7 March 2012—Physicists in Europe have experimentally demonstrated, for the first time, that a theoretical principle limiting modern-day computing is real.
In 1961, Rolf Landauer posited that the act of erasing a bit of information gives off an amount of heat related to the temperature and Boltzmann’s constant—a total of 3 x 10-21 joules at room temperature. Among other things, the theory has been used to address the famous problem of Maxwell’s Demon, a thought experiment that suggested a minute monster could create energy for free by sorting particles by their speed, in apparent violation of the laws of thermodynamics. Prior to 1982, when IBM’s Charles Bennett applied Landauer’s theory to the problem, the thinking was that the demon’s act of making measurements produced heat, eliminating the violation. But Bennett argued that heat was produced because the demon had to erase a bit of information in its memory in order to sort each particle.
Though it sounds like a philosophical argument, the theory has real implications for computing. And debate over its validity, some researchers claim, has influenced the direction of computing and semiconductor research in the past decade.
Eric Lutz, who was at the University of Augsburg when the research was conducted, and a group of European scientists, set out to demonstrate Landauer’s limit by building his thought experiment in a real system. Landauer imagined a memory consisting of a single particle. The particle could be in either of two wells of potential energy. (Imagine a W with a dot in either of the V s.) If the particle is in the left-hand side, the bit is 0; if it’s in the right-hand side, the bit is 1. The bit is erased by forcing it from an unknown starting point into the 1 well.
“It’s the simplest memory you can think of,” says Lutz. “It’s completely generic.”
Lutz and his colleagues built this system using a 2-micrometer-wide glass bead in water as the particle and created the potential wells using a modified version of a laboratory instrument called optical tweezers. The tweezers are a laser system that holds particles in place at the laser’s focal point. The researchers made two wells by alternating the focus between two points. Erasing the bit involves first manipulating the laser to lower the barrier between the wells—imagine the W looking more like a U—then tilting the whole setup slightly to the right so the bead will roll in that direction, and finally reestablishing the potential barrier.
At 3 x 10-21 J, the heat predicted to be released by this action is so small that no laboratory calorimeter could measure it. However, the measurement can be derived by closely following the bead’s trajectory as the bit is erased. And that’s what Lutz, who is now at Freie Universität Berlin, and his collaborators at École Normale Supérieure de Lyon, in France, and the University of Kaiserslautern, in Germany, were able to do. In this week’s issue of Nature, they reported that the heat released was exactly as Landauer predicted.
“This is beautiful experimental work,” says IEEE Fellow Mark Lundstrom, a professor of electrical and computer engineering at Purdue University and an expert on the limits of nanodevices. “It’s remarkable that Landauer’s gedanken [thought] experiment can now actually be done.”
The energy involved in computing today, Lundstrom points out, is hundreds of thousands of times greater than the Landauer limit. Today’s computers use CMOS electronics to implement what’s called irreversible logic—you cannot run the process backward to reproduce the input. By its nature, this kind of logic destroys information with every cycle of the processor clock. In 2000, IEEE Medal of Honor winner James Meindl calculated that the smallest amount of energy such a CMOS computer would need is limited by Landauer’s principle. The new work from Europe proves it experimentally, showing that Landauer’s limit is real for any irreversible operation.
But irreversible logic isn’t the only way to compute. An experimental form of computing called reversible logic doesn’t necessitate destroying information. So, in principle, it could dodge Landauer’s limit.
According to Gregory L. Snider, professor of electrical engineering at the University of Notre Dame, in Indiana, reversible logic does just that. In results to be published in the Japanese Journal of Applied Physics, he and his colleagues demonstrate an electronic reversible logic system that gives off less heat than Landauer’s limit. “There is no limit if you’re doing it reversibly,” says Snider.
As part of the same research, Snider says his group also demonstrates that Landauer’s limit applies in an irreversible electronic system, backing up Lutz’s work in a different kind of experiment.
Reversible electronic logic has received short shrift in the past decade, according to Snider. Influential refutations of Landauer’s principle by Ralph Cavin and others at Semiconductor Research Corp. at the beginning of the last decade, argued that even reversible logic could not go below the limit if that logic used charge as a bit. This, says Snider, led researchers away from reversible logic and toward logic systems whose bits are things other than charge, such as the spin state of electrons. Cavin was not immediately available for comment.
Snider hopes the European experiment and his own lab’s work will show not only that Landauer’s limit is real but that reversible logic can break through it. The need to get past this limit isn’t urgent today, but extrapolating Moore’s Law to that limit leads to some absurd ends. A chip built about a decade from now near the limit would throw off more energy per square centimeter than the surface of the sun, Snider estimates.
This article appeared in May 2012 print as "Computing’s Power Limit Demonstrated."
A correction to this article was made on 14 March 2012.