CQT Colloquium by Rubem Mondaini, Beijing Computational Sciences Research Center
Title: Many-body localization: When thermalization fails and how to observe it experimentally Date/Time: 21-Nov, 04:00PM Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: The observation of many-body localization is a paradigmatic example of the amount of time an idea takes to get mature enough, and the numerical and experimental methods to develop sufficiently, in order to settle its existence. After the original study of Philip Anderson in 1958, demonstrating localization of non-interacting quantum particles in disordered settings, a natural question is on the resulting effects of the inter-particle interactions on this phenomenon. Only after 50 years, substantial theoretical progress was made in solving this puzzle and, in 2016 the first experimental observation was realized. The advent of platforms involving ultracold atoms allowed the inspection of an inherently dynamical quantum phase transition, that goes beyond the standard ground-state classification of the quantum matter, and its associated low-lying excitations. In this talk, after introducing the general conditions where it occurs, and review the experiments tackling it so far, I will show numerical and experimental results using quantum circuits of superconducting qubits that shed light on yet another highly debated aspect: the possible existence of many-body mobility edges.
Abstract: We live in an era where everyone is constantly connected via the internet. The way we work, play, socialise and perform transactions can no longer be dissociated with our smartphones. This is possible only because we could communicate securely over the virtual world, keeping our sensitive information away from prying eyes. However, quantum computers could break our current encryption scheme, completely disrupting our way of life in the current digital age. Fortunately, quantum theory also provides a means of encrypting unconditionally secure messages. In this talk, the art and science of quantum cryptography will be introduced. Our vision of connecting people securely via quantum links in the upcoming quantum internet will also be shared.
Speaker: Dr Goh Koon Tong, Research Fellow, Department of Electrical & Computer Engineering, National University of Singapore (NUS)
Speaker’s Profile: Dr Goh Koon Tong, Research Fellow, Department of Electrical & Computer Engineering, National University of Singapore (NUS) Dr Goh Koon Tong is a Research Fellow at the Department of Electrical & Computer Engineering, NUS and leads a theoretical team of scientists in the Quantum Communications Laboratory. His research interest is centred on quantum cryptography, focusing on the development of practical quantum protocols for the future Quantum Internet.
Abstract: How tocharacterize fundamental states of quantum matter is an important theme in condensedmatter physics. Characterizations of the celebrated Landau symmetry-breakingand topological quantum phases are mainly developed based onthe equilibrium theories. In this talk, I will introduce how to characterize equilibrium symmetry-breakingand topological quantum phases by far-from-equilibrium quantum quench dynamics.For topological phases, a generic theory is established by showing adynamical bulk-surface correspondence, which connects the dD bulk topology of equilibriumphases to topological pattern of quench dynamics emergingin the (d-1)D momentum subspace,dubbed band-inversion surfaces (BISs), similar to the well-knownbulk-boundary correspondence for equilibrium topological phases inthe real space. Further, we consider the Haldane-Hubbard modelwhich hosts both symmetry-breaking orders and topological phases, and showthat the correlated pseudospin quench dynamics exhibits robust universalbehaviors on the BISs: both the topology andsymmetry-breaking orders are then extracted. In particular, thetopology of the post-quench regime is characterized by the emergenttopological pattern of quench dynamics on BISs, which is robust againstdephasing and heating induced by interactions; the pre-quenchsymmetry-breaking orders are read out from a universal scaling of the quenchdynamics emerging on the BIS, which is valid beyond the mean-field theory. Theseresults may show insights into the exploration of thecharacterization of both symmetry-breaking and topological phases byquench dynamics.
References:  L. Zhang, L. Zhang, etal., ScienceBull. 63, 1385 (2018).  W. Sun et al., Phys. Rev. Lett.121, 250403 (2018).  B. Song, C. He, S. Niu et al. Nature Physics, 15, 911 (2019).  L. Zhang et al., Phys.Rev. A 99, 053606 (2019).  L. Zhang et al., arXiv:1903.09144v2.  C. R. Yi, L. Zhang et al., Phys.Rev. Lett. 123, 190603 (2019).
About the speaker: Xiong-Jun Liu received Ph.D in Texas A&M University in 2011,and was a postdoctoral fellow in University of Maryland, IAS HKUST and MIT (2011-2014). He joined the faculty of International Center for Quantum Materials at Peking University (09/2014), becametenured (07/2018), and now a full professor and BoYa distinguished Professor (from01/2019). He works incondensed matter theory and ultracold atoms, focusing on quantum simulationand topological matter:topological superconductors, synthetic gauge fields, non-equilibrium topological quantum systems, and strongly correlated topologicalstates. He has been awarded The National Science Fundfor Distinguished Young Scholars (2018), and AAPPS-APCTP CN Yang Award (2019).
CQT Talk by Nikolaos Proukakis, Newcastle University, UK
Title: Dissipation of Superflow in a Thin Atomic Josephson Junction Date/Time: 22-Nov, 03:00PM Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: We study the onset of dissipation in an atomic Josephson junction in an ultracold atomic gas, and compare our results to the experimentally-observed dynamics of Fermi superfluids in the molecular Bose-Einstein condensation limit of strong attraction, finding excellent agreement. Specifically, we construct a new extended phase diagram which identifies a critical population imbalance (and a corresponding maximum Josephson current) delimiting dissipationless and dissipative transport. We unambiguously link dissipation to vortex ring nucleation and dynamics, and demonstrate that quantum phase slips are responsible for the observed resistive current.
Our work directly connects microscopic features with macroscopic dissipative transport, providing a comprehensive description of vortex ring dynamics in three-dimensional inhomogeneous constricted superfluids. Extending our simulations to a state-of-the-art kinetic model, we show that such features persist at non-zero temperatures (the vortex generation is not significantly affected by the presence of a thermal mean field potential, for equal condensate numbers), but that temperature affects the long-term system evolution adding damping to both the coherent superflow and the generated vortices.