APS physics tip sheet #62

May 23, 2006

A Quantum CPU (the Pentium Q?)
M-H Yung et al.
Physical Review Letters (upcoming article, available to journalists on request)

A new design scheme for a quantum processor core makes potential quantum computers more technically feasible, more efficient, and in many cases faster by keeping all of the quantum bits active all the time, rather than switching them off and on as in most quantum computer designs.

Typical computers store and manipulate information as bits - that is 0's and 1's. Quantum computers are made of quantum bits, or qubits, that are encoded as a superposition of the values 0 and 1 at the same time. In addition, quantum mechanics allows qubits to become entangled, which smears information out among multiple qubits.

Previous schemes for making a quantum computer have sought to harness this process by keeping qubits under strict control - only letting them communicate with each other occasionally. But such tight constraints are hard to achieve in the lab, and experimental progress has been slow. The new idea shows that researchers don't need to be so controlling. Instead they can assemble a processor core where qubits are active all the time, continuously and freely talking with all their neighbors. The whole core becomes entangled and the qubits record and manipulate data as a group. The key to making the new design work is a separate storage bank of qubits that swap information in and out of the quantum processor core.

Although the new design should be easier to implement than other quantum computer layouts, the always-on processor core has yet to be realized in the lab. When researchers iron out all the difficulties, quantum computers - based either on the quantum processor core or other designs - will outperform their classical counterparts in a variety of calculations such as simulations of problems that are inherently quantum mechanical (including many nanoscopic, molecular, and biophysical problems, to name a few). They would also be good at factoring large numbers and tackling other mathematical problems that would take eons for even the most powerful classical computers imaginable to solve.

What will You Do to Avoid the Flu?
T. Gross et al.
Physical Review Letters (upcoming article, available to journalists on request)

Chances are, most people will take some precautions to avoid getting sick if the Avian Flu ever makes the leap to human populations. Staying home from work or school, avoiding crowded stores and theaters, and eating in instead of going out to restaurants change the way people are linked together through social interactions. As a result, epidemiological models based on our normal social networks may fail at predicting the threat and propagation of diseases like a pandemic flu. Researchers in Germany have now devised a model of social networks that takes into account the fact that we are likely to sever ties to infected people and add ties to uninfected people to reduce our risks of contagious diseases.

Although the model radically simplified the rationales people would use to make such decisions, it showed that rewiring of social networks may reduce a population's susceptibility to some epidemics. That is, individuals who take some precautions to avoid infection help protect their whole social network. On the other hand, the model also showed that uninfected people may increase their connections to other healthy people, leading to islands of highly connected groups that are potentially susceptible to the disease. This sort of rewiring can stymie programs that target vaccinations to stem epidemics, if the programs are based on assumptions about social network structures that do not take spontaneous rewiring of social connections into account.

In addition to pandemics, the researchers believe that their model may also help us to understand the spread of information, opinions, and beliefs in populations.

Brownian Refrigerator
C. Van den Brock and R Kawai
Physical Review Letters (upcoming article, available to journalists on request)

Brownian motion is the random jitter of small particles that are bumped about by molecules. Brownian motion can be harnessed to create microscopic motors that are driven by molecular battering (for a recent example of an experimentally realized Brownian motor see Phys. Rev. Lett. 96, 190602 (2006), http://link.aps.org/abstract/PRL/v96/e190602). Now researchers at Hasselt University in Belgium and the University of Alabama in Birmingham have theorized that Brownian motion might be useful in designing molecular-scale refrigerators. Potentially, a Brownian refrigerator could help cool nanoscale machines or control the heat flow in molecular biology experiments.

Differences in the temperatures of molecules in two regions lead to the heat flow that drives Brownian motors. Therefore, the researchers propose, using some external force to drive a Brownian motor in reverse could cause heat to flow from a colder region to a warmer one, much as household heat pumps cool homes with a motor that moves heat outside. Unlike heat pumps that move heat by compressing and expanding large volumes of fluids, a Brownian refrigerator controls heat flow heat by striking molecules to speed them up or slow them down.

The researchers propose a theoretical model of their refrigerator that consists of a rod piercing an insulating membrane. One end of the rod has an arrangement of flat paddles, much like those on a paddle wheel boat. On the other end the paddles are replaced with wedge shapes. Left to itself, the structure will spin, provided the molecules surrounding the wedges are warmer than the molecules surrounding the paddles, and heat will be moved through to the cooler side in the process of running the motor. If the motor could be forced to run backward it would move heat in the other direction - from the cool side to the hot side - forming the smallest possible refrigerator. The authors do not explicitly address the source of the force turning the Brownian refrigerator's rotor, presumably leaving that challenge to future experimentalists.

American Physical Society

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