Electrical control of quantum bits in silicon paves the way to large quantum computers

April 10, 2015 by Rob Gutro, University of New South Wales
Electrical control of quantum bits in silicon paves the way to large quantum computers
Electron wavefunction of a donor under an electrostatic gate. A positive voltage applied to the gate attracts the electron towards the Si-SiO2 interface. This modifies the hyperfine coupling, shifts the resonance frequencies of electron and nucleus, and allows addressing of individual donor qubits in a global microwave field. Credit: A. Laucht, UNSW Australia

A UNSW-led research team has encoded quantum information in silicon using simple electrical pulses for the first time, bringing the construction of affordable large-scale quantum computers one step closer to reality.

Lead researcher, UNSW Associate Professor Andrea Morello from the School of Electrical Engineering and Telecommunications, said his team had successfully realised a new control method for future quantum computers.

The findings were published today in the open-access journal Science Advances.

Unlike conventional computers that store data on transistors and hard drives, quantum computers encode data in the quantum states of microscopic objects called qubits.

The UNSW team, which is affiliated with the ARC Centre of Excellence for Quantum Computation & Communication Technology, was first in the world to demonstrate single-atom spin qubits in silicon, reported in Nature in 2012 and 2013.

The team has already improved the control of these qubits to an accuracy of above 99% and established the world record for how long can be stored in the solid state, as published in Nature Nanotechnology in 2014.

It has now demonstrated a key step that had remained elusive since 1998.

"We demonstrated that a highly coherent qubit, like the spin of a single phosphorus atom in isotopically enriched silicon, can be controlled using electric fields, instead of using pulses of oscillating magnetic fields," explained UNSW's Dr Arne Laucht, post-doctoral researcher and lead author of the study.

The donor position can be triangulated from a combination of donor-gate capacitances and the spin readout criterion. This way, it is possible to locate the donor with an accuracy of 4, 2.5 and 3.5 nm in the three cartesian axis x, y and z. Credit: A. Laucht, UNSW Australia

Associate Professor Morello said the method works by distorting the shape of the electron cloud attached to the atom, using a very localized electric field.

"This distortion at the atomic level has the effect of modifying the frequency at which the electron responds.

"Therefore, we can selectively choose which qubit to operate. It's a bit like selecting which radio station we tune to, by turning a simple knob. Here, the 'knob' is the voltage applied to a small electrode placed above the atom."

Electron wave in a phosphorus atom, distorted by a local electric field. Credit: Dr. Arne Laucht

The findings suggest that it would be possible to locally control individual qubits with electric fields in a large-scale quantum computer using only inexpensive voltage generators, rather than the expensive high-frequency microwave sources.

Moreover, this specific type of quantum bit can be manufactured using a similar technology to that employed for the production of everyday computers, drastically reducing the time and cost of development.

The device used in this experiment was fabricated at the NSW node of the Australian National Fabrication Facility, in collaboration with the group led by UNSW Scientia Professor Andrew Dzurak.

Dr. Arne Laucht (left) and assistant professor Andrea Morello (right). Credit: Paul Henderson-Kelly/UNSW

Key to the success of this electrical control method is the placement of the qubits inside a thin layer of specially purified silicon, containing only the silicon-28 isotope.

"This isotope is perfectly non-magnetic and, unlike those in naturally occurring silicon, does not disturb the quantum bit," Associate Professor Morello said.

The purified was provided through collaboration with Professor Kohei Itoh from Keio University in Japan.

Explore further: Physicists set new records for silicon quantum computing

More information: Electrically controlling single-spin qubits in a continuous microwave field, advances.sciencemag.org/content/1/3/e1500022

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5 / 5 (2) Apr 10, 2015
Awesome news! Quantum computers are becoming a reality! :D

Yes, and with the huge leaps forward they keep taking with their implementation methods, maybe they will actually be capable of being used just like a normal PC, instead of for pure complex calculations. It seem every new discovery or advancement, changes their entire idea of what quantum computing will be capable of. At the moment, they are reducing cost and time required to produce them. Son they will be increasing processing speeds and the functional capabilities of the quantum processors.
It's a very exiting time in the tech fields, with quantum computing now a reality, graphine super capacitors looking more and more viable, and the discovery of being able to grow nano wire on pure graphine, thus be able to control their exact dimensions, and observe the exact structure of the graphine itself by looking at the nano wire. Combining all these things could be the dawn of a new era in computing and tech.
5 / 5 (1) Apr 10, 2015
If this was the 1950's the Australian Government would say the same thing it said about the development of the digital computer, something it helped pioneer then.

"There would only be a need for five of them in the world, it will be a commercial failure. Instead we will stop this research and invest the money into cloud seeding a more promising field."

interestingly, the Australian Government is cutting back on all research to save the budget at a time when traditional exports such as iron ore and coal exports are depressed. Loonies.
not rated yet Apr 13, 2015
Fantastic coherent work on the difficult road to a useful quantum computer, may be in a few years.
Any real system, like a molecule, even life, is quantum and if controled in details is a quantum computer. A 4 qubit quantum computer on a simple molecule has been working since 16 years !
Because real quantum systems of more than 10 particles are impossible to simulate on any classical computer of any size, quantum computers will open quite new possibilities in our lives.
Our real quantum world has possibilities that remains to be used.
not rated yet Apr 13, 2015
I am quite convinced by this approach to build a working quantum computer in a few years, because many nuclear spins like P in silicon are quite isolated from our world and easlly manipulated as fundamentally improved by this beautiful coherent international work .

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