  We demonstrate the miniaturization of perpendicularly magnetized
superparamagnetic tunnel junctions (SMTJs) down to 50 nm in diameter. We
experimentally show stochastic reversals in those junctions, with tunable mean
dwell times down to a few nanoseconds through applied magnetic field and
voltage. The mean dwell times measured at negligible bias voltage agree with
our simulations based on Langer's theory. We shed light on an Arrhenius
prefactor $\tau_0$ of a few femtoseconds, implying that the rates of
thermally-activated magnetic transitions exceed the GHz-to-THz limitation of
macrospin models, whereby $\tau_0\sim1$ ns. We explain the small prefactor
values by a Meyer-Neldel compensation phenomenon, where the prefactor exhibits
a large entropic contribution with an exponential dependence on the activation
energy. These findings pave the way towards the development of ultrafast,
low-power unconventional computing schemes operating by leveraging thermal
noise in perpendicular SMTJs, which are scalable below 20 nm.
