Abstract
We extend the spin-boson model of open quantum systems to the regime of quantum noise characterized by negative exponents of its spectral distribution. Using the numerically exact time-evolving matrix product operator, we find the dynamic regime diagram, including pseudocoherent dynamics controlled by quantum noise. We determine the dephasing rate and find for it an empirical formula valid at zero temperature. The bath reorganization energy depends on the infrared bath cutoff frequency, revealing an increased sensitivity of the dephasing on the measurement time of an experiment. \ep{Our results apply to a qubit as an elementary building block of a quantum computer and pave the way towards a quantum treatment of low-frequency noise in more complex architectures.