along the channel towards the other dot.
Credit: Robert McNeil
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.
