  Controlling the temperature of nano-scale quantum systems is becoming
increasingly important in the efforts to develop thermal devices such as
quantum heat valves, heat engines, and refrigerators, and to explore
fundamental concepts in quantum thermodynamics. In practice, however, it is
challenging to generate arbitrary time-dependent temperatures, similarly to
what has been achieved for electronic voltage pulses. To overcome this problem,
we here propose a fully quantum mechanical scheme to control the time-dependent
environment temperature of an open quantum system. To this end, we consider a
collection of quantum harmonic oscillators that mediate the interactions
between the quantum system and a thermal reservoir, and we show how an
effective time-dependent temperature can be realized by modulating the
oscillator frequencies in time. By doing so, we can apply effective temperature
pulses to the quantum system, and it can be cooled below the temperature of the
environment. Surprisingly, the scheme can be realized using only a few
oscillators, and our proposal thereby paves the way for controlling the
temperature of open quantum systems.
