QSD

QSD is an acronym that can refer to several concepts, organizations, or terms across different fields. Because there is no universal meaning, the intended interpretation depends on the context in which it is used. This article summarizes two widely cited scientific uses and notes that other industry-specific meanings exist.

Quasi-stationary distribution. In the study of Markov processes with absorbing states, a quasi-stationary distribution (QSD) describes the limiting distribution of the process conditional on not having been absorbed. If the initial state is chosen appropriately, the distribution of the system, given that it has not left a specified set, converges to the QSD as time progresses. QSDs are used in population dynamics, epidemiology, ecology, and reliability theory to model systems that can terminate or exit a region but persist long enough for conditioning on continued survival to be meaningful. They are distinct from true stationary distributions because absorption changes the long-run behavior of the process.

Quantum state diffusion. In quantum mechanics and quantum optics, quantum state diffusion (QSD) is a stochastic formalism that unravels the quantum master equation into random pure-state trajectories. This approach models continuous measurement and environmental monitoring, providing a way to simulate open quantum systems and study decoherence. Individual QSD trajectories represent possible evolutions of a system, while the ensemble average reproduces the standard master-equation dynamics.

Other uses. In industry and quality management, QSD may stand for terms such as Quality System Description or Quality System Documentation, referring to documentation or descriptions of an organization’s quality management system. As with many acronyms, exact meaning depends on the domain and organization.