David Kirkpatrick

February 7, 2010

Another step closer to quantum computers

Here’s the release from Friday:

Princeton scientist makes a leap in quantum computing

A major hurdle in the ambitious quest to design and construct a radically new kind of quantum computer has been finding a way to manipulate the single electrons that very likely will constitute the new machines’ processing components or “qubits.”

Princeton University’s Jason Petta has discovered how to do just that — demonstrating a method that alters the properties of a lone electron without disturbing the trillions of electrons in its immediate surroundings. The feat is essential to the development of future varieties of superfast computers with near-limitless capacities for data.

Petta, an assistant professor of physics, has fashioned a new method of trapping one or two electrons in microscopic corrals created by applying voltages to minuscule electrodes. Writing in the Feb. 5 edition of Science, he describes how electrons trapped in these corrals form “spin qubits,” quantum versions of classic computer information units known as bits. Other authors on the paper include Art Gossard and Hong Lu at the University of California-Santa Barbara.

Previous experiments used a technique in which electrons in a sample were exposed to microwave radiation. However, because it affected all the electrons uniformly, the technique could not be used to manipulate single electrons in spin qubits. It also was slow. Petta’s method not only achieves control of single electrons, but it does so extremely rapidly — in one-billionth of a second.

“If you can take a small enough object like a single electron and isolate it well enough from external perturbations, then it will behave quantum mechanically for a long period of time,” said Petta. “All we want is for the electron to just sit there and do what we tell it to do. But the outside world is sort of poking at it, and that process of the outside world poking at it causes it to lose its quantum mechanical nature.”

When the electrons in Petta’s experiment are in what he calls their quantum state, they are “coherent,” following rules that are radically different from the world seen by the naked eye. Living for fractions of a second in the realm of quantum physics before they are rattled by external forces, the electrons obey a unique set of physical laws that govern the behavior of ultra-small objects.

Scientists like Petta are working in a field known as quantum control where they are learning how to manipulate materials under the influence of quantum mechanics so they can exploit those properties to power advanced technologies like quantum computing. Quantum computers will be designed to take advantage of these characteristics to enrich their capacities in many ways.

In addition to electrical charge, electrons possess rotational properties. In the quantum world, objects can turn in ways that are at odds with common experience. The Austrian theoretical physicist Wolfgang Pauli, who won the Nobel Prize in Physics in 1945, proposed that an electron in a quantum state can assume one of two states — “spin-up” or “spin-down.” It can be imagined as behaving like a tiny bar magnet with spin-up corresponding to the north pole pointing up and spin-down corresponding to the north pole pointing down.

An electron in a quantum state can simultaneously be partially in the spin-up state and partially in the spin-down state or anywhere in between, a quantum mechanical property called “superposition of states.” A qubit based on the spin of an electron could have nearly limitless potential because it can be neither strictly on nor strictly off.

New designs could take advantage of a rich set of possibilities offered by harnessing this property to enhance computing power. In the past decade, theorists and mathematicians have designed algorithms that exploit this mysterious superposition to perform intricate calculations at speeds unmatched by supercomputers today.

Petta’s work is using electron spin to advantage.

“In the quest to build a quantum computer with electron spin qubits, nuclear spins are typically a nuisance,” said Guido Burkard, a theoretical physicist at the University of Konstanz in Germany. “Petta and coworkers demonstrate a new method that utilizes the nuclear spins for performing fast quantum operations. For solid-state quantum computing, their result is a big step forward.”

Petta’s spin qubits, which he envisions as the core of future quantum logic elements, are cooled to temperatures near absolute zero and trapped in two tiny corrals known as quantum wells on the surface of a high-purity, gallium arsenide chip. The depth of each well is controlled by varying the voltage on tiny electrodes or gates. Like a juggler tossing two balls between his hands, Petta can move the electrons from one well to the other by selectively toggling the gate voltages.

Prior to this experiment, it was not clear how experimenters could manipulate the spin of one electron without disturbing the spin of another in a closely packed space, according to Phuan Ong, the Eugene Higgins Professor of Physics at Princeton and director of the Princeton Center for Complex Materials.

Other experts agree.

“They have managed to create a very exotic transient condition, in which the spin state of a pair of electrons is in that moment entangled with an almost macroscopic degree of freedom,” said David DiVencenzo, a research staff member at the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y.

Petta’s research also is part of the fledgling field of “spintronics” in which scientists are studying how to use an electron’s spin to create new types of electronic devices. Most electrical devices today operate on the basis of another key property of the electron — its charge.

There are many more challenges to face, Petta said.

“Our approach is really to look at the building blocks of the system, to think deeply about what the limitations are and what we can do to overcome them,” Petta said. “But we are still at the level of just manipulating one or two quantum bits, and you really need hundreds to do something useful.”

As excited as he is about present progress, long-term applications are still years away. “It’s a one-day-at-a-time approach,” Petta said.


May 8, 2009

Zapping “killer” electrons

Filed under: Science — Tags: , , , , — David Kirkpatrick @ 5:10 pm

An interesting release from NASA today:

NASA’s THEMIS: ‘Singing’ Electrons Help Create and Destroy ‘Killer’ Electrons

GREENBELT, Md., May 8 /PRNewswire-USNewswire/ — Scientists using NASA’s fleet of THEMIS spacecraft have discovered how radio waves produced by electrons injected into Earth’s near-space environment both generate and remove high-speed “killer” electrons.

(Logo: http://www.newscom.com/cgi-bin/prnh/20081007/38461LOGO)

Killer electrons are born within Earth’s natural radiation belts, called the Van Allen belts after their discoverer, James Van Allen. Killer electrons are mostly found in the outer belt, which over the equator begins approximately 8,000 miles above Earth and tapers off about 28,000 miles high.

The high-speed electrons pose a threat to satellites in or near the outer belt — those in medium-level and higher (geosynchronous) orbits — like the Global Positioning System and most communications satellites. They are known as “killer” electrons because they can penetrate a spacecraft’s sensitive electronics and cause short circuits.

“This discovery is important to understand the physical processes that shape the radiation belts, so that one day we will be able to predict the moment-by-moment evolution of the radiation belts and be in a position to safeguard satellites in these regions, or astronauts passing through them on the way to the moon or other destinations in the solar system,” said Dr. Jacob Bortnik of the University of California, Los Angeles, lead author of a paper on this research appearing May 8 in Science.

Electrons are subatomic particles that carry negative electric charge, and we harness their flow every day as electricity. Electrons are also present in space in a gas of electrically charged particles called plasma, which is constantly blown from the surface of the sun. When this plasma interacts with Earth’s magnetic field, some of it is shot toward Earth. The magnetic field over Earth’s night side acts like a slingshot, propelling blobs of plasma toward Earth. When this happens, electrons in the plasma blobs release extra energy gained from the slingshot by “singing” — they generate a discrete type of organized radio wave called “chorus,” which sounds like birds singing when played through an audio converter.

Scientists previously discovered that electrons in the outer radiation belt can extract energy from these chorus waves to reach near-light speed and become killer electrons. The new research, confirmed by the team’s THEMIS (Time History of Events and Macroscale Interactions during Substorms) observations, is that the chorus waves can be refracted into the inner portion of the radiation belts by dense plasma near Earth and bounce around from hemisphere to hemisphere within the radiation belts. When this happens, the chorus waves become disorganized and evolve into another type of radio wave called “hiss,” according to the team.

Hiss waves, named for the sound they make when played through a speaker, are of interest to space weather forecasters because earlier research showed they can clear killer electrons from lower altitudes of the outer radiation belt. Hiss deflects the speedy particles into Earth’s upper atmosphere, where they lose energy and are absorbed when they hit atoms and molecules there. Despite its important role, it was not clear how hiss was generated.

“It is not immediately obvious that these two waves are related, but we had a fortuitous observation where the THEMIS spacecraft were lined up just right to make the connection,” said Bortnik. “First we observed chorus on the THEMIS ‘E’ spacecraft, then a few seconds later, we observed hiss on the THEMIS ‘D’ spacecraft, about 20,000 kilometers (almost 12,500 miles) away, with the same modulation pattern as the chorus.”

“Last year, we published a Nature paper that put forward a theory that seemed to explain just about everything we knew about hiss,” adds Bortnik. “We showed theoretically how chorus could propagate from a distant region, and essentially evolve into hiss. We reproduced statistical information about hiss, and a few case-examples published in the literature seemed to agree with what we were predicting. The only problem was that it seemed really difficult to verify the theory directly — to have a satellite in the (distant) chorus source region, to have another satellite in the hiss region, to have both satellites recording in high-resolution simultaneously, for the waves to be active and present at the same time, and for the satellites to be in the right relative configuration to each other to make the measurement possible. That’s where THEMIS came in. It has the right set of instruments, and the right configuration at certain parts of its orbit.”

According to the team, it’s possible other mechanisms could contribute to the generation of hiss as well. “Lightning could certainly contribute, and so could ‘in situ’ growth — the high-speed particles in the belts could generate hiss with their own motion. However, it’s just a question of which mechanism is dominant, and each might dominate at different times and locations. More research is needed to determine this,” said Bortnik.

The research was funded by NASA. For images, refer to:

http://www.nasa.gov/mission_pages/themis/news/themis_singing_electrons.htm l

Photo:  http://www.newscom.com/cgi-bin/prnh/20081007/38461LOGO
AP Archive:  http://photoarchive.ap.org/
PRN Photo Desk photodesk@prnewswire.com
Source: NASA

Web Site:  http://www.nasa.gov/