Probing non-Hermitian Physics with Parity-Time Symmetry --- from coupled microcavities to flying atoms

Non-Hermitian optical settings with Parity-Time (PT) symmetry become one of the most widely studied platforms in probing intriguing physics of non-Hermitian Hamiltonians. These include a variety of exotic phenomena associated with unique exceptional points and spontaneous PT phase transitions that are difficult to be observed in either Hermitian or natural materials. In synthesizing such a compound system, gain and loss are usually placed in a balanced manner. In the first part of this talk, we will briefly discuss our recent work on observing PT symmetry in two active-passive-coupled microtoroid cavities [1]. By balancing gain and loss, a cavity-guided “photonic molecule” could be formed in analogy with, say, Hydrogen atoms. In the second part of this talk, we will report the first experimental realization of anti-PT symmetry [2], a counterpart of PT symmetry, in a hot atomic vapor cell. Different from all previous theoretical proposals and experimental demonstrations with solid materials, this work introduces a novel yet much simpler mechanism for implementing (anti-)PT symmetry without involving any advanced nanotechnologies or sophisticated nanofabrication techniques, but is fully capable of unprecedented resolution on phase transition and many undiscovered effects including nonlocal interference and refractionless propagation. Of importance, this scheme firmly sets up a framework for probing non-Hermitian physics in AMO, where high controllability and excellent tunability offers competitive advantages over solid-material systems. Last but not the least, this scheme substantially reduces the complexity and cost on PT-symmetry implementations, and provides a very clean and simple platform that is reachable to most experimental groups.