Past Events

Astrophysical Black Holes
(Joint Colloquium of the Department of Physics and the HKU-UCAS Joint Institute of Theoretical and Computational Physics)

Speaker: Prof. Cosimo BAMBI
Affiliation: Fudan University
Date: September 7, 2022 (Wednesday)
Time: 5:00 p.m.
Zoom Link: https://bit.ly/3KgAxhu
Meeting ID: 970 4476 0018
Password: 2859
Poster: Download
Abstract:
In this talk, I will review the development of the field, from the first speculations to the current lines of research. According to Einstein's theory of general relativity, black holes are relatively simple objects and completely characterized by their mass, spin angular momentum, and electric charge, but the latter can be ignored in the case of astrophysical macroscopic objects. Search for black holes in the sky started in the early 1970s with the dynamical measurement of the mass of the compact object in Cygnus X-1. In the past 10-15 years, astronomers have developed some techniques for measuring the black hole spins. Recently, we have started using astrophysical black holes for testing fundamental physics.
Key References:
https://ui.adsabs.harvard.edu/abs/2018AnP...53000430B/abstract
https://ui.adsabs.harvard.edu/abs/2020mbhe.confE..28B/abstract

Active and passive non-Hermitian devices

Speaker: Prof. Henning SCHOMERUS
Affiliation: Lancaster University
Date: September 21, 2022 (Wednesday)
Time: 5:00 p.m.
Zoom Link: https://bit.ly/3KyAWfv
Meeting ID: 921 1310 5357
Password: 2859
Poster: Download
Abstract:
In photonic systems, gain and loss can be used to induce intriguing effects that are linked to non-Hermitian and topological physics. Prominent examples are exceptional points and the non-Hermitian skin effect, which can be used for enhanced sensing and directed amplification, as well as symmetry-protected states, which can be addressed by topological mode selection. Many of these applications make explicit use of mode nonorthogonality, which becomes especially intriguing when the system is nonreciprocal. I describe how these effects can be probed in response theory, transport, and scattering, and highlight fundamental practical limits of the observability of some effects.
Key References:
H Schomerus Phys. Rev. Lett. 104, 233601 (2010); Opt. Lett. 38, 1912-1914 (2013)
Phys. Rev. Research 2, 013058 (2020); arXiv:2207.09014 (2022)
Poli et al, Nat. Commun. 6, 6710 (2015); H Zhao et al Nat. Commun. 9, 981 (2018)

Twinkle twinkle little stars, how I wonder how you die

Speaker: Prof. Ming Chung CHU
Affiliation: Department of Physics, The Chinese University of Hong Kong
Date: October 5, 2022 (Wednesday)
Time: 5:00 p.m.
Venue: MWT5, 1F, Meng Wah Complex, The University of Hong Kong
Poster: Download
Abstract:
Supernovae – explosions of dying stars – are remarkable events that play an essential role in stellar evolution, as well as the spreading of elements and reshaping of the matter distribution in the universe. The study of supernovae is entering a golden age with many new observational tools utilizing neutrino, gravitational-wave, and electromagnetic-wave observations, and exciting discoveries are expected. I will introduce three types of supernovae: Thermonuclear, Accretion-induced collapse, and Core-collapse supernovae, and how they might be used to detect dark matter and quark deconfinement.
Key References:
1. Delayed Detonation Thermonuclear Supernovae with an Extended Dark Matter Component, H. S. Chan, M.-C. Chu, S.C. Leung, and L.M. Lin, The Astrophysical Journal 914,138 (2021)
2. Gravitational-wave Signature of a First-order Quantum Chromodynamics Phase Transition in Core-Collapse Supernovae, S. Zha, E. O’Connor, M.–C. Chu, L.M. Lin, S. Couch, Physical Review Letters 125, 051102 (2020)
3. Accretion-Induced Collapse of Dark Matter Admixed White Dwarfs -- II: Rotation and Gravitational-wave Signals, Shuai Zha, Ming-Chung Chu, Shing-Chi Leung, Lap-Ming Lin, The Astrophysical Journal 883, 13 (2019)

Thermodynamics and order beyond equilibrium – from eigentstate thermalisation to time crystals

Speaker: Prof. Roderich MOESSNER
Affiliation: Max Planck Institute for the Physics of Complex Systems in Dresden, Germany
Date: October 19, 2022 (Wednesday)
Time: 5:00 p.m.
Zoom Link: https://bit.ly/3DSdNmV 
Meeting ID: 967 7300 3194 
Password: 2859
Poster: Download
Abstract:
The field of thermodynamics is one of the crown jewels of classical physics. Thanks to the advent of experiments in cold atomic systems with long coherence times, our understanding of the connection of thermodynamics to quantum statistical mechanics has seen remarkable progress. Extending these ideas and concepts to the non-equilibrium setting is a challenging topic, in itself of perennial interest. Here, we present perhaps the simplest non-equilibrium class of quantum problems, namely Floquet systems, i.e. systems whose Hamiltonians depend on time periodically, H(t + T) = H(t). For these, there is no energy conservation, and hence not even a natural concept of temperature. We find that certain structures from equilibrium thermodynamics are lost, while entirely new non-equilibrium phenomena can arise, including a spectacular spatiotemporal `time-crystalline' form of order, recently observed experimentally on google AI's sycamore NISQ platform.
Key References:
For an introductory overview, see Nature Physics 13, 424–428 (2017)
For an in-depth review, see arxiv:1910.10745 . NISQ experiment: Nature 601, 531 (2022).

NISQ: Error correction, mitigation, and noise simulation

Speaker: Prof. Bei ZENG
Affiliation: The Hong Kong University of Science and Technology
Date: November 2, 2022 (Wednesday) November 9, 2022 (Wednesday)
Time: 5:00 p.m. 12:15 p.m.
Hybrid Mode
In Person: Room 522, 5/F, Chong Yuet Ming Physics Building, HKU
Zoom Link: https://bit.ly/3MKejFM
Meeting ID: 945 9522 1667
Password: 2859
Poster: Download
Abstract:
Error-correcting codes were invented to correct errors on noisy communication channels. Quantum error correction (QEC), however, may have a wider range of uses, including information transmission, quantum simulation/computation, and fault-tolerance. These invite us to rethink QEC, in particular, about the role that quantum physics plays in terms of encoding and decoding. The fact that many quantum algorithms, especially hybrid quantum-classical algorithms designed for the near-term noisy-intermediate-scale quantum (NISQ) devices, only use limited types of local measurements on quantum states, leads to various new techniques called Quantum Error Mitigation (QEM). Using some intuitions built upon classical and quantum communication scenarios, we clarify some fundamental distinctions between QEC and QEM. We also examine the connections between QEC, QEM and noisy quantum circuit simulations.
Key References:
1. Ningping Cao, Junan Lin, David Kribs, Yiu-Tung Poon, Bei Zeng, and Raymond Laflamme, “NISQ: error correction, mitigation, and noise simulation”, arXiv:2111.02345 (2021). 
2. Chenfeng Cao, Chao Zhang, Zipeng Wu, Markus Grassl, and Bei Zeng, “Quantum variational learning for quantum error-correcting codes”, arXiv:2204.03560 (2022).
3. Zipeng Wu, Shi-Yao Hou, Chao Zhang, Lvzhou Li, and Bei Zeng, “Variational learning algorithms for quantum query complexity”, arXiv:2205.07449 (2022).

IXPE: The Mission, Initial Discoveries and Future Prospects

Speaker: Prof. Roger W. ROMANI
Affiliation: Stanford University
Date: November 16, 2022 (Wednesday)
Time: 10:00 a.m.
Zoom Link: https://bit.ly/3D7Bm99
Meeting ID: 933 3672 9143
Password: 2859
Poster: Download
Abstract:
IXPE, launched on December 9, 2021, is the first mission devoted to polarimetry of celestial targets in the classical soft X-ray band. Some 50 years after the first (and until recently, only) X-ray polarimetry detection (measuring the Crab nebula), IXPE has increased our polarization sensitivity by a factor of 100x and provided the first imaging polarimetry studies. With over 25 sources observed to date, from a dozen source classes, IXPE is establishing polarimetry as a powerful tool to explore the physics of compact objects and their outflows. After a review of IXPE’s instrumentation and comment on the prospects for improved polarization analysis, we summarize some recent discoveries and outline the opportunities for additional work in the upcoming Guest Observer phase of the project.
Key References:
https://ui.adsabs.harvard.edu/abs/2022JATIS...8b6002W/abstract
https://ui.adsabs.harvard.edu/abs/2021ApJ...920...40P/abstract

Towards a Quantum Internet

Speaker: Prof. Stephanie WEHNER
Affiliation: QuTech, Delft University of Technology
Date: November 30, 2022 (Wednesday) Cancelled
Time: 5:00 p.m. Cancelled
Abstract:
The vision of a Quantum Internet is to provide fundamentally new internet technology by ultimately enabling quantum communication between any two points on earth. Such a Quantum Internet would – in synergy with the ‘classical’ internet that we have today - connect quantum processors in order to achieve new capabilities that are provably impossible using classical communication.
In this talk, we provide an introduction to quantum (internet-) working, and present recent results on the path of taking such a network from a physics experiment to a quantum network system. We report on the realization of the first quantum link layer protocol, now implemented on quantum hardware based on Nitrogen-Vacancy Centers in diamond. We conclude by presenting several tools that can be used to examine the experimental requirements of building larger quantum network, and be used by you to learn more about quantum networking beyond this talk.
Key References:
https://www.science.org/doi/10.1126/science.aam9288
https://www.quantamagazine.org/stephanie-wehner-is-designing-a-quantum-internet-20190925/
https://www.youtube.com/watch?v=XzPi29O6DAc