# Heat flow reversals without reversing the arrow of time: the role of internal quantum coherences and correlations

## TL;DR

This paper demonstrates that heat flow reversal in quantum systems can occur without reversing the arrow of time and without initial system-bath correlations, relying instead on internal quantum coherences and correlations.

## Contribution

It introduces a novel mechanism for heat flow reversal driven solely by internal quantum coherences, independent of system-bath correlations or time arrow reversal.

## Key findings

- Heat flow reversal can occur without reversing the arrow of time.
- Internal quantum coherences enable control of heat flow direction.
- The scheme can be used as a correlations-to-energy converter.

## Abstract

One of the stunning consequences of quantum correlations in thermodynamics is the reversal of the arrow of time, recently shown experimentally in [K. Micadei, et al., Nat. Commun. 10:2456 (2019)], and manifesting itself by a reversal of the heat flow (from the cold system to the hot one). Here, we show that contrary to what could have been expected, heat flow reversal can happen without reversal of the arrow of time. Moreover, contrasting with previous studies, no initial correlations between system and bath is required. Instead, the heat flow reversal only relies on internal quantum coherences or correlations, which provides practical advantages over previous schemes: one does not need to have access to the bath in order to reverse the heat flow. The underlying mechanism is explained and shown to stem from the collective system-bath coupling and the impact of non-energetic coherences (coherences between degenerate energy levels) on apparent temperatures. The phenomenon is first uncovered in a broad framework valid for diverse quantum systems containing energy degeneracy. By the end of the paper, aiming at experimental realisations, more quantitative results are provided for a pair of two-level systems. Finally, as a curiosity, we mention that our scheme can be adapted as a correlations-to-energy converter, which have the particularity to be able to operate at constant entropy, similarly to ideal work sources.