Empowering Maxwell’s Demon with Correlations


Physicists at the University of Vienna, in collaboration with the Federal University of Santa Catarina in Brazil, have empowered Maxwell’s Demon using correlations to provide the Demon with extra information to extract work more efficiently. The scientists used tools from quantum photonics to create a Maxwell’s Demon which sorts hot correlated photons from cold ones. Due to these correlations when the Demon measures one photon it also gains information about unmeasured photons. It can convert this information into additional work. The results of the experiment were published in the current issue of Quantum.

Maxwell’s Demon is a device envisioned by James Clerk Maxwell in 1867, in which a Demon would measure the velocity of individual particles, and then sort the fast particles from the slower particles. This creates a temperature gradient, allowing the generation of work.  Although this thought experiment originally generated confusion regarding violations of the Second Law of Thermodynamics, we now understand that every particle that the Demon measures provides it with information. The Demon must eventually erase this information, expending exactly as much work to perform this erasure as could be extracted from the temperature gradient.  Guilherme L Zanin, who led the study explains that “today, we understand that Maxwell’s Demon converts information into work.”

Building a Maxwell’s Demon is no easy task as it requires the ability to measure single particles and immediately perform a sorting operation which depends on the outcome of that measurement. The sorting operation must be performed extremely quickly before the measured particle moves away. Such experiments have only recently been made possible due to advances in quantum technology, which now allow measurements at the single-particle level.

The original Demon acted on a gas consisting of massive particles, but in 2016 a group of scientists based in Oxford proposed a Photonic Maxwell’s Demon [1].  In a Photonic Demon, the box of gas particles is replaced by two “thermal beams” made up of photons (particles of light). While the two beams have the same temperature on average, due to thermal fluctuations, sometimes one beam is slightly hotter than the other. The Photonic Demon then measures the instantaneous temperature by extracting single photons from each beam. It can then make one beam “hotter” than the other by swapping the remaining “hot” photons with “cool” photons from the other beam. Paulo H Souto Ribeiro, a collaborator from Santa Catarina in Brazil says, “the thing with photons is that they move at the speed of light, so the Demon must decide when and how to swap the paths only nanoseconds after measuring a photon. Back in 2016, this prevented the Photonic Maxwell Demon from actively sorting the particles”. Now, using techniques they originally developed for quantum photonics [2], known as measurement and feed-forward, the team of scientists from Vienna could make their Demon sort the particles in real time.

The team from Vienna then used other techniques from quantum photonics to generate different types of correlations between two the thermal beams, providing the Demon with significantly more information per measured photon. In essence, this meant that the thermal temperature fluctuations were correlated. For example, when the Demon found that one beam was slightly hotter than average it also knew that the other beam was cooler. The team experimentally found that this would allow about ten times as much work to be extracted. Others have theoretically shown that for much hotter average temperatures than those used in this work, such correlations can provide even larger enhancements [3].

According to the authors, when considering the work that could be extracted in the experiment, they neglected the energy cost of running the detectors and operating the optical switches. If they were to include these costs, no net work could be generated.  However, Lee Rozema, an author from the University of Vienna, says, “one day these costs could be removed using photonic quantum logic gates—which are being actively developed for other quantum computing applications—to perform the sorting operations. This highlights how advancements coming from quantum technology often feed back into fundamental research.”  The team is excited to study other fundamental physical effects at the intersection of quantum mechanics and thermodynamics using their approach. They say that their work shows that photonic experiments have a lot to bring to the study of thermodynamics, a field which has traditionally focused more on other physical platforms. Zanin says, “our photonic approach provides not only a unique perspective on thermodynamics, but also a new set of tools and capabilities which could lead to new understandings of quantum thermodynamics.”

  1. “Photonic Maxwell's Demon.” Phys. Rev. Lett., 116, 050401 (2016). 
  2. “Fiber-compatible photonic feed-forward with 99% fidelity.” Opt. Express, 29, 3425–3437 (2021).
  3. “Power of an Optical Maxwell's Demon in the presence of Photon-Number Correlations.” Phys. Rev. A, 95, 022123 (2017).

© Lee Rozema