Exceptional Point Physics

Dissipation is ubiquitous in nature; as in radioactive decay of an atomic nucleus and wave propagation in absorptive media, dissipation results from the coupling of these systems to different environmental degrees of freedom. These dissipative systems can be phenomenologically described by non-Hermitian Hamiltonians, where the non-Hermitian terms are introduced to account for the dissipation. The non-Hermiticity leads to a complex energy spectrum with the imaginary part quantifying the loss of particles or energy from the system. The degeneracies of a non-Hermitian Hamiltonian are known as exceptional points (EPs), where both the eigenvalues and the associated eigenstates coalesce. The existence of EPs has been demonstrated in many classical systems with applications such as chiral laser emission [e.g., PNAS 113, 6845 (2016)] and enhanced sensing [e.g., Nature 548, 192 (2017)].

Dissipative quantum systems are usually described by a Lindblad master equation that contains two dissipative terms: the first is a term that describes quantum jumps between the energy eigenstates of the system, and the second is a term that yields coherent non-unitary evolution. Suppressing the former term, for example, through post-selection to eliminate trajectories that contain quantum jumps, results in an effective non-Hermitian Hamiltonian evolution. Our second approach to investigate EPs utilizes the (generically non-Hermitian) Liouvillian superoperators of the full Lindblad equation. We studied chiral state transfer via dynamically encircling the Hamiltonian and Liouvillian EPs in the parameter space [PRL 128, 110402 (2022); PRL 128, 160401 (2022), PRL 133, 070403 (2024)].

Nonequilibrium Open Quantum Systems

Floquet engineering, which involves control of physical systems through periodic driving, provides a powerful way to explore and control nonequilibrium properties of materials, for example, dramatically altering the band structure of materials, and creating artificial matters such as discrete time crystals in periodically driven many-body-localized systems. The periodic drive will heat up the materials via resonance excitations or multi-photon absorptions; in the long-time limit, the system will reach featureless infinite-temperature states due to the energy injection from the drive. The heating effect, however, can be balanced by dissipation arising from the coupling to environmental degrees of freedom, which leads to nonequilibrium steady states (NESSs). Engineering dissipation by leveraging the synergy between coherent driving and dissipation pathways therefore offers a powerful means to control NESSs, including generating entanglement, tuning phase transitions, and controlling superconductivity. Our recent work [PRL 134 (9), 090402 (2025)] in generating high-purity NESSs through a Floquet drive highlights new opportunities in harnessing nonequilibrium systems for quantum applications.  

AI for Quantum Circuit Design

We are currently developing machine learning algorithms to achieve automated multi-qubit device design, in both forward and reverse engineering.