David Kirkpatrick

May 1, 2010

Quantum computing news

I wouldn’t call this an astounding breakthrough, but sometimes a shift in focus — like this research into alternative materials for quantum computers — can lead to seismic changes in the field down the road.

The release:

UCSB scientists look beyond diamond for quantum computing

IMAGE: David Awschalom is a researcher at University of California – Santa Barbara.

Click here for more information.

(Santa Barbara, Calif.) –– A team of scientists at UC Santa Barbara that helped pioneer research into the quantum properties of a small defect found in diamonds has now used cutting-edge computational techniques to produce a road map for studying defects in alternative materials.

Their new research is published in the online edition of the Proceedings of the National Academy of Sciences (PNAS), and will soon be published in the print edition of the journal. The findings may enable new applications for semiconductors ––materials that are the foundation of today’s information technology. In particular, they may help identify alternative materials to use for building a potential quantum computer.

“Our results are likely to have an impact on experimental and theoretical research in diverse areas of science and technology, including semiconductor physics, materials science, magnetism, and quantum device engineering,” said David D. Awschalom, UCSB physics professor and one of two lead investigators on this project. “Ironically, while much of semiconductor technology is devoted to eliminating the defects that interfere with how today’s devices operate, these defects may actually be useful for future quantum technologies.”

IMAGE: Chris Van de Walle is a researcher at University of California – Santa Barbara.

Click here for more information.

According to PNAS, the researchers have developed a set of screening criteria to find specific atomic defects in solids that could act as quantum bits (qubits) in a potential quantum computer. As a point of reference, they use a system whose quantum properties they themselves have recently helped to discern, the NV or nitrogen-vacancy center defect in diamond. This defect, which the team has shown can act as a very fast and stable qubit at room temperature, consists of a stray nitrogen atom alongside a vacancy in the otherwise perfect stacking of carbon atoms in a diamond.

Electrons trapped at the defect’s center interact with light and microwaves in a predictable way, allowing information to be stored in and read out from the orientation of their quantum-mechanical spins.

The drawback to using diamond, however, is that the material is expensive and difficult to grow and process into chips. This raises the question of whether there may be defects in other materials that have similar properties and could perform equally well.

In this week’s publication, the researchers enumerate specific screening criteria to identify appropriate defects in materials that could be useful for building a quantum computer. Experimental testing of all the potential candidates might take decades of painstaking research, explained Awschalom. To address this problem, the UCSB group employed advanced computational methods to theoretically examine the characteristics of potential defect centers in many different materials, providing a sort of road map for future experiments.

UCSB’s Chris G. Van de Walle, professor of materials and one of the senior investigators on the project, remarked: “We tap into the expertise that we have accumulated over the years while examining ‘bad’ defects, and channel it productively into designing ‘good’ defects; i.e., those that have the necessary characteristics to equal or even outperform the NV center in diamond.” This expertise is backed up by advanced theoretical and computational models that enable the reliable prediction of the properties of defects, a number of which are proposed and examined in the paper.

Awschalom added: “We anticipate this work will stimulate additional collaborative activities among theoretical physicists and materials engineers to accelerate progress towards quantum computing based on semiconductors.”

Current computers are based on binary logic: each bit can be either “one” or “zero.” In contrast, each qubit in a quantum computer is continuously variable between these two states and hence offers infinitely more possibilities to be manipulated and combined with other qubits to produce a desired computational result. “It has been well established that, in theory, quantum computers can tackle some tasks that are completely beyond the capabilities of binary computers,” said Awschalom. “The challenge has been to identify real physical systems that can serve as qubits for future machines.”

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February 27, 2010

Getting closer to a quantum computer

Another incremental step toward functional quantum computing. We don’t need quantum computing just yet, but we will.

The release:

UW-Madison physicists build basic quantum computing circuit

MADISON — Exerting delicate control over a pair of atoms within a mere seven-millionths-of-a-second window of opportunity, physicists at the University of Wisconsin-Madison created an atomic circuit that may help quantum computing become a reality.

Quantum computing represents a new paradigm in information processing that may complement classical computers. Much of the dizzying rate of increase in traditional computing power has come as transistors shrink and pack more tightly onto chips — a trend that cannot continue indefinitely.

“At some point in time you get to the limit where a single transistor that makes up an electronic circuit is one atom, and then you can no longer predict how the transistor will work with classical methods,” explains UW-Madison physics professor Mark Saffman. “You have to use the physics that describes atoms — quantum mechanics.”

At that point, he says, “you open up completely new possibilities for processing information. There are certain calculational problems… that can be solved exponentially faster on a quantum computer than on any foreseeable classical computer.”

With fellow physics professor Thad Walker, Saffman successfully used neutral atoms to create what is known as a controlled-NOT (CNOT) gate, a basic type of circuit that will be an essential element of any quantum computer. As described in the Jan. 8 issue of the journal Physical Review Letters, the work is the first demonstration of a quantum gate between two uncharged atoms.

The use of neutral atoms rather than charged ions or other materials distinguishes the achievement from previous work. “The current gold standard in experimental quantum computing has been set by trapped ions… People can run small programs now with up to eight ions in traps,” says Saffman.

However, to be useful for computing applications, systems must contain enough quantum bits, or qubits, to be capable of running long programs and handling more complex calculations. An ion-based system presents challenges for scaling up because ions are highly interactive with each other and their environment, making them difficult to control.

“Neutral atoms have the advantage that in their ground state they don’t talk to each other, so you can put more of them in a small region without having them interact with each other and cause problems,” Saffman says. “This is a step forward toward creating larger systems.”

The team used a combination of lasers, extreme cold (a fraction of a degree above absolute zero), and a powerful vacuum to immobilize two rubidium atoms within “optical traps.” They used another laser to excite the atoms to a high-energy state to create the CNOT quantum gate between the two atoms, also achieving a property called entanglement in which the states of the two atoms are linked such that measuring one provides information about the other.

Writing in the same journal issue, another team also entangled neutral atoms but without the CNOT gate. Creating the gate is advantageous because it allows more control over the states of the atoms, Saffman says, as well as demonstrating a fundamental aspect of an eventual quantum computer.

The Wisconsin group is now working toward arrays of up to 50 atoms to test the feasibility of scaling up their methods. They are also looking for ways to link qubits stored in atoms with qubits stored in light with an eye toward future communication applications, such as “quantum internets.”

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This work was funded by grants from the National Science Foundation, the Army Research Office and the Intelligence Advanced Research Projects Agency.

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.

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December 7, 2009

Introducing the one-atom transistor

Via KurzweilAI.net — This really is just amazing. A transistor made of one single atom of phosphorus.

Single-atom transistor created
KurzweilAI.net, Dec. 7, 2009

A working transistor whose active region comprises only of a single phosphorus atom in silicon has been built byresearchers from Helsinki University of Technology, University of New South Wales, and University of Melbourne.

The device uses sequential tunneling of single electrons between the phosphorus atom and the source and drain leads of the transistor. The tunneling can be suppressed or allowed by controlling the voltage on a nearby metal electrode with a width of a few tens of nanometers.

The researchers plan to use the spin degree of freedom of an electron of the phosphorus donor as a quantum bit (qubit). They were able to observe spin up and down states for a single phosphorus donor in a magnetic field for the first time–a crucial step towards the control of these states in realizing a qubit.


(a) Scanning electron microscope image of single-atomtransistor
(b) Differential conductance through the transistor with 4 Tesla magnetic field

More informationHelsinki University of Technology news

October 22, 2008

Moving toward quantum computing

Making headway, it seems. This is amazing — storing information inside the nucleus of an atom!

From the link:

The problem: How do you isolate a quantum bit from a noisy environment to protect the deli-cate quantum information, while at the same time allowing it to interact with the outside world so that it can be manipulated and measured?

The team, with scientists and engineers from Oxford and Princeton universities and Lawrence Berkeley National Laboratory, reported a solution to this problem in the Oct. 23 issue of the journal Nature.

The team’s plan was to devise a hybrid system using both the electron and nucleus of an atom of phosphorous embedded in a silicon crystal. Each behaves as a tiny quantum magnet capa-ble of storing quantum information, but inside the crystal the electron is more than a million times bigger than the nucleus, with a magnetic field that is a thousand times stronger. This makes the electron well-suited for manipulation and measurement, but not so good for storing information, which can become rapidly corrupted. This is where the nucleus comes in: when the information in the electron is ready for storage, it is moved into the nucleus where it can survive for much longer times.

Go below the fold for a release from October 23 on this story.

(more…)

February 27, 2008

Nanotechnology advances in quantum computing

Filed under: Science, Technology — Tags: , , , , , — David Kirkpatrick @ 11:52 pm

More cool nanotech. This time from the KurzweilAI.net newsletter. For reference, a qubit is a unit of quantum information.

Physicists Demonstrate Qubit-Qutrit Entanglement
PhysOrg.com, Feb. 26, 2008An international team of physicists entangled a qubit with its 3D equivalent, the “qutrit,” demonstrating a new way to handle higher-dimensional quantum information carriers.

Qubit-qutrit entanglement could lead to advantages in quantum computing, such as increased security and more efficient quantum gates, and enable novel tests of quantum mechanics. A qutrit is the quantum informationanalogue of the classical trit and carries more information: it exists in superpositions of its three basics states, while a qubit can exist in superpositions of its two states.
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