XJTU researchers make progress in ultra-cold atomic physics

2021-08-23  []


Zhang Ren, associate professor of the School ofPhysics of Xi'an Jiaotong University (XJTU), and the research group of Professor Zhou Qi of Purdue University in the United States have made progress in the quantum simulation of ultra-cold atomic physics. Related results were published in top physics journals Physical Reviews Letter and Science Bulletin.        



Based on the ultra-cold atomic physics platform, people can carry out novel quantum physics research and deepen people's understanding of curved space and its symmetry applications.


On one hand, with the flexible maneuver ability of ultra-cold atomic physics, people can study physical phenomena in high-dimensional space.Therefore, the study of artificial dimensions is one of the frontier research directions that people have attached great importance toin recent years.


For example, by using the interaction between lasersand atoms, people have realized geometric structures such as Hall cylinders and Hall bands, and therefore been able to study the novel physical phenomena in them.


The research group of Zhou and Associate Professor Zhang Ren studied the localization phenomenon of the Hall cylinder,    and theoretically pointed out that the boundary conditions in the artificial dimension are crucial to the appearance of the localization state.


The conclusions are consistent with the experimental results of  Professor Ian Spielman's group of the National Bureau of Standards. The research results were published in the Physical Review Letters (2021) under the title of "Localization on a synthetic Hall cylinder".


On the other hand, the research of ultra-cold atomphysics has gradually intersected with other research fields. The Efimov state,with discrete scale invariance, has attracted the interest of physicists fordecades.Recent studies have found that the Efimov effect not only exists in the three-body problem,but also in condensed matter, such as to pological materials and graphene.        


The quantum state studied on the hyperbolic plane has similar properties to the Efimov state.Specifically, the non-zero energy eigenstates on the Poincaré half-plane and the Poincaré disk are infinitely degenerate. There is adiscrete scale invariance similar to the Efimov state between the degeneratestates. Therefore, they call these degenerate eigenstates Efimov-like states.        


By manipulating the non-uniform transition intensitybetween the grid points and the energy occupied by the grid points, the authorpointed out that the grid point model in a flat, two-dimensional space cansimulate Poincaré half-planes, Poincaré disks and other arbitrarytwo-dimensional Riemann surfaces, and provide a new platform for studying novelquantum effects in curved space.


For example, there is a funnel mouth in hyperbolic space. In the hyperbolic coordinate system, the eigenstate index of the systemis distributed near the mouth of the funnel. Therefore, in the process of dynamical evolution, any initial wave packets will gather towards the funnel mouth. This phenomenon is called the quantum funnel phenomenon. This is the first time that physicists have discovered the quantum funnel phenomenon in the Hermitian system.


The research results were published online in the"Chinese Science Bulletin" under the title "Efimov-likestates and quantum funneling effects on synthetic hyperbolic surfaces". Zhang is the  first  author  of the above two papers, and  XJTU  is  the  first signatory unit.


Symmetry is one of the most important tools for people to carry out physics research. With the aid of symmetry analysis, people can get the most thorough understanding of related physical phenomena.


Based on the SU (1,1)symmetry of the two-dimensional interaction condensate, the research group of Zhang and Zhou studied the dynamics of two-dimensional Bose gas, and proposed an SU (1,1) echo method that can modulate the dynamic evolution o f the breathing mode.Based on this method, they provided an intuitive physical image of the behavior of  breathing patterns with special geometric structures.


An experiment conducted by the Research University of Paris (Ecole Normale Superieure), one of France's best post-second aryinstitutions, in 2019 found that the oscillation period of certain shapes of breathing modes is consistent with the traditional theory, while other shapes have longer periods, and even produce irregular, non-periodical motions.


These novel behaviors of breathing modes have aroused great interest in the entire field, but there has been a lack of theoreticalexplanations. As the breathing mode of the eigenstate of any harmonic potential well,  its  dynamic evolution can be visualized   on a single Poincaré disk, corresponding to closedor open orbits under attractive or repulsive potential wells, respectively.        


The SU (1,1) echo method can reverse these dynamic processes and close these orbits. As the eigenstate of an inharmonic potentialwell, the breathing mode needs to rely on multiple disks to describe its dynamics. The periodic behavior is derived from the quantum interference between different disks.


If the dynamic phase in quantum interference is incommensurate, the motion of the breathing mode will not show any periodicbehavior. Different from the breathing mode generated by passive dynamics, the SU (1,1) echoperiodically modulates the frequency of the potential well.        


On one hand, the dynamic process including free expansion  can  be reversed,  and on the other hand, the degree of freedom  in thedynamic evolution of the breathing mode is expanded, which makes the breathingmode SU(1,1) echo method have certain   advantages in detecting the detuning of the harmonic potential well.


The spin echo based on the symmetry of SU (2) has been widely used in medical and scientific research, and has a profound  impact on people's daily lives. Researchers believe that the SU (1,1) echo method is also expected to have an important impact in  the future.


The research results are titled "SU (1,1) Echoes for Breathers inQuantum Gases" and was published in Physical Review Letters(2020) in December 2020. Zhang is the co-first author of the paper.


This series of research was funded by the National Key R&D  Program of China and the National Natural Science Foundation of  China. Zhang's main research interests focus on the few-body problem of ultra-coldatomic physics, the evolution of many-body dynamics, and the quantum simulation of curved space.


Link to the paper: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.253002