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/

Turning up the heat on quantum mechanics

Scientists have made a startling prediction about the quantum world that seems to show that simply taking the temperature of certain types of quantum systems at frequent intervals causes such systems to break one of the hard and fast rules of thermodynamics.

Anyone who has dabbled in quantum mechanics will know just how slippery is the atomic and sub-atomic world of probability wave-functions where particles eddy and swirl like waves.

Gershon Kurizki

Gershon Kurizki

One of the underlying rules of the quantum world is the Time-Energy Uncertainty Principle. Wrapped up in this apparently simple phrase is the notion that it is impossible to know both the precise duration of any process and its exact energy cost in an atomic or subatomic particle with 100 % certainty; the very act of observing one or the other somehow disturbing the counterpart property.

The quantum world is spooky, to say the least.

Quantum systems run hot and cold when you take their temperature regularly (Credit: Gershon Kurizki)

Quantum systems run hot and cold when you take their temperature regularly (Credit: Gershon Kurizki)

Now, the laws of thermodynamics are apparently irrefutable, after all they allow sceptics to see straight through the claims of those inventors who claim perpetual motion machines, they allow us to build power stations, and ultimately they will take us to the ends of the universe.

One law reveals that the interaction between a large heat source and a cluster of smaller systems will, on average, move progressively towards thermal equilibrium – hot moves to cold to even out the temperature, in other words; this is the so-called zero’th law of thermodynamics. But, it ain’t necessarily so in the quantum world claim Weizmann chemists Gershon Kurizki, Noam Erez and Goren Gordon of the Weizmann Institute in Rehovot, Israel, working with Mathias Nest of Potsdam University, Germany. They have shown that an ensemble of quantum systems in thermal contact with a large heat source could buck this thermodynamic trend.

Their predictions suggest that such a quantum ensemble could actually heat up even if it is hotter than a neighbouring large heat source or if it is colder, it could get colder still, but only under certain conditions. The scientists showed that if the energy of these systems is measured repeatedly, both systems and large heat source will undergo a temperature increase or decrease, and this change depends only on the rate of measurement, not on the results of the measurements themselves.

In the classical world, a thermometer does not interfere with the laws of thermodynamics no matter how hot or cold a system nor how often the thermometer is read, but taking the temperature of a quantum system somehow decouples it from the neighbouring heat source. This decoupling, followed by recoupling of the two when measurement ceases, introduces energy (at the expense of the measuring apparatus) into the systems and the heat source alike, and so heats them up. Depending on whether the measurements are repeated at short or long intervals, it should be possible to heat up or cool down the systems.

The predicted effects may be the key to developing novel heating and cooling schemes for microscopic solid-state devices, such as quantum computer chips or in allowing ultrafast temperature control for fast optical measurements in the chemistry laboratory.

Further reading

Nature, 2008, 452, 724-727
http://dx.doi.org/10.1038/nature06873

Weizmann Institute Quantum Optics Group homepage
http://www.weizmann.ac.il/chemphys/gershon/

Mathias Nest homepage
http://tcb16.chem.uni-potsdam.de/nest/

Suggested searches

thermodynamics
quantum mechanics

Wayz to go!

The ultrasound equivalent of a laser could lead to important new discoveries in materials science by providing researchers with a non-destructive way to detect even the subtlest of changes, such as phase transitions, deep in their samples. Now, researchers at the University of Illinois at Urbana-Champaign and at the University of Missouri-Rolla have built just such an ultrasound analogue of the laser – the uaser, pronounced way-zer.

Light amplification by stimulated emission of radiation devices, lasers, are well known, but a sonic analogue had until now not been developed. Richard Weaver and his colleagues set out to change that and to develop a device that would induce ultrasound amplification by stimulated emission of radiation to produce coherent ultrasonic waves of a single frequency. A sonic laser has been possible for some time now, Weaver told us, our method could have been done earlier. I tend to think it wasn’t for two reasons: first no-one saw an application and second few people are expert at both laser physics and ultrasonics.

Richard Weaver

Richard Weaver

Ultrasound penetrates solids without harmful ionizing radiation and researchers have been using it for many years to find cracks, assess material damage or material microstructure, and to look inside the human body. Conventional methods for making ultrasound are already well developed and are better for such applications. However it is in the field of conventional light lasers that the uaser might be used most effectively to study laser activity itself.

“We have demonstrated that the essential nature of a laser can be mimicked by classical mechanics – not quantum mechanics – in sound instead of light,” explains Weaver.

Alexey Yamilov

Alexey Yamilov

To make their uaser, Weaver, Illinois research associate Oleg Lobkis and UMR physics professor Alexey Yamilov mounted several piezoelectric auto-oscillators on to a block of aluminium. This would serve as an elastic, acoustic body. When they then apply an external acoustic source to the body, the oscillators do their thing and synchronize their oscillations to its tone. When the external source is switched off, the tiny ultrasonic transducers lock on to each other because they are connected to the same acoustic body, almost like stringing a guitar with six G strings and plucking just one, the other five would oscillate in unity with the same frequency. We exploit the fact that coherence and stimulated emission are classical concepts and, as such, can be applied to build a mechanical device, explains Yamilov.

An aluminium block interacts with electronic oscillators through piezoelectric transducers to make a sound laser.

An aluminium block interacts with electronic oscillators through piezoelectric transducers to make a sound laser.

By careful design of the transducers, the team can ensure that they all oscillate in phase, something that would be tantamount to plucking all six G strings at exactly the same time. “We can assure the correct phases and produce stimulated emission, Weaver explains, As a result, the power output scales with the square of the number of oscillators.”

The current design of uaser more closely resembles a “random laser” than it does a conventional, highly directional laser, Weaver concedes, “In principle, however, there is no reason why we shouldn’t be able to design a uaser to generate a narrow, highly directional beam.”

Nevertheless, uasers may be useful. With their longer wavelengths and more convenient frequencies, uasers could prove useful for modelling and studying laser dynamics. They could also serve as highly sensitive scientific tools for measuring the elastic properties and phase changes of modern materials, such as thin films or high-temperature superconductors.

“Uasers can produce an ultrasonic version of acoustical feedback – an ultrasonic howl similar to the squeal created when a microphone is placed too close to a speaker,” Weaver adds, “By slowly changing the temperature while monitoring the ultrasonic feedback frequency, we could precisely measure the phase change in various materials.”

Further reading

Alexey Yamilov
http://web.mst.edu/~yamilov/

Richard Weaver
http://physics.illinois.edu/people/profile.asp?r-weaver

Suggested searches

lasers
ultrasonics