In classical stochastic thermodynamics there is a clear separation between the noise source (= the heat bath) and the information source (= the detector). Previous attempts to formulate a theory of quantum stochastic thermodynamics (e.g., the two-point measurement scheme, unravelling of quantum jump trajectories, etc.) identify the noise source with the information source by assuming that one has precise knowledge about the environment. Put differently, the heat bath and the detector are the same object in these proposals. This makes these theories hard to verify in practice and has led to the question which quantum effects they can actually describe.

I will here introduce an alternative approach, called “operational” quantum stochastic thermodynamics [1-4], in which the noise source in form of a heat bath is strictly separated from the actions of an external agent. The basic building blocks of this novel theory are the control operations of the external agent and, provably, it allows to equip any quantum causal model with a consistent thermodynamic interpretation. No detailed control of the bath is necessary in this framework. It further provides a thermodynamic framework to analyze Nobel-prize-winning experiment done in the group of Serge Haroche [5,6], which was previously not possible. Remarkably, the thermodynamic efficiency to prepare a pure photon number (Fock) state of the cavity in these experiments is very high [1].

[1] P. Strasberg, Phys. Rev. E 100, 022127 (2019).
[2] P. Strasberg and A. Winter, Phys. Rev. E 100, 022135 (2019).
[3] P. Strasberg, Phys. Rev. Lett. 123, 180604 (2019).
[4] P. Strasberg, Quantum 4, 240 (2020).
[5] C. Sayrin, et al., Nature 477, 73 (2011).
[6] X. Zhou, et al., Phys. Rev. Lett. 108, 243602 (2012).