Quantum Knowledge Cools Computers: New Understanding of Entropy

Quantum Knowledge Cools Computers: New Understanding of Entropy

  9:20:00 PM    Science Lover

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From a laptop warming a knee to a supercomputer heating a room, the idea that computers generate heat is familiar to everyone. But theoretical physicists have discovered something astonishing: not only do computational processes sometimes generate no heat, under certain conditions they can even have a cooling effect. Behind this finding are fundamental considerations relating to knowledge and a lack of knowledge. The researchers publish their findings in the journal Nature.

According to the latest theoretical studies, the ever-
increasing energy costs caused by supercomputers of 
the kind operated at the Swiss National Scientific 
Computing Centre in Manno (canton of Ticino) 
could be reduced. However, this would need 
a quantum computer. (Credit: Michele 
De Lorenzi / CSCS)

When computers compute, the energy they consume eventually ends up as heat. This isn't all due to the engineering of the computer -- physics has something to say about the fundamental energy cost of processing information.

Recent research by a team of physicists reveals a surprise at this fundamental level. ETH-Professor Renato Renner, and Vlatko Vedral of the Centre for Quantum Technologies at the National University of Singapore and the University of Oxford, UK, and their colleagues describe in the scientific journal Nature how the deletion of data, under certain conditions, can create a cooling effect instead of generating heat. The cooling effect appears when the strange quantum phenomenon of entanglement is invoked. Ultimately, it may be possible to harness this effect to cool supercomputers that have their performance held back by heat generation. "Achieving the control at the quantum level that would be required to implement this in supercomputers is a huge technological challenge, but it may not be impossible. We have seen enormous progress is quantum technologies over the past 20 years," says Vedral. With the technology in quantum physics labs today, it should be possible to do a proof of principle experiment on a few bits of data.

Landauer's principle is given a quantum twist

The physicist Rolf Landauer calculated back in 1961 that during the deletion of data, some release of energy in the form of heat is unavoidable. Landauer's principle implies that when a certain number of arithmetical operations per second have been exceeded, the computer will produce so much heat that the heat is impossible to dissipate. In supercomputers today other sources of heat are more significant, but Renner thinks that the critical threshold where Landauer's erasure heat becomes important may be reached within the next 10 to 20 years. The heat emission from the deletion of a ten terabyte hard-drive amounts in principle to less than a millionth of a joule. However, if such a deletion process were repeated many times per second then the heat would accumulate correspondingly.

The new study revisits Landauer's principle for cases when the values of the bits to be deleted may be known. When the memory content is known, it should be possible to delete the bits in such a manner that it is theoretically possible to re-create them. It has previously been shown that such reversible deletion would generate no heat. In the new paper, the researchers go a step further. They show that when the bits to be deleted are quantum-mechanically entangled with the state of an observer, then the observer could even withdraw heat from the system while deleting the bits. Entanglement links the observer's state to that of the computer in such a way that they know more about the memory than is possible in classical physics.

Similar formulas -- two disciplines

In order to reach this result, the scientists combined ideas from information theory and thermodynamics about a concept known as entropy. Entropy appears differently in these two disciplines, which are, to a large extent, independent of each other. In information theory, entropy is a measurement of the information density. It describes, for instance, how much memory capacity a given set of data would take up when compressed optimally. In thermodynamics, on the other hand, entropy relates to the disorder in systems, for example to the arrangement of molecules in a gas. In thermodynamics, adding entropy to a system is usually equivalent to adding energy as heat.

The ETH physicist Renner says "We have now shown that in both cases, the term entropy is actually describing the same thing even in the quantum mechanical regime." As the formulas for the two entropies look the same, it had already been assumed that there was a connection between them. "Our study shows that in both cases, entropy is considered to be a type of lack of knowledge," says Renner. The new paper in Nature builds on work published earlier in the New Journal of Physics.

In measuring entropy, one should bear in mind that an object does not have a certain amount of entropy per se, instead an object's entropy is always dependent on the observer. Applied to the example of deleting data, this means that if two individuals delete data in a memory and one has more knowledge of this data, she perceives the memory to have lower entropy and can then delete the memory using less energy. Entropy in quantum physics has the unusual property of sometimes being negative when calculated from the information theory point of view. Perfect classical knowledge of a system means the observer perceives it to have zero entropy. This corresponds to the memory of the observer and that of the system being perfectly correlated, as much as allowed in classical physics. Entanglement gives the observer „more than complete knowledge" because quantum correlations are stronger than classical correlations. This leads to an entropy less than zero. Until now, theoretical physicists had used this negative entropy in calculations without understanding what it might mean in thermodynamic terms or experimentally.

No heat, even a cooling effect

In the case of perfect classical knowledge of a computer memory (zero entropy), deletion of the data requires in theory no energy at all. The researchers prove that "more than complete knowledge" from quantum entanglement with the memory (negative entropy) leads to deletion of the data being accompanied by removal of heat from the computer and its release as usable energy. This is the physical meaning of negative entropy.

Renner emphasizes, however, "This doesn't mean that we can develop a perpetual motion machine." The data can only be deleted once, so there is no possibility to continue to generate energy. The process also destroys the entanglement, and it would take an input of energy to reset the system to its starting state. The equations are consistent with what's known as the second law of thermodynamics: the idea that the entropy of the universe can never decrease. Vedral says "We're working on the edge of the second law. If you go any further, you will break it."

Fundamental findings

The scientists' new findings relating to entropy in thermodynamics and information theory may have usefulness beyond calculating the heat production of computers. For example, methods developed within information theory to handle entropy could lead to innovations in thermodynamics. The connection made between the two concepts of entropy is fundamental.

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Silicon Can Be Made to Melt in Reverse

Silicon Can Be Made to Melt in Reverse

  9:39:00 PM    Science Lover

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Like an ice cube on a warm day, most materials melt -- that is, change from a solid to a liquid state -- as they get warmer. But a few oddball materials do the reverse: They melt as they get cooler. Now a team of researchers at MIT has found that silicon, the most widely used material for computer chips and solar cells, can exhibit this strange property of "retrograde melting" when it contains high concentrations of certain metals dissolved in it.

A tiny silicon chip -- the glowing orange square at the center of this special heating device -- is heated to a temperature well below silicon's melting point, and then very slowly cooled down. The chip inside this heating device was placed in the path of a synchrotron beam to probe its changes at a molecular level as it went through the retrograde melting process. (Credit: Patrick Gillooly)

The material, a compound of silicon, copper, nickel and iron, "melts" (actually turning from a solid to a slush-like mix of solid and liquid material) as it cools below 900 degrees Celsius, whereas silicon ordinarily melts at 1414 degrees C. The much lower temperatures make it possible to observe the behavior of the material during melting, based on specialized X-ray fluorescence microprobe technology using a synchrotron -- a type of particle accelerator -- as a source.

The material and its properties are described in a paper just published online in the journal Advanced Materials. Team leader Tonio Buonassisi, the SMA Assistant Professor of Mechanical Engineering and Manufacturing, is the senior author, and the lead authors are Steve Hudelson MS '09, and postdoctoral fellow Bonna Newman PhD '08.

The findings could be useful in lowering the cost of manufacturing some silicon-based devices, especially those in which tiny amounts of impurities can significantly reduce performance. In the material that Buonassisi and his researchers studied, impurities tend to migrate to the liquid portion, leaving regions of purer silicon behind. This could make it possible to produce some silicon-based devices, such as solar cells, using a less pure, and therefore less expensive, grade of silicon that would be purified during the manufacturing process.

"If you can create little liquid droplets inside a block of silicon, they serve like little vacuum cleaners to suck up impurities," Buonassisi says. This research could also lead to new methods for making arrays of silicon nanowires -- tiny tubes that are highly conductive to heat and electricity.

Buonassisi predicted in a 2007 paper that it should be possible to induce retrograde melting in silicon, but the conditions needed to produce such a state, and to study it at a microscopic level, are highly specialized and have only recently become available. To create the right conditions, Buonassisi and his team had to adapt a microscope "hot-stage" device that allowed the researchers to precisely control the rate of heating and cooling. And to actually observe what was happening as the material was heated and cooled, they drew upon high-power synchrotron-based X-ray sources at Lawrence Berkeley National Laboratory in California and at Argonne National Laboratory in Illinois (researchers from both national labs are co-authors of the paper).

The material for the tests consisted of a kind of sandwich made from two thin layers of silicon, with a filling of copper, nickel and iron between them. This was first heated enough to cause the metals to dissolve into the silicon, but below silicon's melting point. The amount of metal was such that the silicon became supersaturated -- that is, more of the metal was dissolved in the silicon than would normally be possible under stable conditions. For example, when a liquid is heated, it can dissolve more of another material, but then when cooled down it can become supersaturated, until the excess material precipitates out.

In this case, where the metals were dissolved into the solid silicon, "if you begin cooling it down, you hit a point where you induce precipitation, and it has no choice but to precipitate out in a liquid phase," Buonassisi says. It is at that point that the material melts.

Funding was provided by the U.S. Department of Energy, the National Science Foundation, the Clare Booth Luce Foundation, Doug Spreng and the Chesonis Family Foundation, and some equipment was provided by McCrone Scientific

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Nanowick at Heart of New System to Cool 'Power Electronics'

Nanowick at Heart of New System to Cool 'Power Electronics'

  6:07:00 PM    Science Lover

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Researchers have shown that an advanced cooling technology being developed for high-power electronics in military and automotive systems is capable of handling roughly 10 times the heat generated by conventional computer chips.

This is a test facility for nanowicks. (Credit: Purdue 
University School of Mechanical Engineering)

The miniature, lightweight device uses tiny copper spheres and carbon nanotubes to passively wick a coolant toward hot electronics, said Suresh V. Garimella, the R. Eugene and Susie E. Goodson Distinguished Professor of Mechanical Engineering at Purdue University.

This wicking technology represents the heart of a new ultrathin "thermal ground plane," a flat, hollow plate containing water.

Similar "heat pipes" have been in use for more than two decades and are found in laptop computers. However, they are limited to cooling about 50 watts per square centimeter, which is good enough for standard computer chips but not for "power electronics" in military weapons systems and hybrid and electric vehicles, Garimella said.

The research team from Purdue, Thermacore Inc. and Georgia Tech Research Institute is led by Raytheon Co., creating the compact cooling technology in work funded by the Defense Advanced Research Projects Agency, or DARPA.

The team is working to create heat pipes about one-fifth the thickness of commercial heat pipes and covering a larger area than the conventional devices, allowing them to provide far greater heat dissipation.

New findings indicate the wicking system that makes the technology possible absorbs more than 550 watts per square centimeter, or about 10 times the heat generated by conventional chips. This is more than enough cooling capacity for the power-electronics applications, Garimella said.

The findings are detailed in a research paper appearing online this month in the International Journal of Heat and Mass Transfer and will be published in the journal's September issue. The paper was written by mechanical engineering doctoral student Justin Weibel, Garimella and Mark North, an engineer with Thermacore, a producer of commercial heat pipes located in Lancaster, Pa.

"We know the wicking part of the system is working well, so we now need to make sure the rest of the system works," North said.

The new type of cooling system can be used to prevent overheating of devices called insulated gate bipolar transistors, high-power switching transistors used in hybrid and electric vehicles. The chips are required to drive electric motors, switching large amounts of power from the battery pack to electrical coils needed to accelerate a vehicle from zero to 60 mph in 10 seconds or less.

Potential military applications include advanced systems such as radar, lasers and electronics in aircraft and vehicles. The chips used in the automotive and military applications generate 300 watts per square centimeter or more.

Researchers are studying the cooling system using a novel test facility developed by Weibel that mimics conditions inside a real heat pipe.

"The wick needs to be a good transporter of liquid but also a very good conductor of heat," Weibel said. "So the research focuses largely on determining how the thickness of the wick and size of copper particles affect the conduction of heat."

Computational models for the project were created by Garimella in collaboration with Jayathi Y. Murthy, a Purdue professor of mechanical engineering, and doctoral student Ram Ranjan. The carbon nanotubes were produced and studied at the university's Birck Nanotechnology Center in work led by mechanical engineering professor Timothy Fisher.

"We have validated the models against experiments, and we are conducting further experiments to more fully explore the results of simulations," Garimella said.

Inside the cooling system, water circulates as it is heated, boils and turns into a vapor in a component called the evaporator. The water then turns back to a liquid in another part of the heat pipe called the condenser.

The wick eliminates the need for a pump because it draws away fluid from the condenser side and transports it to the evaporator side of the flat device, Garimella said.

Allowing a liquid to boil dramatically increases how much heat can be removed compared to simply heating a liquid to temperatures below its boiling point. Understanding precisely how fluid boils in tiny pores and channels is helping the engineers improve such cooling systems.

The wicking part of the heat pipe is created by sintering, or fusing together tiny copper spheres with heat. Liquid is drawn sponge-like through spaces, or pores, between the copper particles by a phenomenon called capillary wicking. The smaller the pores, the greater the drawing power of the material, Garimella said.

Such sintered materials are used in commercial heat pipes, but the researchers are improving them by creating smaller pores and also by adding the carbon nanotubes.

"For high drawing power, you need small pores," Garimella said. "The problem is that if you make the pores very fine and densely spaced, the liquid faces a lot of frictional resistance and doesn't want to flow. So the permeability of the wick is also important."

The researchers are creating smaller pores by "nanostructuring" the material with carbon nanotubes, which have a diameter of about 50 nanometers, or billionths of a meter. However, carbon nanotubes are naturally hydrophobic, hindering their wicking ability, so they were coated with copper using a device called an electron beam evaporator.

"We have made great progress in understanding and designing the wick structures for this application and measuring their performance," said Garimella. He said that once ongoing efforts at packaging the new wicks into heat pipe systems that serve as the thermal ground plane are complete, devices based on the research could be in commercial use within a few years.

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