Electron Spins Talk to Each Other Via a 'Quantum Mediator'

 Cecile G. Tamura

In the esoteric world of quantum computing research, it is relatively easy to get two bits of quantum information to communicate with one another—as long as they are neighbors. Separate them, however, and they can no longer exchange information.

Thanks to a clever work around new Lieven Vandersypen, Ph.D. student Tim Baart, and post-doc Takafumi Fujita, we now have a way to overcome this problem. They hope to use it to make quantum computers more flexible by improving their ability to exchange information over longer distances.

One way quantum computers store information is through electron spin of quantum dots. An “up” spin would be zero; a “down” spin would be one. They communicate spin information when the electrons are next to one another.

The researchers then added an empty quantum dot between the two occupied quantum dots. Lowering the energy barrier of the empty dot enables the occupied dot to send its spin information into the empty dot. The empty dot can then transmit it to the second occupied dot.

The researchers can turn the interaction on and off at will. This could make it possible to transmit information over longer distances in computers by using strings of empty dots.

The unparalleled possibilities of quantum computers are currently still limited because information exchange between the bits in such computers is difficult, especially over larger distances. Lieven Vandersypen, Professor at QuTech and workgroup leader at the Dutch Organization for Fundamental Research on Matter (FOM), have succeeded with his colleagues for the first time in enabling two non-neighbouring quantum bits in the form of electron spins in semiconductors to communicate with each other.

Information exchange is something that we scarcely think about these days. People constantly communicate via e-mails, mobile messaging applications and phone calls. Technically, it is the bits in those various devices that talk to each other. “For a normal computer, this poses absolutely no problem,” says professor Lieven Vandersypen, Co-Director of the Kavli Institute of Nanotechnology at TU Delft. “However, for the quantum computer – which is potentially much faster than the current computers – that information exchange between quantum bits is very complex, especially over long distances.”

Electrons talk with each other

Within Vandersypen's research group, PhD student Tim Baart and postdoc Takafumi Fujita worked on the communication between quantum bits. Each bit consists of a single electron with a spin direction (spin up = ‘0’ and spin down = ‘1’). “From previous research, we knew that two neighbouring electron spins can interact with each other, but that this interaction sharply decreases with increasing distance between them,” says Baart. “ We have now managed to make two non-neighbouring electrons communicate with each other for the first time. To achieve this, we used a quantum mediator: an object that can exchange the information between the two spins over a larger distance.”

Mediator

Chip used to create quantum dots The chip with the electrical contacts used to create the quantum dots. (Source: Tim Baart)
Baart and Fujita positioned the electrons in so-called quantum dots, where they were held in position by an electrical field. Between the two occupied quantum dots, they positioned an empty quantum dot that could form an energy barrier between the two spins. “By adjusting the electrical field around the empty quantum dot, we could enable the electrons to exchange their spin information via the superexchange mechanism: when the energy barrier is lowered, the spin information is exchanged,” says Baart. “This makes the empty quantum dot act as a type of mediator to make the interaction between the quantum bits possible. Furthermore, we can switch this interaction on and off at will.”

Fast quantum computer

The research of Vandersypen and Baart forms an important step in the construction of a larger quantum computer in which the communication between quantum bits over large distances is essential. Now that the concept of this quantum mediator has been demonstrated in practice, the researchers want to increase the distance between electron spins and place other types of ‘mediators’ between the quantum bits as well.

https://en.wikipedia.org/wiki/Quantum_dot

http://www.tudelft.nl/…/deta…/onderhandelen-met-quantumdots/

http://www.trustedreviews.com/…/quantum-dots-explained-what…