If you prepare a cup of tea and leave it on a table, you know what will happen: it will get cold. And if you mix a cup of water at room temperature with some ice it is also quite clear what will happen next: the ice will melt and the water will get a bit colder. In both situations the system will, eventually, reach an equilibrium temperature. Why do these processes only happen in this direction? That is, why can’t room temperature water become warm naturally, without the use of heat? In one word: entropy. Every process will always seek to maximize the value of this quantity. When a warm cup of tea is left in a table it will reach room temperature because this is the state that will maximize the entropy of the universe.

Figure 1. Can quantum systems thermalize just as classical ones do?

Statistical mechanics is the area in physics that studies processes like this one. It relies on the principle of the maximization of entropy in a system at thermal equilibrium. In classical mechanics, chaos plays a major role in leading a system to it’s thermal equilibrium state. Even small classical systems are able to reach thermal equilibrium if they have a chaotic behavior. As times passes the system will visit all the possible states that are associated with that value of entropy and this is one of the major characteristics of classical thermalization.

Quantum systems, however, cannot formally thermalize. If your starting quantum system is in a pure state (the state of an isolated quantum system) it will remain a pure state as times passes. And this means that, in this sense, it is a static system and it will have zero entropy. Even if the system is subjected to a sudden change it will remain a pure system with zero entropy. However, recent theoretical work has shown that quantum systems can also thermalize. And in August 2016 a papers was published in the magazine Nature, showing that, indeed, quantum thermalization can happen.

Figure 2. Scientists show that thermalization can happen in quantum system.

The work was done by researchers in Boston University. They showed experimentally that a small quantum system is capable of thermalizing. The scientists used a quantum microscope to study strings of six rubidium atoms that were confined in an optical well.

They showed that statistical mechanics properties can arise in this system. Even though the system as a whole remains a pure system, smaller subsets of two or three atoms can thermalize. In this setting the force that is responsible for driving the thermalization is quantum entanglement. This is a physical phenomenon in which a group of particles interact in a way that each individual particle cannot be described separately

The scientists were also able to measure directly entanglement entropy which acts in quantum system with the same role thermal entropy has in classical thermalization. In this work the scientists were able to experimentally study the emergence of statistical mechanics in a quantum system. This is a breakthrough that will certainly improve our understanding of how statistical physics works in a molecular level.