There is a common misconception that the decoherence time is the time scale over which quantum mechanics is lost in a quantum system. This is not correct. Open quantum systems can have highly quantum mechanical equilibrium states, even when the decoherence time is very short. Under certain conditions, you can wait the lifetime of the universe and the entanglement in an open quantum system will never “go away”. These systems can have very short decoherence times.
Here is a presentation Mohammad Amin recently gave that explains why this is and how decoherence times affect AQC, and a technique we have implemented for measuring a key figure of merit for qubits used in AQC systems.