Erase entangled memory to cool a computer
Imagine cooling a supercomputer not with fans or freezers, but by deleting some of its memory. New calculations show that this is possible, provided some of the bits that make up the computer’s memory are “entangled”– a spooky property that can link two quantum systems, no matter how far apart they sit in physical space.
The notion of cooling by erasure seemingly violates a principle articulated by physicist Rolf Landauer in 1961. He showed that erasing information is akin to a decrease in entropy or disorder. As entropy overall must always increase, the deletion of bits must therefore be accompanied by an increase in the entropy of the surroundings, which manifests itself as heat.
The heat produced by a computer today is mainly due to processing inefficiencies, and while these can be reduced, Landauer’s insight implies a fundamental limit on how much you can reduce the heat generated by computing.
Now, Lídia del Rio of the Swiss Federal Institute of Technology in Zurich and colleagues have shown that quantum entanglement provides a way to sneak around Landauer’s law.
To understand how this might work, consider two people who are each trying to erase a string of bits in computer memory, which can exist either as 1s or 0s. One of the pair has no knowledge of the stored bits, so to ensure they get erased, he or she must always reset them to “0”, regardless of their original content. The second person, however, knows the content of the string and so need only reset those bits that are 1s.
In this situation, the first person has to do more work on average to erase the string than the second. As a result, the “conditional entropy” of the memory is said to be lower for the first person than the second.
Now imagine that the memory bits to be erased are entangled with other objects. In such a system, observing or determining the state of one part immediately fixes the state of the other. So an observer who has access to the entangled objects could know even more about the memory than would be possible otherwise, causing the conditional entropy of the system to dip and become negative when the memory is erased.
Weirdness at work
Del Rio and colleagues have shown mathematically that this negative conditional entropy is the equivalent of extracting heat from the surroundings, or cooling.
The team envisages future computers containing entangled systems of this kind. Deletion of some of a computer’s memory should lead to cooling. “If you go to this entangled level of operations, then you will be at the limit of what physics allows you to do,” says team member Vlatko Vedral of the University of Oxford.
This does not violate the laws of thermodynamics: there is still an overall increase in entropy because energy is needed to create the entangled system initially.
At the moment, entangled states are not easy to work with: they require extreme cooling and are notoriously fragile. Still, Robert Prevedel of the University of Waterloo, Ontario, Canada, who was not involved in the work, is impressed by the idea. “This demonstrates that the weird features of the quantum world are not only useful as an informational resource, but can actually be used to generate some real, physical work,” he says.
Journal Reference: Nature, DOI: 10.1038/nature10123
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