Q-Farm Quantum Seminar Series presents "Ultrafast Spectroscopy of Quantum Materials" and "Quantum and classical information spreading in many-body systems"
Ultrafast Spectroscopy of Quantum Materials: Quantum materials such as topological insulators, Weyl-semimetals, and atomically thin two-dimensional crystals have intriguing electronic properties, which makes them promising candidates for their potential applications in next-generation technology. Therefore, it is desirable to investigate the electronic properties of these materials through various methods such that their full potential, as well as the limitations, can be identified. In this talk, I will introduce a novel spectroscopic approach based on the use of strong ultrafast laser pulses and the generation of high-order harmonics. Analysis of temporal and spectral properties of high-order harmonics from these materials reveals their electronic band-structure, topology, as well as driven electron dynamics in the natural time scales of electrons. The advantages of this approach over conventional methods include the use of the all-optical setting, no physical contacts to samples, and much of the measurements that can be performed in ambient conditions. More importantly, the non-perturbative interactions between materials and strong laser fields could generate transient novel quantum phases, which can be subsequently probed by analyzing high-order harmonics.
Quantum and classical information spreading in many-body systems: I describe a new theoretical result that constrains how quantum information can spread through large systems. When a small subsystem interacts with a large environment, which information about the subsystem can we detect locally in the environment? In great generality, we show there exists at most an O(1)-sized region of the environment where quantum information about the subsystem can be locally detected, whereas in the rest of the environment, any locally detectible information about the subsystem must be classical information, and moreover this classical information can be modeled by measuring the subsystem in a fixed basis. We will discuss implications for many-body physics. This work builds on earlier work by Brandao et al. in arXiv:1310.8640.