Oct 19 2020
A group of scientists from Trinity College Dublin has identified an exclusive quantum effect in deleting data that may hold major implications for designing quantum computing chips.
A bit of information can be encoded in the position of a particle (left or right). A demon can erase a classical bit (blue) by raising one side until the particle is definitely on the right. A quantum particle (red) can also tunnel under the barrier, which generates more heat. Image Credit: Trinity College Dublin.
This unexpected finding revives the paradoxical “Maxwell’s demon,” which has perplexed physicists for more than 150 years.
The thermodynamics of computation was predominantly introduced in 1961 when Rolf Landauer, who was working at IBM at that time, identified a link between the logically irreversible operations and dissipation of heat.
Landauer is famous for the mantra “Information is Physical,” which reinforces the fact that data is encoded on physical hardware and is not abstract.
The “bit” refers to the currency of data (it can be either 1 or 0) and Landauer found that a minimum amount of heat is discharged upon erasing a bit. This is referred to as Landauer’s bound and is the conclusive relationship between thermodynamics and information theory.
At Trinity College Dublin, Professor John Goold’s QuSys team is investigating this topic keeping quantum computing in mind. In the case of quantum computing, a quantum bit—or a qubit, which can be 0 and 1 concurrently—is removed.
In a study published recently in the Physical Review Letters journal, the team found that the quantum nature of the data to be deleted could cause huge deviations in the dissipation of heat, which does not exist in traditional bit erasure.
Thermodynamics and Maxwell’s Demon
A century before Landauer’s finding, individuals like Viennese researcher Ludwig Boltzmann and Scottish physicist James Clerk Maxwell were developing the kinetic theory of gases, restoring life to an outdated concept of the ancient Greeks by visualizing about matter being made of atoms and by deriving macroscopic thermodynamics from microscopic dynamics.
Statistical mechanics tells us that things like pressure and temperature, and even the laws of thermodynamics themselves, can be understood by the average behavior of the atomic constituents of matter. The second law of thermodynamics concerns something called entropy which, in a nutshell, is a measure of the disorder in a process.
John Goold, Professor, Trinity College Dublin
Professor Goold continued, “The second law tells us that in the absence of external intervention, all processes in the universe tend, on average, to increase their entropy and reach a state known as thermal equilibrium.”
“It tells us that, when mixed, two gases at different temperatures will reach a new state of equilibrium at the average temperature of the two. It is the ultimate law in the sense that every dynamical system is subject to it. There is no escape: all things will reach equilibrium, even you!” Professor Goold further added.
But right from the beginning of the kinetic theory, the founding fathers of statistical mechanics were attempting to find loopholes in the second law of thermodynamics. One can reconsider the case of a gas in equilibrium: Maxwell visualized a hypothetical “neat-fingered” being able to monitor and sort particles in a gas on the basis of their speed.
Maxwell’s demon, as it popularly came to be known, could rapidly open and close a trap door in a box filled with a gas, allowing hot particles through to one side of the box but limiting the cold ones to the other. However, this situation appears to oppose the second law of thermodynamics, because the overall entropy seems to reduce, and maybe the most popular paradox of physics was born.
But what about the discovery made by Landauer regarding the heat-dissipated price of deleting data? This indeed took another two decades until that was completely appreciated, the enigma was resolved, and Maxwell’s demon was ultimately exorcised.
Charlie Bennett, also from IBM, was inspired by Landauer’s work to explore the concept of reversible computing. Earlier in 1982, Bennett contended that Maxwell’s demon should have a memory, and according to him, it is not the measurement but rather the removal of the data in the demon’s memory which is the act that revives the second law in the paradox. And, consequently, this resulted in computation thermodynamics.
New Findings
Now, four decades later, this is where the latest study headed by Professor Goold’s team comes to the fore, with the focus on quantum computation thermodynamics.
In the new article, published with colleague Harry Miller from the University of Manchester and two postdoctoral fellows, Mark Mitchison and Giacomo Guarnieri, in the QuSys Group at Trinity College Dublin, the researchers carefully analyzed an experimentally realistic erasure process that enables quantum superposition (the qubit can be in both 0 and 1 states simultaneously).
In reality, computers function well away from Landauer’s bound for heat dissipation because they are not perfect systems. However, it is still important to think about the bound because as the miniaturisation of computing components continues, that bound becomes ever closer, and it is becoming more relevant for quantum computing machines. What is amazing is that with technology these days you can really study erasure approaching that limit.
John Goold, Professor, Trinity College Dublin
Professor Goold added, “We asked: ‘what difference does this distinctly quantum feature make for the erasure protocol?’ And the answer was something we did not expect. We found that even in an ideal erasure protocol—due to quantum superposition—you get very rare events which dissipate heat far greater than the Landauer limit.”
In the paper we prove mathematically that these events exist and are a uniquely quantum feature. This is a highly unusual finding that could be really important for heat management on future quantum chips—although there is much more work to be done, in particular in analysing faster operations and the thermodynamics of other gate implementations.
John Goold, Professor, Trinity College Dublin
“Even in 2020, Maxwell’s demon continues to pose fundamental questions about the laws of nature,” Professor Goold concluded.
Journal Reference:
Miller, H. J. D., et al. (2020) Quantum Fluctuations Hinder Finite-Time Information Erasure near the Landauer Limit. Physical Review Letters. doi.org/10.1103/PhysRevLett.125.160602.