  We demonstrate that the probability distribution of the net number of
electrons passing through a quantum system in a junction obeys a steady-state
fluctuation theorem (FT) which can be tested experimentally by the full
counting statistics (FCS) of electrons crossing the lead-system interface. The
FCS is calculated using a many-body quantum master equation (QME) combined with
a Liouville space generating function (GF) formalism. For a model of two
coupled quantum dots, we show that the FT becomes valid for long binning times
and provide an estimate for the finite-time deviations. We also demonstrate
that the Mandel (or Fano) parameter associated with the incoming or outgoing
electron transfers show subpoissonian (antibunching) statistics.
