Recent technological advances have allowed the construction of many-body quantum systems that operate below the thermodynamic limit. This has given rise to the field of quantum thermodynamics (QTD), which describes heat, work, and entropy while accounting for quantum effects. Within the framework of QTD, numerous theoretical and experimental quantum thermal machines (QTMs) have been proposed, aiming to exploit quantum effects as a resource. Hence, understanding the relationship between quantum effects and thermodynamic properties is essential for creating efficient QTMs.
We explore the use of Nth-root controlled-not gates (NRCGs) in QTD, modelling our system as multiple QTMs. These gates allow for a fractional application of two-qubit operations, facilitating access to intermediary states inaccessible by full controlled-not (CNOT) gates. The system presented is a closed, out-of-equilibrium multi-qubit quantum system in which we employ NRCGs in three distinct protocols, comparing them using regime-specific performance measures.
First, we consider a quantum battery, where we optimise initial parameters to maximise performance measures related to ergotropy and charging potential. Our results show that, across all protocols considered, initial quantum coherence in a single qubit improves the battery’s performance. Moreover, some NRCG protocols demonstrate better performance than CNOTs. We then characterise our system as a quantum heat engine, where initial quantum coherence increases extractable work which is achieved at high efficiencies. Furthermore, we uncover a strong linear correlation between work production and many-body correlations.
Finally, we demonstrate the system can operate as different thermal machines without changes to protocol design or system size, with regime selection dictated by the initialisation parameters of a single qubit. Two uniquely interesting regimes are accessed: a work producing refrigerator and reverse flow heat engine, both of which produce work and provide cooling simultaneously. These regimes could reduce the energetic cost of cooling in quantum computer architecture.
