"Dark resonance has been interested in many research areas such as nonlinear optics, quantum optics, and laser physics for potential applications. One of the most interesting features of the dark resonance is electromagnetically induced transparency (EIT), in which absorption is cancelled due to destructive quantum interference. Owing to the absorption cancellation at line center, nonlinear optical processes such as sum frequency generations are enhanced greatly. Two-photon coherence indirectly induced by two-color laser fields is another important feature of the dark resonance. Like EIT, the coherence decay time in the two-photon coherence is dominated by phase decay-rate rather than population decay rate. In this talk, current technological problems of optical switching devices in fiber-optic communications are overviewed. For ultrahigh-speed optical communications, I propose quantum switching based on dark resonance for potential optical switching method. Recent experimental proof of the proposed idea is presented for the implementation of hyper-THz all-optical quantum switches. "