  We investigate the efficiency of an isothermal Brownian work-to-work
converter engine, composed of a Brownian particle coupled to a heat bath at a
constant temperature. The system is maintained out of equilibrium by using two
external time-dependent stochastic Gaussian forces, where one is called load
force and the other is called drive force. Work done by these two forces are
stochastic quantities. The efficiency of this small engine is defined as the
ratio of stochastic work done against load force to stochastic work done by the
drive force. The probability density function as well as large deviation
function of the stochastic efficiency are studied analytically and verified by
numerical simulations.
