2021-now:Associate Researcher, Shenzhen Institute of Quantum Science and Engineering (SUSTech)
2017-2021: Postdoc, Beijing Computational Science Research Center
2017: PhD, TU Berlin, Supervisor: Tobias Brandes
2013: Master of Science, TU Berlin, Supervisor: Tobias Brandes
2012: Bachelor of Science, TU Berlin, Supervisor: Tobias Brandes
Motivation and achievements
My research investigates principles of light-matter interaction
in the quantum optical and semiclassical regimes. In this context, I develope protocols for
quantum control and quantum sensing. Besides others, my research has contributed to the
understanding of the exciton-polariton dynamics and the quantum control of Floquet systems.
Research highlights
Polariton Localization and Dispersion Properties of Disordered Quantum Emitters in Multimode Microcavities
Georg Engelhardt and Jianshu Cao
Experiments have demonstrated that the strong light-matter coupling in polaritonic microcavities
significantly enhances transport. Motivated by these experiments, we have solved the disordered
multimode Tavis-Cummings model in the thermodynamic limit and used this solution to analyze its
dispersion and localization properties. The solution implies that wave-vector-resolved spectroscopic
quantities can be described by single-mode models, but spatially resolved quantities require the
multimode solution. Nondiagonal elements of the Green’s function decay exponentially with distance,
which defines the coherence length. The coherent length is strongly correlated with the photon weight and
exhibits inverse scaling with respect to the Rabi frequency and an unusual dependence on disorder. For
energies away from the average molecular energy and above the confinement energy, the
coherence length rapidly diverges such that it exceeds the photon resonance wavelength λ. The rapid
divergence allows us to differentiate the localized and delocalized regimes and identify the transition from
diffusive to ballistic transport.
Unusual dynamical properties of disordered polaritons in microcavities
Georg Engelhardt and Jianshu Cao
The collective light-matter interaction in microcavities gives rise to the intriguing phenomena of cavity-mediated
transport that can potentially overcome the Anderson localization. Yet, an accurate theoretical treatment
is challenging as the matter (e.g., molecules) is subject to large energetic disorder. In this paper, we develop the
Green’s function solution to the Fano-Anderson model and use the exact analytical solution to quantify the effects
of energetic disorder on the spectral and dynamical properties in microcavities. Starting from the microscopic
equations of motion, we derive an effective non-Hermitian Hamiltonian and predict a set of scaling laws: (i) The
complex eigenenergies of the effective Hamiltonian exhibit an exceptional point, which leads to underdamped
coherent dynamics in the weak disorder regime, where the decay rate increases with disorder, and overdamped
incoherent dynamics in the strong disorder regime, where the slow decay rate decreases with disorder. (ii)
The total density of states of disordered ensembles can be exactly partitioned into the cavity, bright-state, and
dark-state local density of states, which are determined by the complex eigensolutions and can be measured via
spectroscopy. (iii) The cavity-mediated relaxation and transport dynamics are intimately related such that both
the energy-resolved relaxation and transport rates are proportional to the cavity local density of states. The ratio
of the disorder-averaged relaxation and transport rates equals the molecule number, which can be interpreted as
a result of a quantum random walk. (iv) A turnover in the rates as a function of disorder or molecule density can
be explained in terms of the overlap of the disorder distribution function and the cavity local density of states.
These findings reveal the significant impact of the dark states on the local density of states and consequently
their crucial role in optimizing spectroscopic and transport properties of disordered ensembles in cavities.
Dynamical Symmetries and Symmetry-Protected Selection Rules in Periodically Driven Quantum Systems
Georg Engelhardt and Jianshu Cao
In recent experiments, the light-matter interaction has reached the ultrastrong coupling limit, which can
give rise to dynamical generalizations of spatial symmetries in periodically-driven systems. Here, we
present a unified framework of dynamical-symmetry-protected selection rules based on Floquet response
theory. Within this framework, we study rotational, parity, particle-hole, chiral, and time-reversal
symmetries and the resulting selection rules in spectroscopy, including symmetry-protected dark states
(spDS), symmetry-protected dark bands, and symmetry-induced transparency. Specifically, dynamical
rotational and parity symmetries establish spDS and symmetry-protected dark band conditions. A particle-hole
symmetry introduces spDSs for symmetry-related Floquet states and also a symmetry-induced
transparency at quasienergy crossings. Chiral symmetry and time-reversal symmetry alone do not imply
spDS conditions but can be combined to define a particle-hole symmetry. These symmetry conditions arise
from destructive interference due to the synchronization of symmetric quantum systems with the periodic
driving. Our predictions reveal new physical phenomena when a quantum system reaches the strong lightmatter
coupling regime, which is important for superconducting qubits, atoms and molecules in optical or
plasmonic field cavities, and optomechanical systems.
