There rarely exist physical systems fully isolated from their environment. People have long been looking for approaches to achieve operations immune to small perturbations in such non-conservative systems. Non-Hermitian Hamiltonians provide a powerful description of many of such open systems. Exploring topological features in non-Hermitian systems is thus of great research interest.

One main research topic that I have been working on is non-Hermitian topological photonics. Based on photonic systems, in particular synthetic dimensions in photonics, we have provided important demonstrations of non-Hermitian topology.

Here are some representative papers on this topic from me and co-authors:

K. Wang*, A. Dutt*, K. Y. Yang, C. C. Wojcik, J. Vučković, and S. Fan, Generating arbitrary topological windings of a non-Hermitian band, Science 371, 1240 (2021). DOI: 10.1126/science.abf6568

This work controls and observes the nontrivial winding of a non-Hermitian energy band. In our experiments, an energy band can “wind” in the complex plane, forming a wide range of nontrivial shapes. Such windings are known to enable nontrivial topological invariants and therefore are potentially useful for exploiting topology to benefit the operation of open systems.

K. Wang, A. Dutt, C. C. Wojcik, and S. Fan, Topological complex-energy braiding of non-Hermitian bands, Nature 598, 59 (2021). DOI: 10.1038/s41586-021-03848-x

This work provides a direct demonstration of the braid-group characterization of non-Hermitian topology and opens a pathway for designing and realizing topologically robust phases in open classical and quantum systems.

The pursuit of extending human vision is always an ongoing theme. The invention of the optical microscope opened the door to a fascinating new world, and so did the telescope in bringing us to touching the stars. Today, with nanotechnology, many conventional optics can be replaced with a thin piece of metasurface consisting of nanostructures. Such flat optics not only can make smartphone cameras thinner than ever but also may help us look into the multi-dimensional quantum world of light.

A major part of my research uses flat optics for the imaging and reconstruction of quantum states of light. This may open a pathway toward quantum photonics’ tremendous applications from quantum satellites to computers, enabled by these unconventional nano-structured imaging elements.

Invited feature article:

K. Wang, M. Chekhova, and Y. Kivshar, Metasurfaces for Quantum Technologies, Physics Today 75, 38 (2022). DOI: 10.1063/PT.3.5062

Highlighted works:

K. Wang, J. G. Titchener, S. S. Kruk, L. Xu, H. Chung, M. Parry, I. I. Kravchenko, Y. Chen, A. S. Solntsev, Y. S. Kivshar, D. N. Neshev, and A. A. Sukhorukov, “Quantum metasurface for multiphoton interference and state reconstruction,” Science 361, 1104–1108 (2018). DOI: 10.1126/science.aat8196

S. Lung, K. Wang, K. Z. Kamali, J. Zhang, M. Rahmani, D. N. Neshev, and A. A. Sukhorukov, Complex-Birefringent Dielectric Metasurfaces for Arbitrary Polarization-Pair Transformations, ACS Photonics 7, 3015 (2020). DOI: 10.1021/acsphotonics.0c01044

• Media release: Tiny camera lens may help link quantum computers to network
• Media coverage: OSA Optics & Photonics News, “Quantum optics goes meta” (by Stewart Wills)

Although we live in a 3+1 dimensional space-time, it is possible to artificially implement higher dimensions using different approaches. An important part of PhD’s research is on photonics in synthetic multidimensional space.

One approach we developed to access physical properties of higher dimensional space is via specially tailored iso-spectral transformations. More specifically, this is applied to multidimensional networks. Networks are everywhere, from the neural networks in our brain to the lattice structure of materials to social networks in our daily life. Dimensionality plays a fundamental role therein, where people usually believe dimensionality is inherently connectivity. Nevertheless, we reveal that fewer connections do not necessarily mean a limited number of dimensions – as long as the connections are made wise use. We develop this entirely new idea for synthesizing multi-dimensional dynamics on optical chips, going beyond the planar 2D limitation of waveguiding structures for a new vision toward many applications such as compact optical computers running artificial intelligence algorithms and advanced sensing chips.

(Equal first authors) L. J. Maczewsky, K. Wang, et al., “Synthesizing multi-dimensional excitation dynamics and localization transition in one-dimensional lattices,” Nature Photonics14, 76 (2020). DOI: 10.1038/s41566-019-0562-8

Media release: Multi-dimensional study offers new vision for optical tech

Reported in News& Views in Nature Photonics, entitled“Optical circuits cross dimensions” (by A. Amo and O. Zilberberg)

Another approach to implementing higher dimensionality is via the concept of synthetic dimensions based on long-range interactions. In a set of works, we achieve such synthesized lattices using discrete frequencies of propagation wave in a nonlinear fiber, using four-wave-mixing Bragg scattering.

K. Wang, B. Bell, J. Titchener, A. S. Solntsev, D. N. Neshev, B. J. Eggleton, and A. A. Sukhorukov, “Multidimensional synthetic chiral-tube lattice via nonlinear frequency conversion,” Light: Science & Applications 9, 132 (2020). DOI: 10.1038/s41377-020-0299-7

J. Titchener, B. Bell, K. Wang, A. S. Solntsev, B. J. Eggleton, and A. A. Sukhorukov, “Synthetic photonic lattice for single-shot reconstruction of frequency combs”, APL Photonics 5 (3), 030805 (2020). DOI: 10.1063/1.5144119

B. A. Bell, K. Wang, A. S. Solntsev, D. N. Neshev, A. A. Sukhorukov, and B. J. Eggleton, “Spectral photonic lattices with complex long-range coupling,” Optica 4, 1433 (2017). DOI: 10.1364/OPTICA.4.001433

Recent advances in putting quantum detectors inside photonic circuits opened up many new possibilities for on-chip quantum information processing devices. Is there anything more we can do apart from pushing forward miniaturization and robustness?

We were trying to make better and judicious use of such integrated detectors for functionalities that were difficult to realize before, introducing the new concept of inline measurement to quantum photonics. We believe this concept is powerful to help check quantum information carried by photons inside the “data superhighway” and find problems in real time.

K. Wang, S. V. Suchkov, J. G. Titchener, A. Szameit, and A. A. Sukhorukov, “Inline detection and reconstruction of multiphoton quantum states,” Optica 6, 41 (2019). DOI: 10.1364/OPTICA.6.000041

Media release: A step closer to a data superhighway for future internet