%A Jing-Min Hou, Wei Chen %T Hidden antiunitary symmetry behind “accidental” degeneracy and its protection of degeneracy %0 Journal Article %D 2018 %J Front. Phys. %J Frontiers of Physics %@ 2095-0462 %R 10.1007/s11467-017-0712-8 %P 130301- %V 13 %N 1 %U {https://journal.hep.com.cn/fop/EN/10.1007/s11467-017-0712-8 %8 2018-02-28 %X

In quantum mechanics, accidental degeneracy refers to energy degeneracy that occurs coincidentally, without any protection by symmetry. Here, we prove a theorem stating that any two-fold degeneracy (accidental or not) in a quantum system is protected by a novel hidden symmetry, which can be expressed by an antiunitary operator with its square being −1. In this sense, the so-called accidental degeneracy is not really accidental, and this actually implies a hidden antiunitary symmetry.