Paris, 8 March 2012
Is there any prospect of designing a perfect computer capable of performing logical operations without using any energy? When this question was put to Rolf Landauer in 1961, his answer was "No". The US physicist had observed that whenever a bit of information is created, the computer's binary memory is reduced to only one of two possible states. By drawing a link to thermodynamics, Landauer argued that for this decrease in disorder to occur, a minimum amount of energy is required, whose value is today known as the Landauer limit(1). This extremely low amount of energy (a billion times smaller than the energy required to heat a cubic micron of water by one degree) has been verified using digital simulations, but had never before been measured under experimental conditions.
For their experiment, the researchers used a two-micron silica bead to represent one bit of information. The particle was immersed in liquid and held in place by an extremely focused laser beam, otherwise known as an "optical tweezer" and commonly used by physicists and biologists. To satisfy the very high precision requirements of the experiment, researchers designed and built their own custom tweezer to ensure that the trapped bead would be completely stable and immune to exterior perturbations. A second identical light trap was then focused next to the first trap. The microscopic bead could thus rest in either one of the two positions available, just as a bit of information can hold a 0 or 1. Researchers then created a small flow in the liquid from right to left, causing the particle to finish its movement in the left well, similar to forcing a bit to the value 1.
The cycle was repeated several times over and filmed using a high-speed camera with over 1 000 frames per second. By knowing the exact position of the particle, the velocity of the liquid flow and its viscosity, researchers were able to measure the average amount of energy required to move the bead from the right well to the left well. They noticed that with very slow flow velocities, the amount of energy was minimal and corresponded exactly to the Landauer limit.
Although the results do not offer any immediate applications for the computing world, since our computers still have a long way to go before they can function at the Landauer limit, the findings could be beneficial to nanotechnologies in the near future. The energy dissipated by a nanometric system is actually comparable to the amount measured by the researchers, meaning that this would be an important parameter if looking to design miniature machines capable of working to high levels of efficiency.
1 - Corresponding to kTln(2), where k is the Boltzmann constant and T is the temperature, and equal to approximately 10-21 joules at room temperature.
"Experimental verification of Landauer's principle linking information and thermodynamics". A. Bérut, A. Arakelyan, A. Petrosyan, S. Ciliberto, R. Dillenschneider, E. Lutz, in the Nature journal of March 8, 2012
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