How disorder at microscopic level reveals important changes in behavior of matter

An article published in the scientific journal Nature Physics by Maciej Lewenstein, ICREA researcher at the Institute of Photonic Sciences (ICFO, Barcelona), affiliated to the Universitat Politècnica de Catalunya (UPC)-Barcelona Tech, and Laurent Sánchez-Palencia, from the Institute of Optics (IO, Palaiseau), provides an overview of the recent history of the field of ultracold gases, one of the safest bets in quantum information implementation, with ambitious projects such as quantum computers and simulators. These simulators offer the best tools for studying disordered systems, which are of the utmost interest in the field of physics, but too complex to be studied analytically.

The article describes the fascinating prospects for this line of research, primarily based on the theoretical predictions made by Lewenstein and Sánchez-Palencia about a new phenomenon known as "disorder-induced order."

Disorder in the physical world is defined as small impurities distributed at random. It is impossible to completely eliminate it in real physical systems, but its effect on the microscopic components of matter is not as negative as it was previously thought to be. Lewenstein and Sánchez-Palencia explain how disorder can lead to new quantum states of matter, such as quantum glass or "dirty" superconductors, which can provide a major contribution towards quantum information processing. One of the differences between macroscopic and microscopic objects is that the former, such as billiard or soccer balls, if we neglect the phenomenon of friction, can roll indefinitely on a flat surface, even if it has small modulations or rough areas. However, they stop when they run into a wall or a hole in the ground.

The situation is completely different in the microscopic world. Quantum particles often behave as wave packets, which enables them to use unique strategies: when they move through a medium with obstacles, they distribute their presence down all possible paths. These possible paths interfere with each other, and if the interference is constructive, the particle reinforces its presence in places it would not have reached if we had been dealing with regular particles. This is how obstacles are avoided in the microscopic world.

However, there is one thing that these fascinating particles cannot resist: disorder, or small rough areas. A group of randomly distributed obstacles, even if they are minute, leaves the particle "out of commission," unable to advance. In this case, its own wave packet works against it, since the irregularity in the order of the obstacles leads the interference between paths to be predominantly destructive, and thus the presence of the particle in most of the points in the box disappears and the particle is only located in one specific point.

This phenomenon is referred to as Anderson Localization, described by the Nobel laureate Philip Warren Anderson more than 50 years ago. Anderson's research laid the groundwork for the development of the information age: the localization processes in disordered systems are what enable us to store data in computer memories.

Maciej Lewenstein is the author of an earlier article, published in 2003, predicting Anderson Localization in ultracold atomic gases with controlled disorder. As Lewenstein and Sánchez-Palencia explain, these localization phenomena in ultracold gases have been subjected to experimental observation. Ultracold atomic gases are of the utmost interest for quantum physicists because they enable macroscopic observation of the microscopic behavior of matter, thus constituting perfect simulators for quantum phenomena. In addition, it is relatively easy to design and control disorder in these gases, and hence they provide the ideal tool for studying an entire family of disorder-induced order phenomena.

Further knowledge of disordered systems could lead to important applications: if computers as we know them today are the result of the knowledge acquired to date about the conductive and insulating properties of matter-stemming from Anderson's work and other studies of disordered systems-it is entirely possible that the quantum computers of the future will process information by managing disorder with ultracold systems.

This induced disorder is what the authors describe in their article, and it can be created and designed on demand by using lasers. The researchers also explain that disorder can lead to new quantum states of matter, such as quantum glass or what are referred to as "dirty" superconductors, a kind of superconductor to which impurities have been added to improve its performance.