  We prove that the solution of the Hudson-Parthasarathy quantum stochastic
differential equation in the Fock space coincides with the solution of a
symmetric boundary value problem for the Schr\"odinger equation in the
interaction representation generated by the energy operator of the environment.
The boundary conditions describe the jumps in the phase and the amplitude of
the Fourier transforms of the Fock vector components as any of its arguments
changes the sign. The corresponding Markov evolution equation (the Lindblad
equation or the ``master equation'') is derived from the boundary value problem
for the Schr\"odinger equation.
