  We experimentally demonstrate the fast generation of thermal states of a
transmon using a single-junction quantum-circuit refrigerator (QCR) as an
in-situ-tunable environment. Through single-shot readout, we monitor the
transmon up to its third-excited state, assessing population distributions
controlled by QCR drive pulses. Whereas cooling can be achieved in the
weak-drive regime, high-amplitude pulses can generate Boltzmann-distributed
populations from a temperature of 110 mK up to 500 mK within 100 ns. As we
propose in our work, this fast and efficient temperature control provides an
appealing opportunity to demonstrate a quantum heat engine. Our results also
pave the way for efficient dissipative state preparation and for reducing the
circuit depth in thermally assisted quantum algorithms and quantum annealing.
