  We explore the quantum version of Brayton cycle with a composite system as
the working substance. The actual Brayton cycle consists of two adiabatic and
two isobaric processes. Two pressures can be defined in our isobaric process,
one corresponds to the external magnetic field (characterized by $F_x$) exerted
on the system, while the other corresponds to the coupling constant between the
subsystems (characterized by $F_y$). As a consequence, we can define two types
of quantum Brayton cycle for the composite system. We find that the subsystem
experiences a quantum Brayton cycle in one quantum Brayton cycle (characterized
by $F_x$), whereas the subsystem's cycle is of quantum Otto in another Brayton
cycle (characterized by $F_y$). The efficiency for the composite system equals
to that for the subsystem in both cases, but the work done by the total system
are usually larger than the sum of work done by the two subsystems. The other
interesting finding is that for the cycle characterized by $F_y$, the subsystem
can be a refrigerator while the total system is a heat engine. The result in
the paper can be generalized to a quantum Brayton cycle with a general coupled
system as the working substance.
