Editing Maxwell's demon (section)

== Experimental work ==
In 2007, [[David Leigh (scientist)|David Leigh]] announced the creation of a nano-device based on the [[Brownian ratchet]] popularized by [[Richard Feynman]]. Leigh's device is able to drive a chemical system out of [[Chemical equilibrium|equilibrium]], but it must be powered by an external source ([[light]] in this case) and therefore does not violate thermodynamics.<ref name="H6rVx">{{cite journal|doi = 10.1038/nature05452|volume=445|issue=7127|title=A molecular information ratchet|journal=Nature|pages=523–527|pmid=17268466|date=February 2007|last1 = Serreli|first1 = V|last2 = Lee|first2 = CF|last3 = Kay|first3 = ER|last4 = Leigh|first4 = DA|bibcode=2007Natur.445..523S|s2cid=4314051}}</ref>

Previously, researchers including Nobel Prize winner [[Fraser Stoddart]] had created ring-shaped molecules called [[rotaxane]]s which could be placed on an axle connecting two sites, ''A'' and ''B''. Particles from either site would bump into the ring and move it from end to end. If a large collection of these devices were placed in a system, half of the devices had the ring at site ''A'' and half at ''B'', at any given moment in time.<ref name="t78Hu">{{cite journal|last1=Bissell|first1=Richard A|last2=Córdova|first2=Emilio|last3=Kaifer|first3=Angel E.|last4=Stoddart|first4=J. Fraser|title=A chemically and electrochemically switchable molecular shuttle|journal=Nature|date=12 May 1994|volume=369|issue=6476|pages=133–137|doi=10.1038/369133a0|bibcode=1994Natur.369..133B|s2cid=44926804}}</ref>

Leigh made a minor change to the axle so that if a light is shone on the device, the center of the axle will thicken, restricting the motion of the ring. It keeps the ring from moving, however, only if it is at ''A''. Over time, therefore, the rings will be bumped from ''B'' to ''A'' and get stuck there, creating an imbalance in the system. In his experiments, Leigh was able to take a pot of "billions of these devices" from 50:50 equilibrium to a 70:30 imbalance within a few minutes.<ref name="bErhf">{{cite journal|author=Katharine Sanderson|date=31 January 2007|title=A demon of a device|journal=[[Nature (journal)|Nature]]|doi=10.1038/news070129-10|s2cid=121130699}}</ref>

In 2009, [[Mark G. Raizen]] developed a laser atomic cooling technique which realizes the process Maxwell envisioned of sorting individual atoms in a gas into different containers based on their energy.<ref name="Bennett87" /><ref name="Raizen2009">{{cite journal|last = Raizen|first = Mark G.|title = Comprehensive Control of Atomic Motion|journal = Science|volume = 324|issue = 5933|pages = 1403–1406|date = June 12, 2009|doi = 10.1126/science.1171506|pmid = 19520950|bibcode = 2009Sci...324.1403R|s2cid = 10235622}}</ref><ref name="Raizen2011">{{cite journal|last = Raizen|first = Mark G.|title = Demons, Entropy, and the Quest for Absolute Zero|journal = Scientific American|volume = 304|issue = 3|pages = 54–59|date = March 2011|url = http://www.scientificamerican.com/article/demons-entropy-and-the-quest/|doi = 10.1038/scientificamerican0311-54|pmid = 21438491|access-date = November 14, 2014|bibcode =2011SciAm.304c..54R}}</ref> The new concept is a one-way wall for atoms or molecules that allows them to move in one direction, but not go back. The operation of the one-way wall relies on an irreversible atomic and molecular process of absorption of a photon at a specific wavelength, followed by spontaneous emission to a different internal state. The irreversible process is coupled to a conservative force created by magnetic fields and/or light. Raizen and collaborators proposed using the one-way wall in order to reduce the entropy of an ensemble of atoms. In parallel, Gonzalo Muga and Andreas Ruschhaupt independently developed a similar concept. Their "atom diode" was not proposed for cooling, but rather for regulating the flow of atoms. The Raizen Group demonstrated significant cooling of atoms with the one-way wall in a series of experiments in 2008. Subsequently, the operation of a one-way wall for atoms was demonstrated by Daniel Steck and collaborators later in 2008. Their experiment was based on the 2005 scheme for the one-way wall, and was not used for cooling. The cooling method realized by the Raizen Group was called "single-photon cooling", because only one photon on average is required in order to bring an atom to near-rest. This is in contrast to other [[laser cooling]] techniques which use the momentum of the photon and require a two-level cycling transition.

In 2006, Raizen, Muga, and Ruschhaupt showed in a theoretical paper that as each atom crosses the one-way wall, it scatters one photon, and information is provided about the turning point and hence the energy of that particle. The entropy increase of the radiation field scattered from a directional laser into a random direction is exactly balanced by the entropy reduction of the atoms as they are trapped by the one-way wall.

This technique is widely described as a "Maxwell's demon" because it realizes Maxwell's process of creating a temperature difference by sorting high and low energy atoms into different containers.  However, scientists have pointed out that it does not violate the [[second law of thermodynamics]],<ref name="Bennett87" /><ref name="Orzel">{{cite web|last = Orzel|first = Chad|title = Single-Photon Cooling: Making Maxwell's Demon|website = Uncertain Principles|publisher = [[ScienceBlogs]] website|date = January 25, 2010|url = http://scienceblogs.com/principles/2010/01/25/single-photon-cooling-making-m/|access-date = November 14, 2014}}</ref> does not result in a net decrease in entropy,<ref name="Bennett87" /><ref name="Orzel" /> and cannot be used to produce useful energy.  This is because the process requires more energy from the laser beams than could be produced by the temperature difference generated.  The atoms absorb low entropy photons from the laser beam and emit them in a random direction, thus increasing the entropy of the environment.<ref name="Bennett87" /><ref name="Orzel" />

In 2014, [[Jukka Pekola|Pekola]] et al. demonstrated an experimental realization of a Szilárd engine.<ref name="Pekola 1">{{cite journal|last1=Koski|first1=J.V.|last2=Maisi|first2=V.F.|last3=Sagava|first3=T.|last4=Pekola|first4=J.P.|title=Experimental Observation of the Role of Mutual Information in the Nonequilibrium Dynamics of a Maxwell Demon|journal=Physical Review Letters|date=14 July 2014|volume=113|issue=3|page=030601|doi=10.1103/PhysRevLett.113.030601|pmid=25083623|arxiv=1405.1272|bibcode=2014PhRvL.113c0601K|s2cid=119311588|url=https://aaltodoc.aalto.fi/handle/123456789/16122}}</ref><ref name="Pekola 2">{{cite journal|last1=Koski|first1=J.V.|last2=Maisi|first2=V.F.|last3=Pekola|first3=J.P.|last4=Averin|first4=D.V.|title=Experimental realization of a Szilard engine with a single electron|journal=Proceedings of the National Academy of Sciences of the United States of America|date=23 Sep 2014|volume=111|issue=38|pages=13786–9|doi=10.1073/pnas.1406966111|pmid=25201966|pmc=4183300|arxiv=1402.5907|bibcode=2014PNAS..11113786K|doi-access=free}}</ref> Only a year later and based on an earlier theoretical proposal,<ref name="Strasberg et al">{{cite journal|last1=Strasberg|first1=P.|last2=Schaller|first2=G.|last3=Brandes|first3=T.|last4=Esposito|first4=M.|title=Thermodynamics of a Physical Model Implementing a Maxwell Demon|journal=Physical Review Letters|date=24 Jan 2013|volume=110|issue=4|page=040601|doi=10.1103/PhysRevLett.110.040601|pmid=25166147|arxiv=1210.5661|bibcode=2013PhRvL.110d0601S|s2cid=5782312|url=http://orbilu.uni.lu/handle/10993/11416|type=Submitted manuscript}}</ref> the same group presented the first experimental realization of an autonomous Maxwell's demon, which extracts microscopic information from a system and reduces its entropy by applying feedback. The demon is based on two capacitively coupled single-electron devices, both integrated on the same electronic circuit. The operation of the demon is directly observed as a temperature drop in the system, with a simultaneous temperature rise in the demon arising from the thermodynamic cost of generating the mutual information.<ref name="Pekola 3">{{cite journal|last1=Koski|first1=J.V.|last2=Kutvonen|first2=A.|last3=Khaymovich|first3=I.M.|last4=Ala-Nissila|first4=T.|last5=Pekola|first5=J.P.|title=On-Chip Maxwell's Demon as an Information-Powered Refrigerator|journal=Physical Review Letters|year=2015|volume=115|issue=26|page=260602|doi=10.1103/PhysRevLett.115.260602|pmid=26764980|arxiv=1507.00530|bibcode=2015PhRvL.115z0602K|s2cid=3393380}}</ref> In 2016, Pekola et al. demonstrated a proof-of-principle of an autonomous demon in coupled single-electron circuits, showing a way to cool critical elements in a circuit with information as a fuel.<ref name="Pekola 4">{{cite journal|last1=Koski|first1=J.V.|last2=Pekola|first2=J.P.|title=Maxwell's demons realized in electronic circuits|journal=Comptes Rendus Physique|date=16 December 2016|volume=17|issue=10|pages=1130–1138|doi=10.1016/j.crhy.2016.08.011|bibcode=2016CRPhy..17.1130K|doi-access=free}}</ref> Pekola et al. have also proposed that a simple qubit circuit, e.g., made of a superconducting circuit, could provide a basis to study a quantum Szilard's engine.<ref name="Pekola 5">{{cite journal|last1=Pekola|first1=J.P.|last2=Golubev|first2=D.S.|last3=Averin|first3=D.V.|title=Maxwell's demon based on a single qubit|journal=Physical Review B|date=5 Jan 2016|volume=93|issue=2|page=024501|doi=10.1103/PhysRevB.93.024501|arxiv=1508.03803|bibcode=2016PhRvB..93b4501P|s2cid=55523206}}</ref>