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/

Golden cat

A more environmentally friendly way to make ethylene (a primary feedstock for the chemical industry, which also goes by the name of ethene) would use natural gas as the raw material rather than cracking crude oil. Now, a golden opportunity in the form of a two-centred gold complex has come to light.

Ethylene is one of the chemical industry’s primary feedstock materials from which a whole range of different compounds are made, among them the coolant and antifreeze compound ethylene glycol and plastics such as polyethylene. Ethylene is currently produced from oil by steam cracking. However, direct conversion of natural gas, methane, to ethylene could also be viable on an industrial scale if a suitable catalyst and reaction setup were developed. One of the big advantages of such an approach is that methane supplies are likely to provide us with raw material long after oil has dwindled.

Scientists working with Thorsten Bernhardt at the University of Ulm, Germany and Uzi Landman at the Georgia Institute of Technology, in Atlanta, Georgia, USA, explain that the problem with the conversion of methane to ethylene is one of bond strength. The bonds between carbon and hydrogen atoms in the methane molecule, are very strong and so difficult to break. To compel the carbon in methane to form bonds with other carbon atoms requires a lot of energy. Additionally, any efforts to do so, usually produce lots of side-products containing more than the requisite two carbon atoms found in ethylene.

Activating methane to allow it to form bigger molecules, such as ethylene, is a complex process. The team explains that in order to find a suitable catalyst a clearer understanding of the process at the molecular level is needed. As such Bernhardt and colleagues investigated different catalytic metal clusters, aggregates of a few metal atoms, as model systems for the process. They found that particles containing two charged gold atoms (ions) in pairs could convert methane to ethylene at low temperatures and pressures in the gas phase without forming significant quantities of side-products.

methane to ethane using a golden cat (Credit: Bernhardt et al/Wiley-VCH)
methane to ethane using a golden cat (Credit: Bernhardt et al/Wiley-VCH)

In order to understand how this process works, the team “trapped” the reaction intermediates and used computational techniques to model the energy levels of starting materials, catalyst, intermediates, and products. They found that each gold ion in the dimer binds to a single methane molecule, this severs hydrogen from the methane and the two carbon atoms can then form a single bond to each other. This is a precursor to ethylene itself which has a strong double bond between its carbon atoms. The precursor initially remains bound to one of the gold ions, while the second gold ion, suddenly finds itself able to bond to a new methane molecule. In the final step, this second methane molecule nudges away the ethylene precursor, the carbon single bond, which can then form a carbon double bond. The newly trapped methane then pairs up with yet another methane and the cycle begins again.

“Both the activation of the carbon-hydrogen bonds of the methane and the subsequent splitting off of the ethylene molecule require cooperative action of several atoms bound to the gold dimer,” Berhnardt explains. “Our insights are not only of fundamental interest, but may also be of practical use.”

LINKS

Angew Chem Int Edn 2010, 49, 980-983
http://dx.doi.org/10.1002/anie.200905643

Bernhardt
http://www.uni-ulm.de/iok/bernhardt/

Black hole

The European Southern Observatory’s Very Large Telescope (VLT) has helped an international team of astronomers to detect a stellar mass black hole that lies at a much greater distance from Earth than any observed before. The black hole is in the spiral galaxy NGC 300, about six million light years away in the constellation Sculptor.

The spiral galaxy NGC 300 lying in the constellation Sculptor (Credit: Galex/NASA)
The spiral galaxy NGC 300 lying in the constellation Sculptor (Credit: Galex/NASA)

Paul Crowther and Vik Dhillon, of the University of Sheffield, UK, Robin Barnard and Simon Clark of the The Open University, Milton Keynes, UK, and Stefania Carpano and Andy Pollock of ESAC, in Madrid, Spain report the black hole which has a mass of about twenty times that of the Sun in the Monthly Notices of the Royal Astronomical Society.

The stellar-mass black holes found in our Milky Way galaxy commonly weigh up to ten times the mass of the Sun. The newly discovered black hole is not only the most distant, but the second most massive stellar-mass black hole ever found. It is also entwined with a star that will soon become a black hole itself.

Lead author Crowther, explains: “This is the most distant stellar-mass black hole ever weighed, and it’s the first one we’ve seen outside our own galactic neighbourhood, the Local Group. The black hole’s curious partner is a Wolf-Rayet star, which also has a mass of about twenty times as much as the Sun. Wolf-Rayet stars are near the end of their lives and expel most of their outer layers into their surroundings before exploding as supernovae, with their cores imploding to form black holes.

An artist's impression of the newly discovered black hole and its stellar companion (Credit: ESO/L. Calçada)
An artist's impression of the newly discovered black hole and its stellar companion (Credit: ESO/L. Calçada)

In less than a million years, a blink of the eye cosmologically speaking, the Wolf-Rayet star will explode as a supernova and its remnants collapse into a black hole. Only one other system of this type has previously been seen, but other systems comprising a black hole and a companion star are not unknown to astronomy. The existence of such systems hints at an underlying galactic chemistry. Astronomers believe that a higher concentration of heavy chemical elements influences how a massive star evolves, increasing how much matter it sheds, resulting in a smaller black hole when the remnant finally collapses.

LINKS

Monthly Notices Royal Astronom Soc, 2010, in press
http://www.eso.org/public/archives/releases/sciencepapers/eso1004/eso1004.pdf

Paul Crowther
http://pacrowther.staff.shef.ac.uk/main.html