The heat exchange fluctuation theorem (XFT) by Jarzynski and Wójcik (Phys Rev Lett 92:230602, 2004) addresses the setting where two systems with different temperatures are brought in thermal contact at time \(t=0\) and then disconnected at later time \(\tau \) . The theorem asserts that the probability of an anomalous heat flux (from cold to hot), while nonzero, is exponentially smaller than the probability of the corresponding normal flux (from hot to cold). As a result, the average heat flux is always normal. In that way, the theorem demonstrates how irreversible heat transfer, observed on the macroscopic scale, emerges from the underlying reversible dynamics. The XFT was proved under the assumption that the coupling work required to connect and then disconnect the systems is small compared to the change of the internal energies of the systems. That condition is often valid for macroscopic systems, but may be violated for microscopic ones. We examine the validity of the XFT’s assumption for a specific model of the Caldeira–Leggett type, where one system is a single classical harmonic oscillator and the other is a thermal bath comprised of a large number of oscillators. The coupling between the system and the bath, which is bilinear, is instantaneously turned on at \(t=0\) and off at \(t=\tau \) . For that model, we found that the assumption of the XFT can be satisfied only for a rather restricted range of parameters. In general, the work involved in the process is not negligible and the energy exchange may be anomalous in the sense that the internal energy of the system, which is initially hotter than the bath, may further increase.
