Quantum computer built inside a diamond

ScienceDaily (Apr. 4, 2012) — Diamonds are forever -- or, at least, the effects of this diamond on quantum computing may be. A team that includes scientists from USC has built a quantum computer in a diamond, the first of its kind to include protection against "decoherence" -- noise that prevents the computer from functioning properly.

The demonstration shows the viability of solid-state quantum computers, which -- unlike earlier gas- and liquid-state systems -- may represent the future of quantum computing because they can be easily scaled up in size. Current quantum computers are typically very small and -- though impressive -- cannot yet compete with the speed of larger, traditional computers.

The multinational team included USC Professor Daniel Lidar and USC postdoctoral researcher Zhihui Wang, as well as researchers from the Delft University of Technology in the Netherlands, Iowa State University and the University of California, Santa Barbara. Their findings will be published on April 5 in Nature.

The team's diamond quantum computer system featured two quantum bits (called "qubits"), made of subatomic particles.

As opposed to traditional computer bits, which can encode distinctly either a one or a zero, qubits can encode a one and a zero at the same time. This property, called superposition, along with the ability of quantum states to "tunnel" through energy barriers, will some day allow quantum computers to perform optimization calculations much faster than traditional computers.

Like all diamonds, the diamond used by the researchers has impurities -- things other than carbon. The more impurities in a diamond, the less attractive it is as a piece of jewelry, because it makes the crystal appear cloudy.

The team, however, utilized the impurities themselves.

A rogue nitrogen nucleus became the first qubit. In a second flaw sat an electron, which became the second qubit. (Though put more accurately, the "spin" of each of these subatomic particles was used as the qubit.)

Electrons are smaller than nuclei and perform computations much more quickly, but also fall victim more quickly to "decoherence." A qubit based on a nucleus, which is large, is much more stable but slower.

"A nucleus has a long decoherence time -- in the milliseconds. You can think of it as very sluggish," said Lidar, who holds a joint appointment with the USC Viterbi School of Engineering and the USC Dornsife College of Letters, Arts and Sciences.

Though solid-state computing systems have existed before, this was the first to incorporate decoherence protection -- using microwave pulses to continually switch the direction of the electron spin rotation.

"It's a little like time travel," Lidar said, because switching the direction of rotation time-reverses the inconsistencies in motion as the qubits move back to their original position.

The team was able to demonstrate that their diamond-encased system does indeed operate in a quantum fashion by seeing how closely it matched "Grover's algorithm."

The algorithm is not new -- Lov Grover of Bell Labs invented it in 1996 -- but it shows the promise of quantum computing.

The test is a search of an unsorted database, akin to being told to search for a name in a phone book when you've only been given the phone number.

Sometimes you'd miraculously find it on the first try, other times you might have to search through the entire book to find it. If you did the search countless times, on average, you'd find the name you were looking for after searching through half of the phone book.

Mathematically, this can be expressed by saying you'd find the correct choice in X/2 tries -- if X is the number of total choices you have to search through. So, with four choices total, you'll find the correct one after two tries on average.

A quantum computer, using the properties of superposition, can find the correct choice much more quickly. The mathematics behind it are complicated, but in practical terms, a quantum computer searching through an unsorted list of four choices will find the correct choice on the first try, every time.

Though not perfect, the new computer picked the correct choice on the first try about 95 percent of the time -- enough to demonstrate that it operates in a quantum fashion.

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The above story is reprinted from materials provided by University of Southern California, via EurekAlert!, a service of AAAS.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

T. van der Sar, Z. H. Wang, M. S. Blok, H. Bernien, T. H. Taminiau, D. M. Toyli, D. A. Lidar, D. D. Awschalom, R. Hanson, V. V. Dobrovitski. Decoherence-protected quantum gates for a hybrid solid-state spin register. Nature, 2012; 484 (7392): 82 DOI: 10.1038/nature10900

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It's the network: Ever wonder why your friends have more friends than you or diamond is harder than graphite?

ScienceDaily (Apr. 19, 2012) — A new article by a Northwestern University complex networks expert discusses how networks governing processes in nature and society are becoming increasingly amenable to modeling, forecast and control.

The article establishes relationships between seemingly disparate topics such as the friendship paradox -- by which our friends have on average more friends than we do -- and why carbon can result in a hard diamond or the softer material graphite.

"Many broadly significant scientific questions, ranging from self-organization and information flow to systemic robustness, can now be properly formalized within the emerging theory of networks," said Adilson E. Motter, the Harold H. and Virginia Anderson Professor of Physics and Astronomy at Northwestern's Weinberg College of Arts and Sciences. "I was thus humbled to be invited to write such a timely piece."

Motter is first author of the article "Networks in Motion," published last week as the cover story in Physics Today, the membership journal of the American Institute of Physics. His co-author is Réka Albert, professor of physics and biology at Penn State University.

The authors argue that, as network research matures, there will be increasing opportunities to exploit network concepts to also engineer new systems with desirable properties that may not be readily available in existing ones. Examples include emerging areas such as synthetic biology and microfluidics, which could be radically changed by rational circuit design, but also established areas such as traffic and materials research.

Motter and Albert consider the problem of network control, particularly in the context of biological networks as a promising new avenue for disease treatment. Cascading processes, in particular, in which successive elements in a complex network fail, are shown to be not as unstoppable as previously thought.

They also discuss at length how collective behavior may depend on properties of the underlying network, even when composed of the exact same nodes -- as in the case of radically different materials made of the same chemical element.

By and large, the recent study of complex systems has been centered on the identification and analysis of network features relevant to a particular phenomenon of interest, ultimately reducing complexity. But, the authors ask, with so many conceivable possibilities, what if one simply fails to look for the right features? Researchers have been thinking about this, too, and, as a result, exploratory methods are now being devised to identify patterns not anticipated by pre-conceptions.

One such method mentioned in the article aims at resolving the internal structure of complex networks by organizing the nodes into groups that share something in common, even if researchers do not know a priori what that thing is.

"This is, of course, only the very tip of the iceberg," Motter said. "A broader undertaking concerns the development of similar exploratory approaches that can also systematically account for network dynamics, which remains widely unexplored."

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The above story is reprinted from materials provided by Northwestern University. The original article was written by Megan Fellman.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Adilson E. Motter, Re´ka Albert. Networks in motion. Physics Today, 2012; 65 (4): 43 DOI: 10.1063/PT.3.1518

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here