IBM Scientists Measure the Heat Emitted From Erasing a Single Bit 111
ananyo writes "In 1961, IBM physicist Rolf Landauer argued that to reset one bit of information — say, to set a binary digit to zero in a computer memory regardless of whether it is initially 1 or 0 — must release a certain minimum amount of heat, proportional to the ambient temperature. New work has now finally confirmed that Landauer was right. To test the principle, the researchers created a simple two-state bit: a single microscopic silica bead held in a 'light trap' by a laser beam. (Abstract) The trap contains two 'valleys' where the particle can rest, one representing a 1 and the other a 0. It could jump between the two if the energy 'hill' separating them is not too high. The researchers could control this height by changing the power of the laser, and could 'tilt' the two valleys to tip the bead into one of them by moving the physical cell containing the bead slightly out of the laser's focus. By monitoring the position and speed of the particle during a cycle of switching and resetting the bit, they could calculate how much energy was dissipated."
This is just entropy, right? (Score:5, Informative)
Re:What a very very stupid test (Score:5, Informative)
Re:What am I missing? (Score:3, Informative)
Say you have two valleys named 0 and 1, and a mountain between. Setting our bit by rolling a ball from 0 to 1 would require energy expenditure, but once the ball is in the valley it is stable and won't roll out again without further input. 0 and 1 may be at different heights relative to each other, but need not be. They might even be at the same altitude. But if 1 were higher than 0, then yes, you would be storing energy in some sort of potential energy form, and may be able to recover that energy when coming back to zero. But you cannot expect to recover all the energy it took to push the ball up the mountain. Any energy required to raise the ball above its destination will have been wasted.
Re:What am I missing? (Score:5, Informative)
It's theoretically possible to change the state of a bit without spending energy. Here's a dumb example: think of a closed system (so no energy is being gained or lost) consisting of a box filled with oxygen and only one molecule of water. Divide the box in two halves and say a bit is "0" if the molecule of water is in the left half and "1" if it's in the right half. If you wait a while, eventually the bit will flip with absolutely no change in energy. That's a dumb example, but it shows that there's nothing that requires a "intrinsic state" and energy loss when you move away from it, like you described.
The only time energy dissipation is unavoidable (in theory) is when you erase information. That's a strange concept because, usually, we don't think about "conservation of information" in the same sense of conservation of energy, but there's a relation [wikipedia.org]. A little more discussion with more relevance to computing can be found here: http://en.wikipedia.org/wiki/Reversible_computing [wikipedia.org].
Re:Slow erasure? (Score:4, Informative)