Szilard engine
The Szilard engine is a thought experiment of 1929 in which a single molecule of gas in a box is made to do work by learning which half of the box it is in. It is the smallest version of Maxwell's demon, and it ties the cost of the demon's knowledge to a definite quantity of energy: each cycle of the engine draws kBT ln 2 of heat from its surroundings and turns it into work, which is 2.87 zJ at 300 K.[1] It was devised by the Hungarian physicist Leo Szilard, who conceived the nuclear chain reaction four years later.
The cycle
A box of volume V contains one molecule and is in contact with a heat bath at temperature T. A partition is slid into the middle of the box; it exerts no force on the molecule and costs no work, and the molecule is now on one side or the other. An observer measures which side, which yields one bit. The partition is then allowed to move as a piston, with a weight attached, and the molecule, pushing on it from its own side, lifts the weight as the piston travels to the far wall.
During this stroke the molecule expands from V/2 to V at constant temperature and draws heat from the bath as it goes. A one-molecule gas obeys pV = kBT, so the work is ∫p dV = kBT ln 2. The piston is then removed and the box is back in its first state, except that the observer holds one bit.
At 300 K the work per cycle is 2.87 × 10−21 J, which is 0.018 eV, about one hundredth of the energy of a photon of visible light. An engine that completed a cycle every microsecond would deliver 2.9 fW. The apparent result is a machine that converts heat from a single bath into work, which the second law forbids.
The accounting
Szilard's resolution put the cost in the measurement: to keep the second law intact the observer has to produce at least kB ln 2 of entropy in learning the answer. This was not the final word. In 1961 Rolf Landauer argued that what costs energy is the erasure of a bit, not its acquisition, and in 1982 Charles Bennett applied the argument to the demon: the observer's memory must be reset before the next cycle, and resetting it releases at least kBT ln 2 of heat, which cancels the work.[2][3] See Landauer's principle. Szilard's paper appeared nineteen years before Claude Shannon's theory of information, and it already ties the answer to a yes-or-no question to a definite quantity of entropy.
Built engines
Two laboratory experiments have realized the principle. In 2010 a group in Japan used a particle made of two polystyrene beads about 0.3 µm across, in water. An electric field gave the particle a potential like a spiral staircase, and a feedback loop watched it and changed the potential when thermal motion carried it up a step, so that it could not slip back. The particle climbed, and gained more free energy than the work performed on it.[4] (See Polystyrene bead.)
In 2014 Koski and colleagues at Aalto University built the engine from a single electron. Two metallic islands in a dilution refrigerator at about 0.1 K shared one extra electron that could sit on either of them, with no wall between. An electrometer measured the side, and the voltage was driven in the direction that trapped the electron there. With the settings chosen, the average heat extracted was about 0.75 of kBT ln 2 per cycle, at a measurement fidelity of about 75 percent.[5]
At Molecular Reality
A pore that reads the current of each molecule and then decides whether to let it through, send it back or hold it has the shape of the engine: measure, then act on the result. The company's name for the idea is the demonpore, and it took its first corporate name from the demon of Maxwell's cartoons (demonpore, inc.). The arithmetic of scale is in Active sensing. A pore carrying 3.1 nA at 200 mV dissipates 0.62 nW, and a decision lasting 10 µs therefore costs 6.2 fJ, about 2.2 million times kBT ln 2.[6] An engine of Szilard's kind cycling at a megahertz would produce roughly 216,000 times less power than the pore uses. The company's character Maxine is, in its words, "named after a tulpa related to Maxwell's demon".[6]
See also
References
- ^ Szilard, L. (1929). "Über die Entropieverminderung in einem thermodynamischen System bei Eingriffen intelligenter Wesen". Zeitschrift für Physik 53 (11-12): 840-856. doi:10.1007/BF01341281.
- ^ Landauer, R. (1961). "Irreversibility and heat generation in the computing process". IBM Journal of Research and Development 5 (3): 183-191. doi:10.1147/rd.53.0183.
- ^ Bennett, C. H. (1982). "The thermodynamics of computation: a review". International Journal of Theoretical Physics 21 (12): 905-940. doi:10.1007/BF02084158.
- ^ Toyabe, S.; Sagawa, T.; Ueda, M.; Muneyuki, E.; Sano, M. (2010). "Experimental demonstration of information-to-energy conversion and validation of the generalized Jarzynski equality". Nature Physics 6 (12): 988-992. doi:10.1038/nphys1821.
- ^ Koski, J. V.; Maisi, V. F.; Pekola, J. P.; Averin, D. V. (2014). "Experimental realization of a Szilard engine with a single electron". Proceedings of the National Academy of Sciences 111 (38): 13786-13789. doi:10.1073/pnas.1406966111.
- ^ a b "Company records". Molecular Reality Corporation. October 2026.