Café com Física



22 de junho de 2016

Prof. Gershon Kurizki
Weizmann Institute of Science, Israel

What is truly quantum about quantum thermodynamics?

Synopsis: I will argue that quantum mechanics endows us with resources that may boost thermodynamic performance: heat-machine power, cooling speed or work, but the basic thermodynamic bounds are still adhered to.
The debated rapport between thermodynamics and quantum mechanics will be addressed in the framework of the theory of periodically driven/controlled quantum thermodynamic machines. The basic model studied by us is that of a two-level system (TLS) or a harmonic oscillator, whose energy is periodically modulated while the system is coupled to two distinct thermal baths. When the modulation interval is short compared to the bath memory time, the system–bath correlations are affected, thereby causing cooling or heating of the TLS, depending on the interval. This setup constitutes the simplest (minimal) quantum heat machine (QHM) that may operate as either an engine or a refrigerator, depending on the modulation/driving rate. It is used by us to scrutinize basic thermodynamic principles in the quantum domain:

(i) Externally driven/modulated QHMs may attain the Carnot efficiency bound, or even surpass it when the driving is done by a quantum device (piston) in an appropriate state.
(ii) An extension of this model to multiple entangled systems shows that the power output can be boosted by the quantum cooperativity of these systems.
(iii) The refrigeration effected by such QHMs persists as the temperature approaches absolute zero for certain quantized baths, e.g., magnons, thus challenging the Third Law (Nernst's unattainability principle).
(iv) System–bath correlations allow more work extraction under ultrafast modulation than currently expected from the Szilard–Landauer principle.
Thus, we may conclude that heat machines in the quantum domain may benefit from hitherto unexploited quantum-thermodynamic resources.