Speaker: Erik Aurell (KTH Royal Institute of Technology, Stockholm)
A well-studied model in open quantum system theory is a system interacting with a thermal bath of harmonic oscillators at finite temperature. This provides a quantum mechanical model of a classical resistive element in a circuit, and includes as famous examples the Caldeira-Leggett theory of quantum Brownian motion, and the „spin-boson model”. Such environments however also include baths of thermal photons and phonons, and putative baths of gravitons. As long as the environment consists of harmonic oscillators interacting linearly with the system, and starting in a thermal state, the environmental degrees of freedom can be integrated out using the Feynman-Vernon method.
I will first present the open system dynamics of a test particle interacting linearly with a thermal bath of photons, following  and . I will then discuss the resulting energy change of the bath (quantum heat) using the Feynman-Vernon approach. I will discuss what one can say if the bath temperature is very low or zero, i.e. if the test particle interacts with the electromagnetic vacuum.
I will then consider a test particle interacting with a gravitational field quantized in the weak-field (linear) approximation. I will review the recent theory of Parikh, Wilczek and Zahariade  describing an arm of a gravitational wave detector interacting with this kind of quantized gravitational field. Following Parikh et al I will show that an effective nonlinear friction force follows analogously to the way ordinary friction appears in the Caldeira-Leggett theory. I will discuss the random force from the vacuum on the test particle, and the heating of such a gravitational vacuum by the interaction with the test particle.
I will end by discussing what this says or does not say about the entropy production in the electro-magnetic vacuum and gravitational vacuum.
 Heinz-Peter Breuer and Francesco Petruccione, „Destruction of quantum coherence through emission of bremsstrahlung”, Phys. Rev. A 63, 032102 (2001)
 Heinz-Peter Breuer and Francesco Petruccione, Theory of Open Quantum Systems (2002), Chapter 12
 Maulik Parikh, Frank Wilczek and George Zahariade, „Signatures of the quantization of gravity at gravitational wave detectors”, Phys. Rev. D 104, 046021 (2021)