WE'RE used to the waste heat produced by electrical wires and car brakes. Not so familiar is the heat created by erasing a digital memory. Now an experiment inspired by a metaphorical demon has measured this fundamental heat, which will one day impose a limit on the power of computers.
Maxwell's demon, named after the 19th-century physicist James Clerk Maxwell, is a conundrum that seems to break a basic law of physics by creating a perpetual motion machine. Maxwell reasoned that his demon could control a gate dividing a box of gas molecules, some moving fast, others slow. By opening the gate at opportune moments, the demon could fill one side of the box with hot gas, the other with cold, creating a temperature difference. That difference could drive an engine, producing useful work without appearing to expend enough energy.
In 1961, Rolf Landauer proposed that the key to the conundrum was the demon's memory. As the creature gathers information on the motion of molecules, it must erase a previous memory. Landauer suggested that the process of erasure dissipates heat. This expended heat could balance out the useful work gained by the demon and ensure it does not, in fact, get something for nothing.
Not everyone agreed with this explanation. Now Eric Lutz of the University of Augsburg in Germany and colleagues have shown that there is indeed a minimum amount of heat produced per bit of erased data. This so-called Landauer limit is proof that the demon does not get a free lunch. "We exorcise the demon," says Lutz.
Rather than bell, book and candle, the "exorcists" use a laser that can set the position of a small glass bead. The laser is focused to give the bead two stable positions, left and right or 0 and 1. The resulting one-bit memory can store a 0 or 1, but memories are always erased by resetting to 0. The team found that the heat generated by erasing the bit is never less than the Landauer limit (Nature, DOI: 10.1038/nature10872).
That has deep implications for the microchip industry, says Lutz. Right now, chips produce about 1000 times more heat per bit than the limit, due to resistance in their wires. Chipmakers are working on this but there will come a point where it can be reduced no more. "Silicon-based technology is predicted to attain the Landauer limit in 20 to 30 years," says Lutz. Then the ability to squeeze ever more bits on a chip will depend on finding better ways to cool them, as they glow with the fundamental heat of forgetting.
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