Atomic circuitry and quantum computing

Conventional supercomputers have limitations: they are logical and fast, certainly, can be run in parallel grids across the globe, but when it comes down to solving problems with no logical answer, such as cracking sophisticated encryption, working out the travelling sales-rep problem of logistics and deliveries, or modelling the climate, they have serious limitations.

A quantum computer, on the other hand, could find all the answers almost instantaneously and pluck out the most appropriate based on probabilities and quantum mechanics. Building such a quantum computer is not proving simple. Now, US researchers have demonstrated that they can exert delicate control over a pair of atoms within a mere seven-millionths-of-a-second window that suggests the necessary atomic circuitry for a quantum computer might one day be possible.

“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 physicist Mark Saffman of the University of Wisconsin-Madison. “You then have to use the physics that describes atoms – quantum mechanics.” In the quantum realm, new possibilities for processing information emerge that mean certain types of problems could be solved exponentially faster on a quantum computer than on any foreseeable classical computer.

Mark Saffman
Mark Saffman

Working with colleague Thad Walker, Saffman and co-workers have successfully used atoms to create a controlled-NOT (CNOT) gate, a basic type of circuit that will be an essential element of any quantum computer. They describe details of the work in the journal Physical Review Letters and explain that this is the first demonstration of a quantum gate formed between two uncharged atoms.

The use of neutral rubidium atoms chilled to a fraction of a degree above absolute zero, rather than charged ions or other materials, distinguishes this 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,” explains 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 reactive, which makes them difficult to control.

Thad Walker
Thad Walker

“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 is now working towards arrays of up to 50 atoms to test the feasibility of scaling up the system.

LINKS

Phys. Rev. Lett. 2010, 104, 010503 http://prl.aps.org/abstract/PRL/v104/i1/e010503

Mark Saffman
http://hexagon.physics.wisc.edu/marksaffman.htm

Thad Walker
http://www.physics.wisc.edu/people/faculty/twalker/

3D astrophysics

Astrophysicists are using a novel 3D computer visualization technique to help them understand the role of gravity in the formation of vast, stellar nurseries, also known as molecular clouds.

Computer simulations are critical tools in understanding the behaviour of these clouds and of star formation, explains Alyssa Goodman of Harvard’s Faculty of Arts and Sciences. They are the only method by which astronomers can watch what happens over the millions of years it takes to form a star.

Alyssa Goodman

Alyssa Goodman

Earlier models of star formation assumed that because gravity is a relatively weak force over large distances that its effects would be negligible in these clouds until the particles are very close together. These earlier models thus explain accretion of particles prior to star formation based on turbulence rather than gravity. Once denser groupings of molecules are formed and gravity becomes a factor, they attract more and more particles until either something disrupts them or they have enough mass to collapse and form a star.

However, Goodman and colleagues have examined the process up to the point where the dense groupings form. Their analysis shows that, rather than turbulence being the only significant force pushing these gas molecules around, their gravitational influence on each other is also significant. That finding means that existing models, which leave gravity out until very dense clumps have formed, would over-predict the rate of star formation in these clouds.

This image shows a very long-exposure view of a 1-degree-square area within the Perseus star-forming region. (Credit: Jaime Pineda & Jonathan Foster, Harvard University)

This image shows a very long-exposure view of a 1-degree-square area within the Perseus star-forming region. (Credit: Jaime Pineda & Jonathan Foster, Harvard University)

The team developed imaging technology borrowed from medicine to visualize the molecular cloud in three dimensions and applied a new computer algorithm that creates a dendrogram, This branching representation of the astronomical data from COMPLETE (COordinated Molecular Probe Line Extinction Thermal Emission) Survey of Star Forming Regions, allowed the researchers to build a sophisticated 3D display of the data, which they could then rotate and examine from many different angles.

Goodman explains that the earlier modelling technology ignores hierarchical structure, regions within regions, and so obscures specific details of the molecular clouds, such as nested areas of varying density and physical breaks from one area to another. There’s no way of noticing this without being able to see this in 3-D, she says.

Further reading

Nature, 2009, 457, 63-66
http://dx.doi.org/10.1038/nature07609

Alyssa Goodman homepage
http://www.cfa.harvard.edu/~agoodman/

Suggested searches

stars formation

Bottom up to nanotech

Two back-to-back papers published by IBM scientists could herald the long-awaited advent of molecular or nanotech computing devices.

There has been much hyperbole written about the potential of nanoscience and supramolecular compounds that could one day be used as the building blocks of computer memory and other devices. But, at the end of August, computing pioneer IBM announced that it has passed the first two milestones in understanding atomic magnetism, bringing single-atom data storage closer to reality and in how to control single molecules as tiny switches. The exploratory research, published in the journal Science hints for the first time at a realistic strategy for developing nanotech computing with devices built from clusters of atoms or molecules.

Schematic tunnelling logical molecules (Courtesy of IBM)

Schematic tunnelling logical molecules (Courtesy of IBM)

In the first report from IBM’s Almaden laboratory, the scientists describe major progress in probing a property called magnetic anisotropy in individual atoms. This fundamental measurement has important technological consequences because it determines an atom’s ability to store information. Previously, nobody had been able to measure the magnetic anisotropy of a single atom.

This work suggests that it may be plausible to build structures consisting of small clusters of atoms, or even individual atoms, which could reliably store magnetic information or lead to entirely novel devices that go beyond conventional computing.

Single-Molecule Logic Switch (Courtesy of IBM)

Single-Molecule Logic Switch (Courtesy of IBM)

In the second paper, IBM’s Zurich team describe the first single-molecule switch that operates flawlessly without disruption of the molecule’s overall structure. Moreover, the team demonstrated that they could switch atoms within one molecule using atoms in an adjacent molecule, which represents a rudimentary logic element, made possible only because the molecular framework is left undisturbed by the switching process.

IBM says both research threads represent a significant step towards building molecular-scale computing elements that are far smaller, faster and use less energy than any current semiconductor technology.

Further reading

Science, 2007, 317, 1199-1203;
http://dx.doi.org/10.1126/science.1146110

Science, 2007, 317, 1141;
http://dx.doi.org/10.1126/science.317.5842.1141a

Suggested searches

nanoscience
supramolecular