In physics, non-Hermitian systems are systems that cannot be described by standard (i.e., Hermitian) laws of quantum mechanics, or more precisely, that can only be described by non-Hermitian Hamiltonians. Non-Hermitian systems are ubiquitous in nature. Many open systems, i.e., systems that are not fully isolated from the rest of the world, belong to this class. The topology of these systems (i.e., robust properties that are immune to any changes of parameters) is fundamentally shaped by the so-called "non-Hermitian skin effect," which leads to unconventional bulk-boundary correspondence, which has never been observed in Hermitian systems.
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