Time: 12:10, Monday, October 19, 2026
Speaker:Dr. Yi-Te Huang
               黃亦得 博士
               Dept. of Physics, NCKU
Title: General input-output framework for constructing quantum exceptional deficiency
Place: Room 36173, 1F, Department of Physics, Science Building, NCKU

Abstract:
Exceptional points are spectral singularities of non-Hermitian matrices at which both eigenvalues and eigenvectors coalesce. This concept has been extended to exceptional deficiency (ED), involving complete eigenspace coalescence and enabling synergistic dynamics beyond conventional singularities. Because ED relies strictly on unidirectional couplings, achieving it in the quantum regime typically requires engineered non-reciprocity to manage inherently bidirectional system-environment interactions. Here, we introduce a fundamentally different approach based on natural causality. By employing the exact input-output formalism, we show that the influence of input and output fields on the reduced system dynamics can be captured by auxiliary degrees of freedom whose strict chronological ordering reveals a unidirectional coupling structure. We harness this inherent one-way interaction to introduce input-output exceptional deficiency (IOED) and demonstrate our approach by tailoring a non-Gaussian single-photon input wave packet. This framework establishes environmental input-output fields as a controllable resource for engineering IOED and higher-order exceptional points in the fully quantum regime.