So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a quantum simulator. Their experiments in this system, presented in July 2017 at a conference in Trieste, revealed completely unexpected periodic oscillations in the dynamics of the interacting atoms.
Ball bouncing chaotically in a stadium (top). If it starts near an unstable trajectory, it remains close to this trajectory for some time but eventually escapes (bottom).
IST Austria/Maksym Serbyn
Now, an international team of researchers, including Alexios Michailidis and Maksym Serbyn from the Institute of Science and Technology Austria (IST Austria) as well as researchers from the University of Leeds and the University of Geneva, have solved the mystery of these previously inexplicable oscillations.
The theoretical explanation they proposed introduces a concept of a “quantum many-body scar” that alters our understanding of the dynamics that are possible in many-body quantum systems.
Imagine a ball bouncing around in an oval stadium. It will bounce around chaotically, back and forth through the available space. As its motion is random, it will sooner or later visit every place in the stadium.
Amidst all the chaos, however, there might be a potential for order: if the ball happens to hit the wall at a special spot and at the “correct” angle of incidence, it might end up in a periodic orbit, visiting the same places in the stadium over and over and not visiting the others. Such a periodic orbit is extremely unstable as the slightest perturbation will divert the ball off its track and back into chaotic pondering around the stadium.
The same idea is applicable to quantum systems, except that instead of a ball bouncing around, we are looking at a wave, and instead of a trajectory, we are observing a probability function. Classical periodic orbits can cause a quantum wave to be concentrated in its vicinity, causing a “scar”-like feature in a probability that would otherwise be uniform.
Such imprints of classical orbits on the probability function have been named “quantum scars”. The phenomenon, however, was only expected to happen with a single quantum particle, as the complexity of the system rises dramatically with every additional particle, making periodic orbits more and more unlikely.
“Generally people assumed that it was impossible for many-body systems to have quantum scars, and when people first saw the oscillations they could not explain it,” says Maksym Serbyn, Professor at IST Austria and co-author of the study. “By extending the concept of scars to quantum many-body systems, we were able to explain why these oscillations are there,” he adds.
In the study, which was published in Nature Physics, the researchers explain the experimental observation with the occurrence of quantum many-body scars. They also identify the many-particle unstable periodic orbit behind the scar behavior as the coherent oscillation of atoms between the excited and ground states.
Intuitively, the quantum many-body scar may be envisioned as a part of configuration space that is to some extent “shielded” from chaos, thus leading to a much slower relaxation. In other words: the system takes longer to return to chaos—the equilibrium state.
“We still don’t know how common quantum many-body scars are, but we have found one example, and this is a paradigm shift,” Serbyn says. But there is a lot left to find out. “We don’t yet understand all the properties of many-body quantum scars, but we have successfully explained the data. We hope that a better understanding of quantum scars will provide a way of protecting quantum systems from relaxation.”
Weak ergodicity breaking from quantum many-body scars, C. J. Turner, A. A. Michailidis, D. A. Abanin, M. Serbyn & Z. Papić, Nature Physics (2018), doi:10.1038/s41567-018-0137-5
The Institute of Science and Technology (IST Austria) is a PhD-granting research institution located in Klosterneuburg, 18 km from the center of Vienna, Austria. Inaugurated in 2009, the Institute is dedicated to basic research in the natural and mathematical sciences. IST Austria employs professors on a tenure-track system, postdoctoral fellows, and doctoral students. While dedicated to the principle of curiosity-driven research, the Institute owns the rights to all scientific discoveries and is committed to promote their use. The first president of IST Austria is Thomas A. Henzinger, a leading computer scientist and former professor at the University of California in Berkeley, USA, and the EPFL in Lausanne, Switzerland. The graduate school of IST Austria offers fully-funded PhD positions to highly qualified candidates with a bachelor’s or master’s degree in biology, neuroscience, mathematics, computer science, physics, and related areas. www.ist.ac.at
University of Leeds
The University of Leeds is one of the largest higher education institutions in the UK, with more than 33,000 students from more than 150 different countries, and a member of the Russell Group of research-intensive universities.
We are a top ten university for research and impact power in the UK, according to the 2014 Research Excellence Framework, and are in the top 100 for academic reputation in the QS World University Rankings 2018. Additionally, the University was awarded a Gold rating by the Government’s Teaching Excellence Framework in 2017, recognising its ‘consistently outstanding’ teaching and learning provision. Twenty-six of our academics have been awarded National Teaching Fellowships – more than any other institution in England, Northern Ireland and Wales – reflecting the excellence of our teaching. www.leeds.ac.uk
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