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.”

Source

Physicists at UC Riverside have made an accidental discovery in the lab that has potential to change how information in computers can be transported or stored. Dependent on the “spin” of electrons, a property electrons possess that makes them behave like tiny magnets, the discovery could help in the development of spin-based semiconductor technology such as ultrahigh-speed computers.

The researchers were experimenting with ferromagnet/semiconductor (FM/SC) structures, which are key building blocks for semiconductor spintronic devices (microelectronic devices that perform logic operations using the spin of electrons). The FM/SC structure is sandwich-like in appearance, with the ferromagnet and semiconductor serving as microscopically thin slices between which lies a thinner still insulator made of a few atomic layers of magnesium oxide (MgO).

The researchers found that by simply altering the thickness of the MgO interface they were able to control which kinds of electrons, identified by spin, traveled from the semiconductor, through the interface, to the ferromagnet.

Study results appear in the June 13 issue of Physical Review Letters.

Experimental results

The spin of an electron is represented by a vector, pointing up for an Earth-like west-to-east spin; and down for an east-to-west spin.

In the researchers’ experiment with the FM/SC structures, both spin up and spin down electrons were allowed to travel from the semiconductor to the ferromagnet.

The researchers found that when the structure’s MgO interface is very thin (less than two atomic layers), spin down electrons pass through to the ferromagnet, while spin up electrons are reflected back, leaving only spin up electrons in the semiconductor.

They also found that when the interface is thicker than six atomic layers, both spin up and spin down electrons are reflected back, leaving electrons with zero net spin in the semiconductor.

But the surprising result for the researchers was that at an intermediate thickness, ranging from two to six atomic layers, the selectivity of the interface completely changes.

“We see a dramatic and complete reversal in the spin of electrons that pass through the interface,” said Roland Kawakami, an assistant professor of physics who led the research team. “This time, spin up electrons pass through while spin down electrons are reflected back to the semiconductor. In other words, the thickness of the MgO interface determines whether spin up or spin down electrons are allowed to pass through it.”

According to his research team, such a “spin reversal” can be used to control current flow.

Significance of the discovery

“Electron spins are oriented at random in an ordinary electric circuit, and, therefore, do not affect current flow,” explained Yan Li, the first author of the research paper, who made the discovery. “But if spin is polarized, that is, aligned in one direction, you can manipulate the flow of current and the transport of information — a feature that would be of great interest to the semiconductor industry. What is amazing is that only a couple of atomic layers of MgO can completely reverse the spin selection of the interface. This is unexpected because MgO is not a magnetic material.”

Li, a graduate student in the Department of Physics and Astronomy working toward her doctorate in physics, said the research team will work next on making electronic devices based on the spin reversal. “This will not only test its feasibility for applications, but also help determine the cause of the spin reversal, which is still unclear,” she said.

Kawakami’s lab is one of very few labs in the world that perform both the advanced material synthesis and pulsed laser measurements needed for experiments with FM/SC structures.

“Without the strong interplay between the materials development and optical measurements, the type of discovery we made probably would not have been possible,” Kawakami said.

A new area of research, spintronics already has helped develop disk-drive read heads and non-volatile memory chips. Researchers believe spintronics also will make “instant-on” computers one day, as well as chips that can store and process data.

Kawakami, who also is a member of UCR’s Center for Nanoscale Science and Engineering, and Li were joined by UCR’s Y. Chye, Y.F. Chiang, K. Pi and W. H. Wang; and UC Santa Barbara’s J.M. Stephens, S. Mack and D.D. Awschalom.

Grants from the Office of Naval Research, the National Science Foundation and the Center for Nanoscience Innovation for Defense supported the two-year study.

Source

Deposits of nearly pure silica discovered by the Mars Exploration Rover Spirit in Gusev Crater formed when volcanic steam or hot water (or maybe both) percolated through the ground. Such deposits are found around hydrothermal vents like those in Yellowstone National Park. That’s the conclusion of planetary scientists working with data collected by the rover’s Miniature Thermal Emission Spectrometer (Mini-TES), developed at Arizona State University.

The silica discovery, announced briefly by NASA in 2007, is fully described in a multi-author paper that appears in the May 23, 2008 issue of the scientific journal Science. The lead author is Steven Squyres of Cornell University, principal investigator for the rover science payload.

The silica finding turns a spotlight on an important site that may contain preserved traces of ancient Martian life.

“On Earth, hydrothermal deposits teem with life and the associated silica deposits typically contain fossil remains of microbes,” says Jack Farmer, professor of astrobiology in ASU’s School of Earth and Space Exploration, part of the College of Liberal Arts and Sciences. Farmer is one of the paper’s co-authors.

“But we don’t know if that’s the case here,” Farmer notes, “because the rovers don’t carry instruments that can detect microscopic life.” He adds, “What we can say is that this was once a habitable environment where liquid water and the energy needed for life were present.”

NASA landed the two Mars rovers, Spirit and Opportunity, on opposite sides of the planet in January 2004 to look for rocks showing the presence of water. As of now, the rovers are more than four Earth years into a mission designed to last just three months. Despite dust collecting on their solar panels and mechanical wear-and-tear, both are continuing to explore.

Dawning realization

The silica discovery unfolded in slow motion as Spirit emerged from hibernation after its second Martian winter. The rover spent those months on the edge of a football-field-size feature dubbed Home Plate.

Home Plate lies in the Columbia Hills, a range of low hills in the middle of Gusev Crater, which spans 100 miles (170 kilometers) wide. The Hills rise about 300 feet (100 meters) above the flat lava plain that fills Gusev, but their structure and origin remain unclear to scientists.

“We were going back to an area of exposed soil called the Tyrone site, which we didn’t have time to investigate before winter began,” notes Steven Ruff, a faculty research associate at ASU’s Mars Space Flight Facility. Ruff is another of the paper’s co-authors.

The Tyrone soil proved rich in sulfate minerals, a phenomenon seen by Spirit at other locations in the Columbia Hills, where Spirit has been exploring since late 2004. While sulfates can form in several ways, water is involved in most.

“While parked next to Tyrone, we used the Mini-TES to look at some nearby light-toned and knobby outcrops,” says Ruff.

Ruff is the geologist in charge of day-to-day operations for Mini-TES, which was designed by ASU’s Philip Christensen, a Regents’ Professor of Geological Sciences and director of the Mars Space Flight Facility. Each rover is outfitted with a Mini-TES.

Silica surprise

Ruff continues, “It wasn’t clear what we were seeing in the knobby outcrops because they were contaminated with dust and wind-blown soil. But I thought they might be silica-rich.” Additional surveys with Mini-TES identified other outcrops, similarly contaminated but likewise hinting at silica.

As it happened, the rover’s jammed right front wheel inadvertently produced the “Aha!” moment. Ruff and others on the science team noticed that the stuck wheel had gouged a trench a few inches deep through the soil as the rover drove ahead in reverse, dragging the crippled wheel behind.

“The trench looked bright white,” Ruff recalls, “but we thought initially it was just more sulfate minerals.”

While parked nearby, however, Ruff got curious. “We aimed Mini-TES at the trench and it showed a clear silica spectrum. This prompted us to drive back to it, where the rover’s Alpha Particle X-Ray Spectrometer told us the white soil was more than 90 percent silica. That’s a record high for silica on Mars.”

Fumaroles and hot springs

Making such pure silica requires a lot of water, says Ruff. “On Earth, the only way to have this kind of silica enrichment is by hot water reacting with rocks.”

This, Ruff says, links the silica with Home Plate, which the rover team already knew was a volcanic feature. “Home Plate came from an explosive volcanic event with water or ice being involved,” he explains. “We saw where rocks were thrown into the air and landed to make small indentations in the soft, wet ash sediment around the vent.”

Once alerted what to look for, the scientists found more silica in many places nearby.

As Ruff explains, “It’s not just the soil in a trench in one place. It’s a broader story of outcrops that extend 50 meters [about 150 feet] away from Home Plate. It’s not a small scale, modest phenomenon.”

The combination of geothermal heat and water produces a hydrothermal system like that which powers the hot springs, geysers, mudpots, and fumaroles (steam vents) of Yellowstone National Park.

Capturing evidence

Astrobiologist Farmer helped with the mineral identification by supplying a variety of high silica rock samples from his laboratory collection. They included rocks from hot spring and fumarole deposits in Yellowstone and New Zealand. These rocks provided reference spectra for Mini-TES. “The best fit we got was with siliceous sinter,” he says, referring to deposits of “opal,” a type of amorphous silica laid down by hot springs.

Farmer explains that hydrothermal systems generally precipitate silica and other minerals as heated groundwater rises, cools, and gives off dissolved gases. “If there were organisms living there,” he says, “our terrestrial experience shows that microbes can easily be entrapped and preserved in the deposits.” Silica, he notes, is an excellent medium for capturing and preserving traces of microbial life.

Whether Mars ever had life is unknown. But if there was once a Martian biosphere, both Ruff and Farmer say the deposits around fumaroles and hot springs are ideal places to start hunting for it.

Although the microscopic imagers on the current rovers cannot resolve the microbial remains seen in terrestrial hot spring deposits, Farmer notes that the new microscopic imagers now in development for future rovers should let scientists detect such features in situ.

Says Farmer, “We just need to deliver such instruments to the right place. The discoveries at Home Plate have helped us know where to go next.”

Ruff adds, “This discovery has us really excited. This site is clearly the best example of a habitable environment that we’ve found in Gusev.”

Source