Quantum Matter Seminar by Dr. Chang Po-Yao on February 9, 2023

Please enter the seminar report here!On February 13, 2023, Assistant Professor Po-Yao Chang of the Department of Physics at National Tsing Hua University in Taiwan gave an online seminar entitled "Entanglement in non-Hermitian quantum systems and non-unitary conformal field theories."

In the seminar, Professor Chang began by discussing the motivation for studying non-Hermitian systems, such as open quantum systems and those with imaginary self energy induced by interaction or disorder. He then introduced the basic concepts of entanglement entropy, arguing that while the quantity is defined through the ground state, it can be used to characterize the entire system similarly to how Boltzmann's thermal entropy characterizes a classical system. In particular, it can be used to characterize topological systems with anyons. Although general non-Hermitian systems have complex eigenvalues, he focused on systems that preserve parity and time-reversal (PT) symmetry, so that all of the eigenvalues must be real in order to determine the ground state and the entanglement entropy. He showed that in a non-Hermitian model, the entropy is negative and corresponds to the negative central charge, which uniquely characterizes conformal field theories (CFTs).

As the main result of his recent work, Professor Chang proposed a generic entanglement entropy to characterize non-Hermitian systems and showed how it could be used to correctly obtain the entanglement properties of several non-Hermitian systems, such as the non-Hermitian Su-Schrieffer-Heeger (SSH) model, the q-deformed XXZ model with imaginary boundary terms, and the Affleck-Kennedy-Lieb-Tasaki (AKLT) model, using numerically extracted central charges.

In conclusion, Professor Chang's seminar provided valuable insights into the field of entanglement in non-Hermitian quantum systems and non-unitary conformal field theories. The seminar was well-received by the audience, who appreciated the clarity of the presentation and the relevance of the research topic.

Reported by Chen-Hsuan Hsu (Academia Sinica, Taiwan) and Ching-Kai Chiu