Posted in | Quantum Physics

Researchers Reveal Unique Nature of Quantum Solitary Waves

Solitary waves, called solitons, appear in many forms. Maybe the most recognizable form is a tsunami, which forms after a disruption on the ocean floor and can travel, without any reduction in intensity, at high speeds for hundreds of miles.

Zoran Norcic/

By definition, a soliton can retain its shape while spreading at a constant velocity. But what happens when two, or more, solitons act together? The general consensus from previous studies indicates that solitons are essentially unchanged by this interaction and pass through one another; however, Erich Mueller, Physics Professor, and Shovan Dutta, a Graduate Student, have challenged that concept in a report just published in Physical Review Letters.

Their paper, “Collective Modes of a Soliton Train in a Fermi Superfluid,” was published on June 29th. Both men work in the Laboratory of Atomic and Solid State Physics at Cornell University.

The research team found something much different for solitons interacting in a superfluid, which forms when a gas made of atoms is cooled to near absolute zero. In addition to affecting one another, the solitons can also collide and destroy each other.

Latest experiments have developed single, long-lived solitons in a superfluid. Both Dutta and Mueller theoretically looked at the interactions within a large array of these solitons in a superfluid, such as Lithium-6. To their surprise, they found an instability where pairs of solitons collide and destroy one another. Mueller and Dutta also discovered a range of novel collective oscillations of the solitons.

The rate of instability is sensitive to the interaction between atoms and the separation of solitons, both of which can be tuned in experiments. They also found that instability can be prevented by magnetizing the gas, forming an exotic quantum state that was first discussed in the context of superconductors with magnetic impurities in the 1960s.

Dutta and Mueller started this work by searching for supersymmetry in condensed-matter physics; in particle physics, the supersymmetry theory links the two basic classes of elementary particles – fermions and bosons – and states that for each particle from one group, a “superpartner” exists from the other.

One direction that we were running in was that we thought we had a way of explicitly seeing this symmetry [in condensed matter].

Erich Mueller, Physics Professor, Cornell University

Mueller stated that it turned out it did not exist; however, what he and Dutta did find, formed the basis of their paper. By comparing fermionic and bosonic excitations of the superfluid, they explored the collective motion of an array of solitons and discovered that the waves, which were formed in one dimension, took on numerous collective motions. While some of them were expected, others, including the instability, were not.

Dutta and Mueller also found that it is possible to overcome the instability by magnetization, which effectively forms a spatially modulated, imbalanced superfluid phase – called the FFLO state – that had been theoretically discussed 50 years ago but never directly accomplished in experiments. Dutta said that this opens the door to further research into novel quantum states and other related areas, such as exotic superconductivity.

It has been a long-standing challenge for a large community of people, to create this quantum state and our findings show that one can directly engineer it in cold atomic gases.

Shovan Dutta, a Graduate Student

Dutta and Mueller have submitted a related paper on their process for direct engineering of this novel quantum state. Their work can broaden the understanding of the non-equilibrium dynamics of many-body quantum systems.

“If you can establish the basic elements of the dynamics for this system, which could be seen as a prototype for more complicated systems, then that gives you some understanding of how the quantum world works,” Dutta added.

The National Science Foundation and the Army Research Office’s Multidisciplinary University Research Initiative provided support for this work.

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