  We derive cooling rate and coefficient of performance as well as their
variances for a quantum Otto engine proceeding in finite-time cycle period.
This machine consists of two driven strokes, where the system isolated from the
heat reservoir undergoes finite-time unitary transformation, and two isochoric
steps, where the finite-time system-bath interaction durations take the system
away from the equilibrium even at the respective ends of the two stages. We
explicitly calculate the statistics of cooling rate and coefficient of
performance for the machine operating with an analytically solvable two-level
system. We clarify the role of finite-time durations of four processes on the
machine performance. We show that there is the trade-off between the
performance parameter and its corresponding variance, thereby indicating that
the cooling rate or coefficient of performance can be enhanced, but at the cost
of increasing the corresponding fluctuations.
