Invited Talks
Supercurrent diode effect, Josephson diode effect and finite-momentum superconductivity
袁凡奇 Fanqi Yuan When both inversion and time-reversal symmetries are broken, the critical current of a superconductor can be nonreciprocal. In certain classes of two-dimensional superconducting systems with spin-orbit coupling, Cooper pairs acquire a finite momentum upon the application of an in-plane magnetic field, and as a result, critical currents in the direction parallel and antiparallel to the Cooper pair momentum become unequal. The nonreciprocal superconducting transport phenomena, supercurrent diode effect (for a single superconductor) and Josephson diode effect (for a Josephson junction of two superconductors) have been reported in a growing number of recent experiments, which will be addressed in this talk in terms of finite-momentum superconductivity.
Time 2:00 PM, Nov 23, 2022
The Lefschetz thimble and its application to the spin foam model
黄子鬯 Zichang Huang The Lefschetz-thimble method is a way to evaluate the complex path integral in a system with complex-valued action. In this talk, two interesting applications of this method will be given: to compute the observables in the system suffering from the sign problem and to find the complex saddle points of an analytically continued action. In particular, the covariant formulation of the Loop Quantum Gravity, i.e., the spin foam model, is used as our proofing ground. We use the thimble method to compute 2-point correlation function and to find the complex saddle points in the spin foam model.
Time 2:00 PM, Oct 25, 2022
Optical responses and manipulation in strongly correlated and topological materials
顾铭强 Mingqiang Gu Light-matter interaction offers opportunities not only to probe different responses in a wide temporal range, but also to manipulate the material properties in an ultrafast manner. It has been identified as an essential route to discover dynamical phases in materials that are unexpected (hidden) in the equilibrium phase diagram. In this talk, I will go through some recent progress in both theoretical design and experimental realization of optical manipulation and detection of novel phases in quantum materials. In oxide perovskites, we have designed an ultrafast route to control the microstructure, which might eventually lead to magnetic phase transition. Similar method can be applied in the Weyl semimetal to switch the inversion symmetry on and off, in this sense the topological electronic band structure can be controlled. We have also observed the light-induced magnetic/charge order-disorder phase transition in a strongly correlated oxide, an ultrafast and non-contact route to control the multiferroics. Finally, I would like to discuss the proposal of using optical Faraday/Kerr rotation as the signature for identifying the novel Axion insulators.
Time 2:00 PM, Oct 18, 2022
Nishimori’s cat: stable long-range entanglement from finite-depth unitaries and weak measurements
朱国毅 Guoyi Zhu In the field of monitored quantum circuits, it has remained an open question whether finite-time protocols for preparing long-range entangled (LRE) states lead to phases of matter which are stable to gate imperfections. Here we show that such gate imperfections effectively convert projective into weak measurements and that, in certain cases, long-range entanglement persists, even in the presence of weak measurements and gives rise to novel forms of quantum criticality. We demonstrate this explicitly for preparing the two-dimensional (2D) GHZ cat state and the three-dimensional (3D) toric code as minimal instances. In contrast to previous studies on measurement-induced phases and transitions, our circuit of gates and measurements is deterministic; the only randomness is in the measurement outcomes. We show how the randomness in these weak measurements allows us to track the solvable Nishimori line of the random-bond Ising model, rigorously establishing the stability of the glassy LRE states in two and three spatial dimensions. Away from this exactly solvable construction, we use hybrid tensor network and Monte Carlo simulations to obtain a non-zero Edwards-Anderson order parameter as an indicator of long-range entanglement in the 2D scenario. We argue that our protocol admits a natural implementation in existing quantum computing architectures, requiring only a depth-3 circuit on IBM’s heavy- hexagon transmon chips.
Time 2:00 PM, Sep 15, 2022