Physicists create first photonic Maxwell's demon

February 12, 2016 by Lisa Zyga feature
Experimental setup of the photonic Maxwell’s demon: The demon’s measurement is implemented by high transmittance beam splitters (BS) and highly sensitive avalanche photodiodes (APDs). The two photodiodes are the work extraction mechanism that use the imbalance in the pulse energies created by the demon to charge a capacitor. Credit: Vidrighin, et al. ©2016 American Physical Society

(—Maxwell's demon, a hypothetical being that appears to violate the second law of thermodynamics, has been widely studied since it was first proposed in 1867 by James Clerk Maxwell. But most of these studies have been theoretical, with only a handful of experiments having actually realized Maxwell's demon.

Now in a new paper, physicists have reported what they believe is the first photonic implementation of Maxwell's demon, by showing that measurements made on two light beams can be used to create an energy imbalance between the beams, from which work can be extracted. One of the interesting things about this experiment is that the extracted work can then be used to charge a battery, providing direct evidence of the "demon's" activity.

The physicists, Mihai D. Vidrighin, et al., carried out the experiment at the University of Oxford and published a paper on their results in a recent issue of Physical Review Letters.

"Our work shows how photonics can be used as a platform to investigate the relation between energy and information," coauthor Oscar Dahlsten, at the University of Oxford and the London Institute for Mathematical Sciences, told

In the original thought experiment, a demon stands between two boxes of gas particles. At first, the average energy (or speed) of gas molecules in each box is the same. But the demon can open a tiny door in the wall between the boxes, measure the energy of each gas particle that floats toward the door, and only allow high-energy particles to pass through one way and low-energy particles to pass through the other way. Over time, one box gains a higher average energy than the other, which creates a pressure difference. The resulting pushing force can then be used to do work. It appears as if the demon has extracted work from the system, even though the system was initially in equilibrium at a single temperature, in violation of the second law of .

Over the years, physicists have resolved this apparent paradox by explaining that, even though the demon may not do work directly on the system, the demon does gain information from its measurements. Erasing this information from the demon's memory requires work, so that overall there can be no net gain in work.

In the photonic version, the physicists replaced the boxes of gas particles with two pulses of light. They implemented the demon using a combination of a photodetector, which can measure the number of photons from each pulse, and a feed-forward operation, which like the open door can escort the brighter beam (with more photons) in one direction and the dimmer beam (with fewer photons) in the other. The different beams fall on different photodiodes, which generate an electric current that goes to a capacitor, but from opposite directions. If the pulse energies were equal, they would cancel out. But the imbalance in the pulse energies—and in the resulting photoelectric charge—is what charges the capacitor.

Even though the researchers did not aim to realize optimal work extraction, it's possible that some type of Maxwell's demon could one day have practical applications.

"Often we have more information available than thermodynamics supposes," Dahlsten said, explaining that things are normally not fully random and have a degree of predictability. "We can then use demon set-ups such as this one to extract work, making use of that information. Similarly, we can use extra information to reduce work costs of, for example, cooling systems. Personally I think that sort of technology will have a real impact on meeting the energy challenge facing the world."

Due to differences between the photonic implementation and previous implementations of Maxwell's demon, traditional theoretical models do not provide a clear path for connecting work extraction to the information acquired by measurement in a fundamental way. So the researchers derived a new model that accounts for the subtleties of the new set-up, in which they relate extraction to the information acquired by measurement.

The researchers hope that the new model will lead to a better understanding of the link between information and thermodynamics, which is necessary for understanding thermodynamics at the microscale and below. As the scientists explain, recent developments of technologies consisting of just a single or few particles require a better understanding of microscale thermodynamics, similar to how the steam engine drove scientists to better understand macroscopic thermodynamics in the 19th century.

A theory of of microscale thermodynamics could have a variety of applications, including making energy-harvesting technology more efficient. It could also allow researchers to investigate the role of quantum coherence in thermodynamics, with applications in quantum information technologies.

"We are already thinking of ways in which features such as entanglement can be introduced in future experiments based on this one, as our interests gravitate around quantum ," Dahlsten said.

Explore further: Maxwell's demon as a self-contained, information-powered refrigerator

More information: Mihai D. Vidrighin, et al. "Photonic Maxwell's Demon." Physical Review Letters. DOI: 10.1103/PhysRevLett.116.050401 , Also at arXiv:1510.02164 [quant-ph]

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1 / 5 (3) Feb 12, 2016
Considering the positive logarithmic values as the measure of entropy and the negative logarithmic values as the measure of information we get the Information – Entropy Theory of Physics, used first as the model of the computer chess program built in the Hungarian Academy of Sciences. Applying this model to physics we have an understanding of the perturbation theory of the QED and QCD as the Information measure of Physics. We have an insight to the current research of Quantum Information Science. The generalization of the Weak Interaction shows the arrow of time in the associate research fields of the biophysics and others. We discuss also the event horizon of the Black Holes, closing the information inside.
5 / 5 (1) Feb 12, 2016
The information paradox is easily solved by using the same technique that a real animal would use in the same situation: the demon compares the energy of two particles approaching the door and only allows them to pass if one has a higher energy than other and they low energy particle is on the high energy side.

There is no need to store and erase information, only to compare measurements and check the rule. As the original thought experiment allows measurement and comparison and only the stored information is problematic the approach I suggest would solve that problem.

Real animals behave in this way, taking local measurements (sensing) of their environment and making local decisions eg bacterial behaviour.
5 / 5 (2) Feb 12, 2016
You know. thinking about it, I don't know if the analogy works with photonics. Look at the diagram and read the description. The fallacy is light is an energy source and if intensity (or high photon current) triggers the gate to open an electric current is created with a +current on one side and a equal -current on the other. The low photodiode would be biased with a higher negative current and the lower photon current would cause the lower photodiode to avalanche on the -current side. The end result is +current on one side, a -current on the other and the cap get's charged. Synced of course.

I don't think it qualifies as a Maxwell's daemon. However, done right, you may be able to charge a cap with radio spectrum on two powerful frequencies. Synced of course.

Feb 13, 2016
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1 / 5 (1) Feb 13, 2016
There's two accounts PO can just delete straight away if the site has any integrity.
Feb 13, 2016
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Feb 13, 2016
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5 / 5 (1) Feb 13, 2016
I'm sure many religious conservatives will be upset that scientists are conjuring demons.
not rated yet Feb 15, 2016
A very well-written article, thank you.

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