Maxwell's Demon - Experimental Work

Experimental Work

Though scientists have not created Maxwell's demon, they are coming surprisingly close. In the 1 February 2007 issue of Nature, David Leigh, a professor at the University of Edinburgh, announced the creation of a nano-device based on Feynman's thought experiment. Leigh's device is able to drive a chemical system out of equilibrium, but it must be powered by an external source (light in this case) and therefore does not violate thermodynamics.

Previously, other researchers created a ring-shaped molecule which could be placed on an axle connecting two sites (called 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.

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, thus restricting the motion of the ring. It only keeps the ring from moving, however, if it is at site A. Over time, therefore, the rings will be bumped from site B to site 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.

The March 2011 issue of Scientific American features an article by Professor Mark G. Raizen of the University of Texas, Austin which discusses the first realization of Maxwell's demon with gas phase particles, as originally envisioned by Maxwell. In 2005, Raizen and collaborators showed how to realize Maxwell's demon for an ensemble of dilute gas-phase atoms or molecules. The new concept is a one-way wall for atoms or molecules that allows them 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 to use 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 to regulate 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 laser cooling which uses the momentum of the photon and requires 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 with the one-way wall. Therefore, single-photon cooling is a physical realization of Maxwell’s Demon in the same sense envisioned by Leo Szilard in 1929.

The importance of single photon cooling is that it provides a general method for cooling multi-level atoms or molecules. It circumvents the limitation of laser cooling which requires a two-level cycling transition, and hence is limited to a small set of atoms in the Periodic Table. The experimental realization of Maxwell's demon is a key step towards general control of atoms in gas phase. Beyond basic scientific research, these methods will enable efficient isotope separation for medicine and basic research, as well as controlling atoms in gas phase for nanoscale deposition on surfaces. This new, bottom-up, approach to nanoscience is called Atomoscience and is enabled by the realization of Maxwell's demon.

Regarding Landauer's principle, the minimum energy dissipated by deleting information was experimentally measured by Eric Lutz et al. in 2012. Although a demon could, in principle, observe the particle, save the result and act on it, deleting the result would necessarily dissipate heat and thus increase entropy. Without an infinite memory, the demon would eventually have to overwrite its previous results. Additionally, the deletion became more energy-efficient the slower it was, thus also requiring the demon to asymptotically approach zero processing speed.

Though presently theoretical, another near-realization of Maxwell's demon is a water filter studied by David Cohen-Tanugi and Jeffrey C. Grossman of MIT. Shown to the right is a simulated image of a hydrophilic coated hole in a nanoporous graphene desalination pressure-powered reverse osmosis filter which would allow only water through, efficiently rejecting the sodium and chloride ions that make sea water non-potable.

In 2011 W. Christensen argued that the universe can be shown to be conscious via a cosmological model based on Maxwell's demon and information theory.