An international team of researchers, led by the University of Leeds and in cooperation with the Institute of Science and Technology Austria and the University of Geneva, has provided a theoretical explanation for the unusual behavior of individual atoms that were trapped and manipulated in a recent Harvard-MIT experiment. In this experiment, the researchers were able to manipulate a notable number of atoms through a quantum simulator. The experiment used a system of finely tuned lasers acting as optical tweezers to assemble a chain of 51 atoms. When the quantum dynamics of the atom chain were measured, there were surprising oscillations that persisted for much longer than expected and that could not be explained. Quantum systems can exist in many possible states, here illustrated by groups of spins, each pointing along a certain direction. Thermalization occurs when a system evenly explores all allowed configurations. But when a “quantum scar” forms (as shown in the figure), some configurations emerge as special. This feature allows scarred systems to sustain memory of the initial state despite thermalization. Courtesy of Zlatko Papic, University of Leeds. “The previous Harvard-MIT experiment created surprisingly robust oscillations that kept the atoms in a quantum state for an extended time," said University of Leeds researcher Zlatko Papic. "We found these oscillations to be rather puzzling because they suggested that atoms were somehow able to ‘remember’ their initial configuration while still moving chaotically." “Our goal was to understand more generally where such oscillations could come from, since oscillations signify some kind of coherence in a chaotic environment — and this is precisely what we want from a robust quantum computer," Papic said. "Our work suggests that these oscillations are due to a new physical phenomenon that we called ‘quantum many-body scar.’” A quantum scar occurs when a special configuration or pathway leaves an imprint on the particles’ state that keeps the particles from filling an entire space. Consequently the systems are prevented from reaching thermalization and are thus allowed to maintain some quantum effects. “We are learning that quantum dynamics can be much more complex and intricate than simply thermalization," Papic said. "The practical benefit is that extended periods of oscillations are exactly what is needed if quantum computers are to become a reality. The information processed and stored on these computers will be dependent on keeping the atoms in more than one state at any time, it is a constant battle to keep the particles from settling into an equilibrium.” “Previous theories involving quantum scars have been formulated for a single particle," said researcher Christopher Turner. "Our work has extended these ideas to systems that contain not one but many particles, which are all entangled with each other in complicated ways. Quantum many-body scars might represent a new avenue to realize coherent quantum dynamics.” The quantum many-body scars theory sheds light on the quantum states that underpin the strange dynamics of atoms in the Harvard-MIT experiment. Understanding this phenomenon could also pave the way for protecting or extending the lifetime of quantum states in other classes of quantum many-body systems. The research was published in Nature Physics (doi:10.1038/s41567-018-0137-5).