A quantum entanglement between two physically separated ultra-cold atomic clouds

The prestigious journal Science has echoed a novel experiment in the field of quantum physics in which several members of the Quantum Information Theory and Quantum Metrology research group of the Department of Theoretical Physics and History of Science at the UPV/EHU's Faculty of Science and Technology participated, led by Geza Toth, Ikerbasque Research Professor, and carried out at the University of Hannover.

In the experiment, they achieved quantum entanglement between two ultra-cold atomic clouds, known as Bose-Einstein condensates, in which the two ensembles of atoms were spatially separated from each other.

Quantum entanglement was discovered by Schrodinger and later studied by Einstein and other scientists in the last century. It is a quantum phenomenon that has no counterparts in classical physics.

The groups of entangled particles lose their individuality and behave as a single entity. Any change in one of the particles leads to an immediate response in the other, even if they are spatially separated.

"Quantum entanglement is essential in applications such as quantum computing, since it enables certain tasks to be performed much faster than in classical computing," explained the leader of the Quantum Information Theory and Quantum Metrology group Geza Toth.

Unlike the way in which quantum entanglement between clouds of particles has been created up to now, and which involves using incoherent and thermal clouds of particles, in this experiment they used a cloud of atoms in the Bose-Einstein condensate state.

As Toth explained, "Bose-Einstein condensates are achieved by cooling down the atoms to very low temperatures, close to absolute zero. At that temperature, all the atoms are in a highly coherent quantum state; in a sense, they all occupy the same position in space. In that state quantum entanglement exists between the atoms of the ensemble." Subsequently, the ensemble was split into two atomic clouds.

"We separated the two clouds from each other by a distance, and we were able to demonstrate that the two parts remained entangled with each other," he continued.

The demonstration that entanglement can be created between two ensembles in the Bose-Einstein condensate state could lead to an improvement in many fields in which quantum technology is used, such as quantum computing, quantum simulation and quantum metrology, since these require the creation and control of large ensembles of entangled particles.

"The advantage of cold atoms is that it is possible to create highly entangled states containing quantities of particles outnumbering any other physical systems by several orders of magnitude, which could provide a basis for large scale quantum computing," said the researcher.

Research Report: Entanglement between two spatially separated atomic modes

Explanation for puzzling quantum oscillations has been found
Vienna, Austria (SPX) May 17, 2018 - 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.

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."

Research paper

Institute of Science and Technology Austria

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