Harnessing hybrid frequency-entangled qudits through quantum interference
Sheng-Hung Wang, Po-Han Chen, Chengyu Yang, Yen-Hung Chen, Pin-Ju Tsai · Physical Review Research · 2025
High-dimensional (HD) quantum entanglement expands the Hilbert space, offering a robust framework for quantum information processing with enhanced capacity and error resilience. In this work, we present an HD frequency-domain entangled state, the hybrid frequency-entangled qudit (HFEQ), generated via Hong-Ou-Mandel (HOM) interference, exhibiting both discrete-variable (DV) and continuous-variable (CV) characteristics in the frequency domain. By tuning HOM interference, we generate and control HFEQs with dimensions D = 5 , 7 , 9 , and 11, confirming their DV nature. Franson interferometry verifies global frequency correlations with a visibility of 97.6 ± 0.6 % and CV entanglement within individual frequency modes with a visibility exceeding 95 % . Furthermore, based on the developed theoretical model, the generated HFEQs exhibit a Schmidt number that exceeds the upper bound D of conventional frequency-entangled qudits, certifying additional entanglement inherent to the hybrid structure. Our findings provide deeper insight into the physical nature of frequency-entangled qudits generated by quantum interference and introduce a resource for HD time-frequency quantum information processing.