  We present a general formalism based on scattering theory to calculate
quantum correlation functions involving several time-dependent current
operators. A key ingredient is the causality of the scattering matrix, which
allows one to deal with arbitrary correlation functions. Our formalism might be
useful in view of recent developments in full counting statistics of charge
transfer, where detecting schemes have been proposed for measurement of
frequency dependent spectra of higher moments. Some of these schemes are
different from the well-known fictitious spin-detector and therefore generally
involve calculation of non-Keldysh-contour-ordered correlation functions. As an
illustration of our method we consider various third order correlation
functions of current, including the usual third cumulant of current statistics.
We investigate the frequency dependence of these correlation functions
explicitly in the case of energy-independent scattering. The results can easily
be generalized to the calculation of arbitrary n-th order correlation
functions, or to include the effect of interactions.
