Controlling Quantum States

Dr. Sven Rogge (Credit: Delft University of Technology)

The odd feature of a molecule in an experimental silicon computer chip opened the door to quantum computing in semiconductors. The researchers explain the creation of a new hybrid molecule whereby its quantum state can be calculatedly influenced. This can in turn place them a step closer to building a quantum computer.

The breakthrough occurred when Sven Rogge and his colleagues at Delft University of Technology in the Netherlands were researching nano-scale transistors that illustrate the properties of unintentional impurities, or dopants. The researchers established that the current-voltage characteristics of the transistor revealed that electrons were being transported by a single atom but it was ambiguous to which impurity triggered this effect. Using the concept of an individual impurity Physicist Lloyd Hollenberg and colleagues at the University of Melbourne in Australia were able to create a theoretical silicon-based quantum computer chip.

“The team found that the measurements only made sense if the molecule was considered to be made of two parts,” Hollenberg says. “One end comprised the arsenic atom embedded in the silicon, while the ‘artificial’ end of the molecule forms near the silicon surface of the transistor. A single electron was spread across both ends.”

“Up to now large scale quantum computing has been a dream,” says Gerhard Klimeck, professor of electrical and computer engineering at Purdue University and associate director for technology for the national Network for Computational Nanotechnology. “This development may not bring us a quantum computer 10 years faster, but our dreams about these machines are now more realistic.”

Standard computers still use bits of information, 1s and 0s, to store and process information. Using the strange behaviours found in quantum physics, quantum computers could be created to carry data using quantum bits, or qubits. These computers would then be able to deal with an exponential amount of information.

As the electric field induced by a silicon nanowire (gray) increases, an electron in an arsenic atom moves from its ground state (left) to an excited state (right). During this transition, the electron enters a hybridized state (middle) in which it is in both of the other states simultaneously. In theory, such an electron could serve as a “qubit” in a quantum computer. (Credit: Insoo Woo and Rajib Rahman, Purdue University)

There are also some counter intuitive behaviours of quantum mechanics which are hard to fathom even for physicists. For example, two quantum computers could, in concept, communicate instantaneously across any distance imaginable, even across solar systems. It would be truly ingenious if quantum computers could take advantage of this behaviour.

By altering the voltage of the transistor the researchers could control the molecule to produce a “neither here nor there” quantum state. Until now the challenge had been to create a computer semiconductor in which the quantum state could be controlled. “If you want to build a quantum computer you have to be able to control the occupancy of the quantum states,” Klimeck says. The team could control the location of the electron in this artificial atom and therefore control the quantum state with an externally applied electrical field.

What is strange about the “surface” end of the molecule is that it occurs as an artifact when electrical current is applied across the transistor and hence can be considered “manmade.” There is no equivalent form existing naturally in the world. “Our experiments made us realize that industrial electronic devices have now reached the level where we can study and manipulate the state of a single atom,” Rogge says. “This is the ultimate limit; you can not get smaller than that.”

TFOT has previously written about nano-diamonds that might lead to quantum computing where nanometer-sized diamonds may help in creating quantum computers capable of performing parallel computing tasks that cannot be carried out by conventional computers. TFOT covered several quantum computing issues including electron traps that compute using superimposed quantum dots, which are able to “trap” single electrons. This imperative result moves scientists another step closer to the creation of an ultimate quantum computer. TFOT also covered a D-Wave demonstration of a 28-qubit quantum computer which supposedly showed an image recognition algorithm running on a quantum computer. The computer carries out multiple calculations simultaneously, thus achieving faster computation results than conventional computers.

Additional information about Quantum Leaps can be found at Purdue University’s website.