A-CUHK402/18
Classification and realization of topological phases in strongly correlated systems: a tensor network approach (TNTOP)

Hong Kong Principal Investigator: Prof Gu Zhengcheng (The Chinese University of Hong Kong)
French Principal Investigator: Prof Poilblanc Didier (University of Touloouse)

It was long believed that Landau symmetry-breaking theory describes all possible orders and all possible (continuous) phase transitions in materials. However, Nature is always full of surprises. In last three decades, it has become more and more clear that Landau symmetry-breaking theory fails to describe all possible orders in strongly correlated systems. One famous example is the fractional quantum Hall (FQH) effect. It turns out that FQH states are topological phases of quantum mater and contain a new class of order – topological order.

Topological order is believed to exist in general strongly correlated systems and will make great impact on future industry, e.g, the realization of topological quantum computation. Unfortunately, a unified analytical and numerical framework to study topological phenomena in strongly correlated systems has not been developed so far due to the lack of principle analogous to symmetry breaking and mathematical approach analogous to mean field theory. Recent years, it has been realized that topological phases of quantum matter can be systematically characterized by long-range entanglement, which can be efficiently encoded by the so-called tensor network states. Therefore, the analytic and numerical study of tensor network state will become a very powerful tool to understand topological phases of quantum matter.

However, great challenges still need to be overcome for implementing tensor network states to systematically study topological phenomena in strongly correlated systems. Analytically, it is still unclear if tensor network states can faithfully represent all topologically ordered states or not. Numerically, it turns out that it can be exponentially hard to calculate physical quantities (ground state energy, correlation functions, etc.) exactly for general tensor network states.

This joint project will form a strong-strong union of an theoretical team in France and Hong Kong to solve the key difficulties in tensor network approach to topological phases of quantum matter in strongly correlated systems. We will team up to classify and realize topological phases of quantum matter in strongly correlated systems using tensor network states approach. We will use tensor network states to systematically classify topological phases of quantum matter in strongly correlated systems. Then we will construct physically motivated tensor network variational wavefunctions to study topological phase transitions. Finally, we will develop new algorithm and implement tensor network states with large bond dimension to study realistic topological materials, such as frustrated magnets and doped Mott-insulator.

A-CUHK404/18
Nanoscale thermal sensing using hybrid diamond sensors

Hong Kong Principal Investigator: Prof Li Quan (The Chinese University of Hong Kong)
French Principal Investigator: Prof Gergely Csilla (Montpellier University)

The project aims at implementing an atomic-scale thermometer surpassing the performances of present nanoscale temperature probes, by combining unprecedentedly high sensitivity and nanoscale spatial resolution with operation over a broad range of temperature and in various environments (including biological media). At the same time, the applicability of such sensors will be demonstrated in two fields of applications: thermoplasmonics and biothermics under exogeneous heating.

The sensor will be based on a novel design of hybrid architecture, building on a conversion of temperature changes to magnetic changes probed by electronic spins in a diamond. Beyond-state-of-the-art capabilities will be demonstrated with sensitivities down to sub-mK/Hz1/2. By relying on single spins, high spatial resolution down to a few tens of nanometers will be achieved. Unprecedentedly, such sensors shall also operate over a broad range of temperature and in various environments including biological media. The sensor's utmost performance will be evidenced by mapping the temperature gradients in the vicinity of gold particles and arrays in connection with thermoplasmonics applications. The possibility to extend such mapping to biological objects at a single cell level, will be explored, by providing a lower bound of the probe's sensitivity in a medium as complex as a cell under exogeneous heating and by tracking temperature during physiological changes subjected to thermal stress.

This project promises significant progress in the rapidly-growing field of nanoscale thermometry while opening up new prospects still out-of-reach in nanoscale thermodynamics.

A-HKUST601/18
METARooms: deep subwavelength reconfigurable acoustic treatments for room acoustics

Hong Kong Principal Investigator: Prof Sheng Ping (The Hong Kong University of Science and Technology)
French Principal Investigator: Dr Groby Jean-Philippe (Acoustics Laboratory of the University of Maine)

The extraordinary functionalities of acoustic metamaterials have led to the realization of wave manipulation techniques previously regarded as impossible with deep subwavelength structures. Unfortunately, since much of metamaterials' properties originate from the resonance phenomenon, the novel functionalities are necessarily restricted to narrow frequency ranges; yet broadband is usually a necessity in practical applications. Recently, breakthroughs in the designed integration strategy have overcome the narrow frequency limitation and showed that some functionalities, such as sound absorption, can be made to be tunable in accordance with the target absorption spectrum. Such designed integration scheme has already led to the formation of a Hong Kong startup company, Acoustic Metamaterials Group (AMG), which has achieved mass production capability of the designed prototypes, and of a French startup company, Metacoustic, which proposes acoustic and vibration solutions consisting in metaporous and metaporoelastic layers. In this project, we would like to extend the previous success in acoustic absorption to open a new frontier in room acoustics, by constructing walls that can passively switch from a totally absorbing to a spatially modulated reflection phase, by utilizing resonances to tune the impedance of the walls. Such change can significantly alter the audio experience of a room, from anechoic-like to the audio feel of a larger room than it is in reality. Traditionally, acoustic wall constructs are static and achieve only one functionality. Moreover, they are efficient at high frequencies but results in bulky and heavy structures at low frequencies. This project aims at designing deep subwavelength re-configurable acoustic metamaterials for altering the room acoustics. We intend to draw on the combined expertise of the Hong Kong and French teams to prove experimentally the effectiveness of such a system in two demonstration rooms, one in Hong Kong and one in Le Mans. The two demonstration rooms will use different integration approaches and different resonators (Fabry-Perot resonators for the Hong Kong team, and Helmholtz resonators for the Le Mans team) in achieving the same goal. The research and development experience of both teams in theory, simulation, sample fabrication, and implementation will be enriched through each others' approaches, as well as through mutual visits and student exchanges.

Both teams will collaborate with AMG and Metacoustic in this project. AMG's role will be to provide space for the demonstration room in Hong Kong, while the PI, Co-I and students in Hong Kong and in Le Le Mans will design, fabricate and test the acoustic metamaterials in collaboration with AMG and Metacoustic. We envision new commercial opportunities with the successful completion of this project.