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The research team found a unique quantum effect in deleting information



The research team found a unique quantum effect in deleting information

Little information can be encoded in the particle position (left or right). A daemon can erase a classic bit (blue) by lifting one side until the particle is definitely on the right. The quantum particle (red) can also tunnel under the barrier, which generates more heat. Credit: Trinity College Dublin

Researchers at Trinity have found a unique quantum effect in deleting information that can have significant implications for the design of quantum computing chips. Their surprising discovery revives the paradoxical “Maxwell demon”

; that has plagued physicists for more than 150 years.


The thermodynamics of computation came to the fore in 1961 when Rolf Landauer, then at IBM, discovered the connection between heat dissipation and logically irreversible operations. Landauer is known for the mantra “Information is physical”, which reminds us that information is not abstract and is encoded on physical hardware.

The “bit” is the currency of the information (it can be zero or one) and Landauer found that when a little is erased, a minimal amount of heat is released. This is known as the Landauer connection and is the final link between information theory and thermodynamics.

Professor John Gould’s QuSys group at Trinity analyzes this topic in terms of quantum calculations, where the quantum bit (a qubit that can be zero and simultaneously) is erased.

In the just published work in the magazine, Physical examination letters, the group found that the quantum nature of the information to be erased can lead to large deviations in heat dissipation, which is not present in conventional bit deletion.

Thermodynamics and the Maxwell Demon

One hundred years before Landauer’s discovery, people such as the Viennese scientist Ludwig Boltzmann and the Scottish physicist James Clerk Maxwell formulated the kinetic theory of gases, reviving an old idea of ​​the ancient Greeks, thinking that matter was made of atoms and producing macroscopic thermodynamics.

Professor Gould says: “Statistical mechanics tells us that things like pressure and temperature and even the laws of thermodynamics themselves can be understood from the average behavior of the atomic constituents of matter. The second law of thermodynamics refers to something called entropy, which in short The second law tells us that in the absence of external interference, all processes in the universe tend to increase their entropy on average and reach a state known as thermal equilibrium.

“This tells us that when mixed, two gases at different temperatures will reach a new state of equilibrium at the average temperature of the two. This is the supreme law in the sense that every dynamical system is subject to it. There is no salvation: all things will reach balance, even you. “

However, the founding fathers of statistical mechanics tried to find holes in the second law at the beginning of kinetic theory. Think again of the example of a gas in equilibrium: Maxwell imagined a hypothetical creature with “clean soil” with the ability to track and sort particles in a gas based on their speed.

Maxwell’s demon, as the creature became known, could quickly open and close the door trap in a gas canister and let hot particles through one side of the can, but limit the cold to the other. This scenario seems to contradict the second law of thermodynamics, as the total entropy seems to decrease and perhaps the most famous paradox of physics is born.

But what about Landauer’s discovery of the heat dissipation of the cost of erasing information? Well, it was another 20 years before that was fully appreciated, the paradox resolved, and Maxwell’s demon finally cast out.

Landauer’s work inspired Charlie Bennett – also at IBM – to investigate the idea of ​​reversible calculations. In 1982, Bennett argued that the demon must have a memory, and that it was not the measurement but the erasure of information in the demon’s memory that was the act that restored the second law to the paradox. And as a result, computational thermodynamics was born.

New discoveries

Now, 40 years later, a new work, led by Professor Gould’s group, focuses on the thermodynamics of quantum computing.

In a recent article published with collaborator Harry Miller of the University of Manchester and two PhD students from the Trinity group, Mark Michison and Giacomo Guarnieri, the team examined very carefully an experimentally realistic erasure process that allows a quantum superposition (qubit can be able to and simultaneously simultaneously).

Professor Gould explains: “In fact, computers operate far from the Landauer limit for heat dissipation, as they are not perfect systems. However, it is important to think about connectivity, because as the miniaturization of computing components continues, this limit becomes ever closer, and becoming more and more suitable for quantum computing machines.What’s amazing is that with technology these days, you can really study the erasure approaching that limit.

“We asked, ‘What’s the difference between this distinctly quantum characteristic of the delete protocol?’ “And the answer was something we didn’t expect. We found that even in an ideal erasure protocol – due to quantum superposition – you get very rare events that dissipate heat far beyond the Landauer boundary.

“In this paper, we prove mathematically that these events exist and are a unique quantum characteristic. This is an extremely unusual finding that can be really important for heat management of future quantum chips – although there is much more work, especially in analysis of faster operations and the thermodynamics of other gate designs.

“Even in 2020, Maxwell’s demon continues to ask basic questions about natural laws.”


Less is more about Maxwell’s Demon in quantum heat engines


More information:
Harry JD Miller et al. Quantum fluctuations prevent the deletion of information as soon as possible near the Landauer boundary, Physical examination letters (2020). DOI: 10.1103 / PhysRevLett.125.160602

Provided by Trinity College Dublin

Quote: The research team finds a unique quantum effect in deleting information (2020, October 16), extracted on October 16, 2020 from https://phys.org/news/2020-10-team-uniquely-quantum-effect- erasing.html

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