arXiv subjects

Qian Liang

Publications and source records attributed to Qian Liang.

25 records · Page 2Linked to original sources

arXiv2022

Observation of Non-Hermitian Skin Effect and Topology in Ultracold Atoms

The non-Hermitian skin effect (NHSE), the accumulation of eigen wavefunctions at boundaries of open systems, underlies a variety of exotic properties that defy conventional wisdom. While NHSE and its intriguing impact on band topology and dynamics have been observed in classical or photonic systems, their demonstration in a quantum many-body setting remains elusive. Here we report the experimental realization of a dissipative Aharonov-Bohm chain -- a non-Hermitian topological model with NHSE -- in the momentum space of a two-component Bose-Einstein condensate. We identify unique signatures of NHSE in the condensate dynamics, and perform Bragg spectroscopy to resolve topological edge states against a background of localized bulk states. Our work sets the stage for further investigation on the interplay of many-body statistics and interactions with NHSE, and is a significant step forward in the quantum control and simulation of non-Hermitian physics.

cond-mat.quant-gas↗

arXiv2022

Aharonov-Bohm Caging and Inverse Anderson transition in Ultracold Atoms

Aharonov-Bohm (AB) caging, a special flat-band localization mechanism, has spurred great interest in different areas of physics. AB caging can be harnessed to explore the rich and exotic physics of quantum transport in flatband systems, where geometric frustration, disorder and correlations act in a synergetic and distinct way than in ordinary dispersive band systems. In contrast to the ordinary Anderson localization, where disorder induces localization and prevents transport, in flat band systems disorder can induce mobility, a phenomenon dubbed inverse Anderson transition. Here, we report on the experimental realization of the AB cage using a synthehtic lattice in the momentum space of ultracold atoms with tailored gauge fields, demonstrate the geometric localization due to the flat band and the inverse Anderson transition when correlated binary disorder is added to the system. Our experimental platform in a many-body environment provides a fashiinating quantum simulator where the interplay between engineered gauge fields, localization, and topological properties of flat band systems can be finely explored.

cond-mat.quant-gas↗

arXiv2022

Doppler cooling of buffer-gas-cooled Barium monofluoride molecules

We demonstrate one-dimensional Doppler cooling of a beam of buffer-gas cooled Barium monofluoride (BaF) molecules. The dependences of the cooling efficiency with the laser detuning, the bias filed and the laser intensity are carefully measured. We numerical simulate our experiment with a Monte Carlo method, and find the theoretic predictions consists with our experimental data. This result represents a key step towards further cooling and trapping of BaF molecules.

physics.atom-ph↗

arXiv2019

A general Zeeman slower for type-II transitions and polar molecules

We proposed a general Zeeman slower scheme applicable to the majority of the laser-coolable molecules. Different from previous schemes, the key idea of our scheme lies in that the compensation of the detuning with the magnetic field is done for the repumping laser instead of the cooling laser. Only atoms or molecules with the right velocity will be repumped and laser slowed. Such scheme is more feasible for molecules with complex energy sturcutres. We apply this scheme for molecules with large Landé g-factor of the excited states and polyatomic molecules, and it shows a better slowing efficiency.

physics.atom-ph↗

arXiv2019

Laser cooling with adiabatic passage for diatomic molecules

We present a magnetically enhanced laser cooling scheme applicable to multi-level type-II transitions and further diatomic molecules with adiabatic transfer. An angled magnetic field is introduced to not only remix the dark states, but also decompose the multi-level system into several two-level sub-systems in time-ordering, hence allowing multiple photon momentum transfer. For complex multi-level diatomic molecules, although the enhancement gets weakened, our simulations still predict a $\sim 4\times$ larger value of the maximum achievable cooling force and a wider coolable velocity range compared to the conventional Doppler cooling. A reduced dependence on spontaneous emission of this scheme makes laser cooling a molecule with leakage channels become a feasibility.

physics.atom-ph↗

arXiv2009

Theoretical Analysis of Cyclic Frequency Domain Noise and Feature Detection for Cognitive Radio Systems

In cognitive radio systems, cyclostationary feature detection plays an important role in spectrum sensing, especially in low SNR cases. To configure the detection threshold under a certain noise level and a pre-set miss detection probability Pf, it's important to derive the theoretical distribution of the observation variable. In this paper, noise distribution in cyclic frequency domain has been studied and Generalized Extreme Value (GEV) distribution is found to be a precise match. Maximum likelihood estimation is applied to estimate the parameters of GEV. Monte Carlo simulation has been carried out to show that the simulated ROC curve is coincided with the theoretical ROC curve, which proves the efficiency of the theoretical distribution model.

cs.IT↗

arXiv2009

Fast Cycle Frequency Domain Feature Detection for Cognitive Radio Systems

In cognitive radio systems, one of the main requirements is to detect the presence of the primary users' transmission, especially in weak signal cases. Cyclostationary detection is always used to solve weak signal detection, however, the computational complexity prevents it from wide usage. In this paper, a fast cycle frequency domain feature detection algorithm has been proposed, in which only feature frequency with significant cyclic signature is considered for a certain modulation mode. Simulation results show that the proposed algorithm has remarkable performance gain than energy detection when supporting real-time detection with low computational complexity.

cs.IT↗