Dr. Jen’s group recently investigated nonequilibrium dynamics in a chirally-coupled atomic chain [1-4] which can be utilized as a quantum interface to retain the memory of initial quantum states and provide a means for photon storage or photon routing. A chirally-coupled atomic chain comprises many one-dimensional quantum emitters that exhibit chiral coupling and exhibit nonreciprocal decay channels along with light-induced dipole-dipole interactions mediated via an atom-waveguide interface. In this system, collective decays of the excited atoms can be subradiant and bound in space when they are strongly coupled to the guided modes supported in the interface.
Dr. Jen’s group considered various regimes to explore the capability of this quantum interface theoretically. In a driven-dissipative setup , the steady-states of interaction-driven crystalline orders, edge or hole excitations, and a dichotomy of chiral flow can be generated. Nonergodic butterflylike system dynamics in the phase of extended hole excitations with a signature of persistent subharmonic oscillations can also be presented. These results demonstrate the interaction-induced quantum phases of matter with chiral couplings, and they provide opportunities to simulate many-body states in nonreciprocal quantum optical systems. When this quantum interface is under position disorder , a phase transition from excitation delocalization to localization with interplay between the directionality of decay rates and the strength of light-induced dipole-dipole interactions can be identified. Deep in the localization phase, its characteristic length decreases and saturates toward a reciprocal coupling regime, leading to system dynamics whose ergodicity is strongly broken. This interface also allows interaction-driven reentrant behavior for the localization phase and a reduction of level repulsion under strong disorder. When multiply excited atoms are considered , their average density-density and modified third-order correlations via Kubo cumulant expansions can arise and finite correlations can be sustained for a long time. This suggests shape-preserving dimers and trimers of atomic excitations emerging in the most subradiant coupling regime and suggests a potential application to quantum information processing and quantum storage in the encoded nonreciprocal spin diffusion.
“Light-matter interacting quantum systems are always fascinating since they show rich phenomena that can be applied to next-generation technology and offer many opportunities in new directions of physical studies” said Dr. Jen. The “quantum supremacy” associates the capacity of the states that a quantum system can access, which underlies the power of future quantum computers. However, the powerfulness of a quantum system is fragile since it is easily attenuated by the outside world, which leads to an unwanted dissipation and leaves behind an idealistic isolated realm, a shelter where a ‘powerful’ quantum system is supposed to stay. Many novel perspectives to understand the interacting quantum systems under dissipations or non-Hermitian quantum systems have been suggested. When a full count of system and reservoir degrees of freedom is involved, an ultimate understanding of dynamical quantum systems may be achieved.
This is not the only way toward complete comprehension of a quantum system, and “I believe new methods and machineries will be invented continuously to understand the quantum world, in a sense to probe the essence of it via different angles and gradually fill the patches of a whole understanding” said Dr. Jen. We expect to hear of more interesting and exciting investigations on unraveling and harnessing the potentiality of quantum interfaces from Dr. Jen in the near future.
Fig. 1. Schematic plot of a chirally-coupled atomic chain and time evolutions of the atomic excitations of bound dimers and trimers. (a) Atomic excited state populations evolved from two atomic excitations side-by-side (left) and separated by two lattice sites (right) for N = 40. (b) Time evolutions of the excitation populations from three atomic excitations side-by-side for N = 21. 
Fig. 2. Members and collaborators of the theoretical team. From left to right:
Yi-Cheng Wang, Chi-Chih Chen,Tzu-Hsuang Chang, Dr. Hsiang-Hua Jen, Prof. Jhih-Shih You, Chin-Yang Lin,Wei-Seng Hiew, Nai-Yu Tsai, and Dr. Kuldeep Suthar.
- H. H. Jen. Phys. Rev. Research (2020) DOI:10.1103/PhysRevResearch.2.013097
- H. H. Jen, et. al. Phys. Rev. A (2020) DOI: 10.1103/PhysRevA.101.023830
- H. H. Jen. Phys. Rev. A (2020) DOI: 10.1103/PhysRevA.102.043525
- H. H. Jen. Phys. Rev. A (2021) DOI: 10.1103/PhysRevA.103.063711