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Quantum's delete freeze

Pete Wilton

Erasing data, rather like cleaning a house, should be hard, hot work.

But now a team including Oxford University’s Vlatko Vedral have shown that, in the quantum world at least, it doesn’t always have to be.

They report in this week’s Nature that, under certain conditions, deleting data held in a quantum computer could actually cool the environment down rather than heating it up.

‘Everyone who has ever worked with a computer knows that they get hotter the more we use them,’ Vlatko writes in Scientific American.

‘Physicist Rolf Landauer argued that this needs to be so, elevating the observation to the level of a principle,’ Vlatko comments. ‘The principle states that in order to erase one bit of information, we need to increase the entropy of the environment by at least as much. In other words we need to dissipate at least one bit of heat into the environment (which is just equal to the bit of entropy times the temperature of the environment).’

This heat threshold is something that could potentially hold supercomputers back: that there comes a point when deleting data to perform fresh computations will create so much heat that any system will no longer be able to cool itself down.

Saying that you can erase information from a system and cool the environment at the same time is, he says, a bit like telling a physicist that perpetual motion is possible.

However, the team’s new theoretical study has turned up an exception based on the idea that entropy, the measure of disorder of a system which should always increase, can be ‘negative’ in quantum mechanics.

Vlatko writes: ‘Adding negative entropy is the same as taking entropy away. The key phenomenon behind it is the spookiest of all quantum phenomena, entanglement.’

In a Nature commentary Patrick Hayden describes the team’s findings like this, that: ‘instead of having to invest work to erase the qubit [quantum bit], the process of erasing the qubit can actually generate work, like a tiny quantum-logical wind turbine.’

Vlatko adds: ‘The implications of our result could be important for superfast and superefficient computers. Current computers waste about 10,000 units of heat per computational step. If we can somehow control and manipulate entanglement between the microprocessor and the computer memory, then we could erase computations to make room for new ones, but keep the environment cool.’

More about this work in Vlatko Vedral's article for Scientific American.

Professor Vlatko Vedral is based at Oxford University's Department of Physics.