We explore the use of fractional controlled-not gates in quantum thermodynamics. The Nth-root gate allows for a paced application of two-qubit operations. We apply it in quantum thermodynamic protocols for charging a quantum battery. Circuits for three (and two) qubits are analysed by considering the generated ergotropy and other measures of performance. We also perform an optimisation of initial system parameters, e.g.,the initial quantum coherence of one of the qubits strongly affects the efficiency of protocols and the system's performance as a battery. Finally, we briefly discuss the feasibility for an experimental realization.
We explore the use of fractional controlled-not gates in quantum thermodynamics. The Nth-root gate allows for a paced application of two-qubit operations. We apply it in quantum thermodynamic protocols for charging a quantum battery. Circuits for three (and two) qubits are analysed by considering the generated ergotropy and other measures of performance. We also perform an optimisation of initial system parameters, e.g.,the initial quantum coherence of one of the qubits strongly affects the efficiency of protocols and the system's performance as a battery. Finally, we briefly discuss the feasibility for an experimental realization.
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.
Nth-root gates allow for a paced application of two-qubit operations. We apply them in quantum thermodynamic protocols for operating a quantum heat engine. A set of circuits for two and three qubits is compared by considering maximum work production and related efficiency. Our results show that for all circuits considered and most regions of initial parameter space, quantum coherence of one of the qubits strongly increases the maximum work production and improves the system's performance as a quantum heat engine. In such circuits, coherence is initially imprinted into one of the qubits, improving the overall maximum extractable work. We focus here on the efficiency of such work extraction, assuming the initialisation of the qubits is a free resource. For the novel protocol that employs fractional control gates, work is generated with 84% up to 100% efficiency. Further, we uncover a strong linear correlation between work production and many-body correlations in the working medium generated by these gates.
We discuss a one-dimensional coupled qubit-array quantum battery model under Born-Karman boundary conditions and investigate both the charging and discharging processes. Applying the stored energy, charging power, and ergotropy as the essential physical indicators of quantum battery, it is observed that minimizing the hopping interaction between the nearest-neighbor qubits in the qubit-array and increasing the number of qubits during battery setup are crucial. Additionally, we employ a classical driving field to optimize battery performance and explore the optimal quantum battery performance by adjusting the driving strength of the classical field. Finally, we have discovered that the initial energy in the charger no longer needs to be higher than the energy in the battery in our protocol, the charger will continue to supply energy to the battery even when there is limited initial available energy in the charger. And the conventional approach of preparing the battery's initial state in its ground state, as observed in previous studies, may not necessarily be the optimal choice. By introducing a strong classical driving field, it is possible to enhance energy storage by allowing for an initial presence of some energy within the battery.
Abstract In this paper, we study the performance of a quantum battery composed of three non-interacting central qubits, which are charged through interactions with three distinct thermal baths, each bath consisting of interacting qubits. We utilize the Holstein-Primakoff transformation and thermodynamic limits to map the collective spin operators of the baths to bosonic modes. Additionally, we employ the finite trio coherent state as the initial state for the charging process to explore its impact on the overall system performance. The study focuses on evaluating the stored energy efficiency and the entropy of the charger modes to assess the overall charging performance. By examining the system parameters such as anisotropy, dimension and shift parameters, and the intensity of the trio coherent state, the optimal conditions for efficient energy storage are identified. Our results provided the role of thermodynamic processes and quantum correlations in enhancing the performance of our quantum battery model.
We reconsider the quantum energetics and quantum thermodynamics of the charging process of a simple, two-component quantum battery model made up of a charger qubit and a single--cell battery qubit. We allow for the initial quantum state of the charger to lie anywhere on the surface of the Bloch sphere, and find the generalized analytical expressions describing the stored energy, ergotropy and capacity of the battery, all of which depend upon the initial Bloch sphere polar angle in a manner evocative of the quantum area theorem. The origin of the ergotropy produced, as well as the genesis of the battery capacity, can be readily traced back to the quantum coherences and population inversions generated (and the balance between these two mechanisms is contingent upon the starting Bloch polar angle). Importantly, the ergotropic charging power and its associated optimal charging time display notable deviations from standard results which disregard thermodynamic considerations. Our theoretical groundwork may be useful for guiding forthcoming experiments in quantum energy science based upon coupled two-level systems.
Quantum battery, as a novel energy storage device, offers the potential for unprecedented efficiency and performance beyond the capabilities of classical systems, with broad implications for future quantum technologies. Here, we experimentally \RefC{demonstrate quantum charging advantage (QCA)} in a scalable solid-state quantum battery. More specifically, we show how double-excitation Hamiltonians for two-level systems promote scalable QCA \RefB{with standard methods.} We effectively implement the collective evolution of quantum systems with 2 up to 12 battery cells in a superconducting quantum processor, and study the performance of quantum charging compared to its uncorrelated classical counterpart. The model considered is a linear chain of superconducting transmon qubits with only \textit{nearest-neighbor} and \textit{pairwise} interactions, which constitute the simplest model of a multi-cell quantum battery. Our results empirically realize substantial QCA without the necessity of adopting long-range and many-body interactions \RefB{ and showcase the quantum features of the QB charging processes with measurements of non-zero coherent ergotropy, incoherent ergotropy and entanglement,} revealing a promising prospect for further developments of efficient and experimentally feasible protocols for QCA.