  Starting from the quantum stochastic differential equations of Hudson and
Parthasarathy (Comm. Math. Phys. 93, 301 (1984)) and exploiting the
Wiener-Ito-Segal isomorphism between the Boson Fock reservoir space
$\Gamma(L^2(\mathbb{R}_+)\otimes (\mathbb{C}^{n}\oplus \mathbb{C}^{n}))$ and
the Hilbert space $L^2(\mu)$, where $\mu$ is the Wiener probability measure of
a complex $n$-dimensional vector-valued standard Brownian motion
$\{\mathbf{B}(t), t\geq 0\}$, we derive a non-linear stochastic Schrodinger
equation describing a classical diffusion of states of a quantum system, driven
by the Brownian motion $\mathbf{B}$. Changing this Brownian motion by an
appropriate Girsanov transformation, we arrive at the Gisin-Percival state
diffusion equation (J. Phys. A, 167, 315 (1992)). This approach also yields an
explicit solution of the Gisin-Percival equation, in terms of the
Hudson-Parthasarathy unitary process and a radomized Weyl displacement process.
Irreversible dynamics of system density operators described by the well-known
Gorini-Kossakowski-Sudarshan-Lindblad master equation is unraveled by
coarse-graining over the Gisin-Percival quantum state trajectories.
