The odd behavior of a molecule in an experimental silicon computer chip has led to a discovery that opens the door to quantum computing in semiconductors.

In a Nature Physics journal paper currently online, the researchers describe how they have created a new, hybrid molecule in which its quantum state can be intentionally manipulated - a required step in the building of quantum computers.

“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.”

The workings of traditional computers haven’t changed since they were room-sized behemoths 50 years ago; they still use bits of information, 1s and 0s, to store and process information. Quantum computers would harness the strange behaviors found in quantum physics to create computers that would carry information using quantum bits, or qubits. Computers would be able to process exponentially more information.

If a traditional computer were given the task of looking up a person’s phone number in a telephone book, it would look at each name in order until it found the right number. Computers can do this much faster than people, but it is still a sequential task. A quantum computer, however, could look at all of the names in the telephone book simultaneously.

Quantum computers also could take advantage of the bizarre behaviors of quantum mechanics - some of which are counterintuitive even to physicists - in ways that are hard to fathom. For example, two quantum computers could, in concept, communicate instantaneously across any distance imaginable, even across solar systems.

Albert Einstein, in a letter to Erwin Schrödinger in the 1930s, wrote that in a quantum state a keg of gunpowder would have both exploded and unexploded molecules within it (a notion that led Schrödinger to create his famous cat-in-a-box thought experiment).

This “neither here nor there” quantum state is what can be controlled in this new molecule simply by altering the voltage of the transistor.

Until now, the challenge had been to create a computer semiconductor in which the quantum state could be controlled, creating a qubit.

“If you want to build a quantum computer you have to be able to control the occupancy of the quantum states,” Klimeck says. “We can control the location of the electron in this artificial atom and, therefore, control the quantum state with an externally applied electrical field.”

The discovery began when Sven Rogge and his colleagues at Delft University of Technology in the Netherlands were experimenting with nano-scale transistors that show the effects of unintentional impurities, or dopants. The researchers found properties in the current-voltage characteristics of the transistor that indicated electrons were being transported by a single atom, but it was unclear what impurity was causing this effect.

Physicist Lloyd Hollenberg and colleagues at the University of Melbourne in Australia were able to construct a theoretical silicon-based quantum computer chip based on the concept of using an individual impurity.

“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.

“What is strange about the ’surface’ end of the molecule is that it occurs as an artifact when we apply electrical current across the transistor and hence can be considered ‘manmade.’ We have no equivalent form existing naturally in the world around us.”

Klimeck, along with graduate student Rajib Rahman, developed an updated version of the nano-electronics modeling program NEMO 3-D to simulate the material at the size of 3 million atoms.

“We needed to model such a large number of atoms to see the new, extended quantum characteristics,” Klimeck says.

The simulation showed that the new molecule is a hybrid, with the naturally occurring arsenic at one end in a normal spherical shape and a new, artificial atom at the other end in a flattened, 2-D shape. By controlling the voltage, the researchers found that they could make an electron go to either end of the molecule or exist in an intermediate, quantum, state.

This model was then made into an image by David Ebert, a professor of electrical and computer engineering at Purdue, and graduate student Insoo Woo.

Delft’s Rogge says the discovery also highlights the current capabilities of designing electronic machines.

“Our experiment 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.”

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Weizmann Institute physicists have demonstrated, for the first time, the existence of ‘quasiparticles’ with one quarter the charge of an electron. This finding could be a first step toward creating exotic types of quantum computers that might be powerful, yet highly stable.

Fractional electron charges were first predicted over 20 years ago under conditions existing in the so-called quantum Hall effect, and were found by the Weizmann group some ten years ago. Although electrons are indivisible, if they are confined to a two-dimensional layer inside a semiconductor, chilled down to a fraction of a degree above absolute zero and exposed to a strong magnetic field that is perpendicular to the layer, they effectively behave as independent particles, called quasiparticles, with charges smaller than that of an electron. But until now, these charges had always been fractions with odd denominators: one third of an electron, one fifth, etc.

The experiment done by research student Merav Dolev in Prof. Moty Heiblum’s group, in collaboration with Drs. Vladimir Umansky and Diana Mahalu, and Prof. Ady Stern, all of the Condensed Matter Physics Department, owes the finding of quarter-charge quasiparticles to an extremely precise setup and unique material properties: The gallium arsenide material they produced for the semiconductor was some of the purest in the world.

The scientists tuned the electron density in the two-dimensional layer – in which about three billion electrons were confined in the space of a square millimeter – such that there were five electrons for every two magnetic field fluxes. The device they created is shaped like a flattened hourglass, with a narrow ‘waist’ in the middle that allows only a small number of charge-carrying particles to pass through at a time. The ’shot noise’ produced when some passed through and others bounced back caused fluctuations in the current that are proportional to the passing charges, thus allowing the scientists to accurately measure the quasiparticles’ charge.

Quarter-charge quasiparticles should act quite differently from odd fractionally charged particles, and this is why they have been sought as the basis of the theoretical ‘topographical quantum computer.’ When particles such as electrons, photons, or even those with odd fractional charges change places with one another, there is little overall effect. In contrast, quarter-charge particle exchanges might weave a ‘braid’ that preserves information on the particles’ history. To be useful for topologically-based quantum computers, the quarter-charge particles must be shown to have ‘non-Abelian’ properties – that is the order of the braiding must be significant. These subtle properties are extremely difficult to observe. Heiblum and his team are now working on devising experimental setups to test for these properties.

Prof. Moty Heiblum’s research is supported by the Joseph H. and Belle R. Braun Center for Submicron Research. Prof. Heiblum is the incumbent of the Alex and Ida Sussman Professorial Chair in Submicron Electronics.

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A deeper fundamental understanding of complex materials may now be possible, thanks to a pair of Princeton scientists who have uncovered a new insight into how crystals form.

The researchers’ findings reveal a previously unknown mathematical relationship between the different arrangements that interacting particles can take while freezing. The discovery could give scientists insight into the essential behaviors of materials such as polymers, which are the basis of plastics.

Molecules in a material cooled to absolute zero can take on a multitude of different configurations. Historically, scientists’ difficulty with identifying crystallized molecules’ spatial arrangements from this high number of possible configurations has blocked theoretical efforts to understand these materials’ qualities, but the new findings could offer the tool that science needs.

“We believe our ‘duality relations’ will be a useful theoretical tool to understand how individual particles come together to form a crystal,” said Salvatore Torquato, a professor of chemistry who co-wrote the paper with senior chemist Frank Stillinger. “If we can tune the interactions among particles that form a crystal, we might be able to create materials that respond to light or mechanical stress in novel ways.”

A material that maintains its exact size and shape through extremes in temperature, for example, might be valuable in the manufacture of orbiting space telescopes, whose mirrors need to retain their shape as they pass from sunlight into the Earth’s shadow.

A crystal is the state of matter that is easiest to analyze because its frozen molecules are motionless and often regularly organized. A crystal’s properties — its ability to bend light, for example — generally reveal valuable information about how its constituent molecules will behave at higher temperatures, such as when they become a liquid.

The challenge is that many complex materials can crystallize into a multitude of different structures. When a substance is cooled to nearly absolute zero, and it can take on an enormously large number of possible “ground states” — the term for the molecular arrangement with the lowest possible energy. Scientists seek to determine the true ground state because it provides a fundamental understanding of matter in the solid state and its possible uses. However, determining which molecular pattern is the true ground state requires mathematical proof that is hard to come by.

“We resort to approximations,” said Christos Likos, a professor of theoretical physics at the University of Dusseldorf in Germany. “They help us produce meaningful results sometimes, but we need to have a lighthouse occasionally to show us we’re on the right path. Such lighthouses are rare in this business, but Sal and Frank have found one.”

Torquato and Stillinger’s findings explore particles’ behavior as they attract and repel each other over varying distances. By analyzing this behavior, the scientists were able to conceive a precise mathematical correspondence — called duality relations — between possible arrangements of particles. The work will enable the researchers to draw important conclusions about how particles at very low temperatures interact over great distances, a situation that is very difficult to treat theoretically.

“Once ground states can be determined and controlled with certainty, scientists might create materials with properties virtually unknown in nature,” Torquato said.

The Department of Energy funded the team’s research, which appears in the Jan. 16 edition of the journal Physical Review Letters.

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