In a new study published September 5th, physicists at Penn State report that they have successfully implemented an experimental version of Maxwell’s demon, a physics thought experiment meant to challenge the second law of thermodynamics. By using complicated procedures involving lasers and magnetic fields, the team created a method for sorting a supercooled cloud of cesium atoms into a repeating 3-D lattice structure.
The resulting ordered array of atoms has decreased entropy than the previous configuration, which seems to be a realization of Maxwell’s hypothetical sorting mechanism.
The new atom building technology will go lengths towards creating quantum computers. Unlike regular computers which store information in bits, 1s or 0s, quantum computers exploit quantum mechanical phenomena to create “qubits”, a quantum analog of the classical bit.
Qubits can store an arbitrarily large amount of classical information, and so could potentially be used to perform computations that would be impossible or take an infinite amount of time on regular digital computers. Atoms packed into a neat and periodic 3-D grid would make quantum computer faster and more efficient, so the new study represents a step forward towards practical quantum computer manufacturing.
Maxwell’s Demon And Entropy
In 1867 the physicist James Clerk Maxwell proposed the following thought experiment: Imagine an enclosed box full of gasses that is internally divided into two sections. In the middle of the box is a small demon next to a door that opens one side up to the other. As individual gas molecules approach the door, the demon opens and shuts it in such a way so as to allow all the fast moving molecules to the left and the slow moving molecules to the right. Because faster molecules are hotter, the behavior of the demon causes the temperature on the left side to increase and the temperature on the right side to decrease. This difference in temperature signifies decreased entropy and is an apparent violation of the second law of thermodynamic—generally formulated as the statistical law that over time all closed systems tend towards thermodynamic equilibrium.
Several responses have been offered to counter Maxwell’s thought experiment. Most responses point out that the mechanical operations of the demonic device, such as measuring the speed of molecules or the opening/closing of the door, require energy and would counteract the entropy-decreasing effects of the setup. Others point out that, yes, for a brief period of time the system’s entropy would decrease, but eventually the demon would have to expend any stored heat energy, thus returning the system to a state of equilibrium. This previous response does not involve a violation of the second law as the second law is a statistical maxim. Systems might undergo random fluctuations in which entropy slightly decreases but these fluctuations are insignificant, thermodynamically speaking. However, at the level of quantum mechanics and at extremely cold temperatures, the traditional laws of thermodynamics may not apply so straightforwardly.
Quantum Lattices And Decreased Entropy
In order to construct the 3-D cesium lattice, the Penn State researcher first began by using counter-propagating lasers to trap a randomly arranged 3-D assortment of cesium atoms. The cesium atoms were then supercooled to temperatures close to absolute zero. At such low temperatures, the entropy of the arrangement of cesium atoms is almost entirely determined by their random arrangement. Then, by minutely altering the polarization of these lasers, the team could sort this random arrangement of cesium atoms into either a 5-by 5-by 2 or 4 by 4 by 3 lattice. Changing the polarization of light changes the quantum state of the cesium atoms, which allows them to be “moved” without increasing entropy. The resulting sublattice structures showed a decrease in entropy by a factor of 2.4 relative to the initial arrangement of atoms.
The key to decreasing entropy was to super cool the atoms before arranging them. According to the kinetic theory of heat, heat is identical to the movement of atoms and molecules. In normal situations, most of the entropy of any particular system is due to the heat from vibrations of atoms and molecules. At extremely low temperatures close to absolute zero, molecules essentially cease to vibrate and thus have negligible entropy associated with their movement. Instead, the entropy of such a system will be determined almost entirely by the spatial arrangement of the atoms. Since the initial spatial arrangement of atoms is random and unstructured, there is a relatively high measure of entropy.
In order to move the cesium atom into their desired positions, the team used lasers to perform a series of reversible operations—processes that operate at thermodynamic equilibrium that do not increase the entropy of a system. The resulting ordered lattice structure has a lower measure of entropy than the random arrangement. Moreover, the new structure is stable and show little degradation over time. According to David Weiss, head researchers on the authored study, “There have been some successes at very small scales, but we’ve created a system in which we can manipulate a large number of atoms, organizing them in a way that reduces the system’s entropy, just like the demon.” It is thought that new low entropy quantum devices will allow for quantum computers that can operate a room temperature without getting too hot.
There is still the lingering question though: does the reported phenomenon constitute a genuine violation of the second law of thermodynamics? It is hard to say. The relatively new field of quantum thermodynamics seeks to explain the foundations of classical thermodynamics and might eventually give an explanation for the presence of classical thermodynamic laws in the macroscopic world. The past 60 years of science have really driven home the lesson that the quantum world plays by different rules than the classical macroscopic world. So perhaps it is not a stretch to say that at the extreme scales of existence, even rock-solid foundations like the second law of thermodynamics can be shaken.
It would seem that the fundamental basis of entropy — that the aggregate probability of all variations of the state of the system, except the one you are seeking, will be more likely to occur — is and still will be in effect, and thus should still apply to quantum thermodynamics.