A hypothetical entity first proposed in the mid-1800s that appears to violate the second law of thermodynamics has for the first time been implemented in photonic experiments carried out at Oxford University and published in a recent edition of the journal Physical Review Letters.
Named Maxwell’s demon in honor of James Clerk Maxwell, the mathematician who according to Auburn University first hypothesized its existence in 1867, the being has been at the center of countless theoretical studies over the years, but only a few were able to actually realize it.
Maxwell’s hypothesis assumes you have a box filled with a gas at some temperature, meaning that molecules will travel at a certain average speed based on how hot or cold the inside of the box is (with some moving faster and others slower). Now suppose that a partition is placed across the middle of the box to separate it into two sides, and that both the left and right halves of the box are filled with gas at the same temperature.
In his hypothesis, Maxwell imagined that the partition contained a tiny trap door controlled by a miniscule demon that was observing individual gas molecules. As they moved closer to the door, the demon quickly opened and shut the entry, allowing faster molecules to enter the other side of the chamber and leaving slower ones behind.
Since faster molecules are hotter, the demon’s behavior would cause one chamber to warm up and the other to cool down, according to Phys.org. This would decrease entropy and effectively violate the Second Law of Thermodynamics in the process. If the box is warm on one side and cool on the other, it could be used to run a heat engine by using the separation of temperature by allowing heat to flow from the hot half of the container to the cold side.
Findings could ultimately be used to improve cooling systems
While the majority of experiments focusing on Maxwell’s demon have been theoretical, a team of physicists from Oxford University, the London Institute for Mathematical Sciences, Imperial College London, Roma Tre University and the National University of Singapore have managed to produce what they are calling the first-ever photonic implementation of the demon.
According to Phys.org, lead author Mihai D. Vidrighin and his colleagues demonstrated that measurements made on two beams of light can be used to generate an energy imbalance between the beams, from which they are able to extract work. Furthermore, their findings indicates that the work extracted from the beams can be used to change a battery, thus effectively providing proof of the so-called “demon’s” activity.
Co-author Oscar Dahlsten from Oxford and the London Institute for Mathematical Sciences told the website that the team’s research demonstrates how photonics could be used to probe the link between energy and information. In their version of the Maxwell’s demon experiment, they used the light pulses in place of the gas particles, and used the combination of a photodetector and a feed-forward operation to simulate the activity of the hypothesized demon.
The photodetector measures the number of photons from each pulse, while the feed-forward operation acted like the open door by escorting the brighter, more photon-rich beam one way and the dimmer beam with fewer photons in the opposite direction, Phys.org explained. Each beam ends up falling on a different photodiodes, generating electrical currents that both travel towards a capacitor but do so at different directions.
Since the pulse energies are not equal, they produce a photoelectric charge and provide energy to the capacity rather than simply cancelling each other out. While the authors noted that they did not set out to realize optimal work extraction, they are confident that some version of Maxwell’s demon could eventually be used to reduce the work costs of cooling systems or in other ways.
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