摘要
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.
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.
