  A novel mechanism of asymmetric frequency conversion is investigated in
nonlinear dispersive devices driven parametrically with a biharmonic pump. When
the relative phase between the first and second harmonics combined in a
two-tone pump is appropriately tuned, nonreciprocal frequency conversion,
either upward or downward, can occur. Full directionality and efficiency of the
conversion process is possible, provided that the distribution of pump power
over the harmonics is set correctly. While this asymmetric conversion effect is
generic, we describe its practical realization in a model system consisting of
a current-biased, resistively-shunted Josephson junction (RSJ). Here, the
multiharmonic Josephson oscillations, generated internally from the static
current bias, provide the pump drive.
