many-body localization

Scientists have found a new way to create disturbances that do not fade away. Instead of relying on disorder to freeze things in place, they tipped a quantum container to one side—a trick that is easier to conjure in the lab. A collaboration between the experimental group of College Park Professor Christopher Monroe and the theoretical group of JQI Fellow Alexey Gorshkov, who is also a Fellow of the Joint Center for Quantum Information and Computer Science, has used trapped ions to implement this new technique, confirming that it prevents their quantum particles from reaching equilibrium. The team also measured the slowed spread of information with the new tipping technique for the first time. They published their results recently in the journal Nature.

The quantum world blatantly defies intuitions that we’ve developed while living among relatively large things, like cars, pennies and dust motes. The quantum behavior of dynamical localization bucks the assumption that a cold object will always steal heat from a warmer object. Until now, dynamical localization has only been observed for single quantum objects, which has prevented it from contributing to attempts to pin down where the changeover occurs. JQI researchers and colleagues have investigated mathematical models to see if dynamical localization can still arise when many quantum particles interact. To reveal the physics, they had to craft models to account for various temperatures, interaction strengths and lengths of times. The team’s results, published in Physical Review Letters, suggest that dynamical localization can occur even when strong interactions are part of the picture.

Consider, for a moment, the humble puddle of water. If you dive down to nearly the scale of molecules, it will be hard to tell one spot in the puddle from any other. You can shift your gaze to the left or right, or tilt your head, and the microscopic bustle will be identical—a situation that physicists call highly symmetric.That all changes abruptly when the puddle freezes. In contrast to liquid water, ice is a crystal, and it gains a spontaneous rigid structure as the temperature drops. Freezing fastens neighboring water molecules together in a regular pattern, and a simple tilt of the head now creates a kaleidoscopic change.In 2012, Nobel-prize winning physicist Frank Wilczek, a professor at the Massachusetts Institute of Technology, proposed something that sounds pretty strange. It might be possible, Wilczek argued, to create crystals that are arranged in time instead of space. The suggestion prompted years of false starts and negative results that ruled out some of the most obvious places to look for these newly named time crystals.Now, five years after the first proposal, a team of researchers led by physicists at the Joint Quantum Institute and the University of Maryland have created the world's first time crystal using a chain of atomic ions. The result, which finally brings Wilczek's exotic idea to life, was reported in Nature on March 9.

Nature doesn’t have the best memory. If you fill a box with air and divide it in half with a barrier, it’s easy to tell molecules on the left from molecules on the right. But after removing the barrier and waiting a short while, the molecules get mixed together, and it becomes impossible to tell where a given molecule started. The air-in-a-box system loses any memory of its initial conditions.The universe has been forgetting its own initial state since the Big Bang, a fact linked to the unrelenting forward march of time. Systems that forget where they started are said to have thermalized, since it is often—but not always—an exchange of heat and energy with some other system that causes the memory loss. For example, a melting ice cube forgets its orderly arrangement of water molecules when heat from its surroundings splits the cube’s crystal bonds. In some sense, the initial information about the ice cube—the structure of the crystal, the distance between molecules, etc.—leaks away.The opposite case is localization, where information about the initial arrangement sticks around. Such a situation is rare, like an ice cube that never melts, but one example is Anderson localization, in which particles or waves in a crystal are trapped near impurities. They tend to bounce off defects in the crystal and scatter in random directions, yielding no net movement. If there are enough impurities in a region, the particles or waves never escape.Since the discovery of Anderson localization in 1958, it has been an open question whether interacting collections of quantum particles can also localize, a phenomenon known as many-body localization. Now, researchers working with JQI and QuICS Fellow Christopher Monroe have directly observed this localization in a system of 10 interacting ions, trapped and zapped by electric fields and lasers. Their findings are one of the first direct observations of many-body localization in a quantum system, and they open up the possibility of studying the phenomenon with more ions. The results were published June 6 in Nature Physics.