Quantum Coherence Optimises Work Extraction from Superconducting Qubits

Research demonstrates that coherence, a quantum property encoded within a system’s density matrix, directly influences work extraction from a superconducting qubit. Experiments utilising three protocols reveal that initial state design optimises ergotropy, or maximum extractable work, contingent on dominant decoherence channels like energy relaxation and dephasing, improving device efficiency.

The pursuit of efficient energy utilisation increasingly focuses on harnessing quantum coherence, a phenomenon where a quantum system exists in multiple states simultaneously. This property represents a potential resource for performing tasks beyond the capabilities of classical systems. Still, its practical application requires careful management of coherence and an understanding of the energetic costs involved in its manipulation. Researchers Li Li, Silu Zhao, Kai Xu, Heng Fan, Dongning Zheng, and Zhongcheng Xiang, all affiliated with the Beijing National Laboratory for Condensed Matter Physics at the Institute of Physics, Chinese Academy of Sciences, address these challenges in their work, titled ‘Experimental Extraction of Coherent Ergotropy and Its Energetic Cost in a Superconducting Qubit’. Their investigation details the experimental extraction of ergotropy, a measure of the maximum work obtainable from a quantum system, from a superconducting transmon qubit. It quantifies the energy expenditure associated with maintaining and utilising coherence.

Researchers detail a novel investigation into the manipulation of quantum coherence to enhance energy utilisation in their recent work, addressing fundamental challenges in harnessing quantum resources. The study focuses on extracting ergotropy – the maximum work obtainable from a quantum system – from a superconducting transmon qubit and quantifying the associated energetic costs.

Quantum thermodynamics explores the interplay between quantum mechanics and thermodynamics, revealing possibilities for energy conversion and storage not available in classical systems. Coherence, a fundamental quantum property describing the superposition of quantum states, limits the maximum extractable work, making its control paramount for efficient energy processes. A system possessing coherence exhibits wave-like behaviour, allowing for interference effects that can be exploited for work extraction.

This experiment investigates how initial-state coherence impacts work extraction from a superconducting transmon qubit, employing a rigorous methodology to quantify the relationship between coherence and energy conversion. Researchers prepared a variety of pure quantum states – states described by a single wavefunction – and implemented three distinct work extraction protocols to determine how coherence governs the distribution of ergotropy. They discovered that the optimal initial state depends on the dominant decoherence channel – energy relaxation or dephasing – highlighting the importance of understanding and mitigating environmental noise. By accounting for associated costs, the team identified initial states that maximise extraction efficiency, demonstrating a pathway towards practical coherence control.

The experimental setup involved a superconducting transmon qubit, a widely used platform for quantum information processing, carefully isolated and controlled within a cryogenic environment. Researchers meticulously prepared a range of pure states, ensuring minimal unwanted quantum noise, and then subjected these states to three distinct work extraction protocols, each designed to probe different aspects of coherence. These protocols allowed them to precisely measure the amount of work extracted from the qubit and to determine how that work is partitioned, revealing the contribution of initial coherence versus other factors.

The experimental design also accounted for the inevitable presence of decoherence, the process by which quantum systems lose their coherence and behave more classically, a significant challenge in quantum technologies. Two primary decoherence channels were considered: energy relaxation, where the qubit loses energy to its environment, and dephasing, where the phase relationship between quantum states is disrupted. Researchers found that the optimal initial state for maximising work extraction depends on which decoherence channel is dominant, a significant finding with implications for practical device design. Furthermore, the study extends beyond simply measuring work extraction to consider the energetic costs associated with preparing the initial state and implementing the extraction protocol, providing a holistic assessment of efficiency. Computational methods, detailed in publications such as those in Physical Review Letters and Physical Review B, were employed.

These results offer valuable guidance for developing efficient quantum devices, suggesting that future devices could be designed to actively control and optimise coherence, leading to significant improvements in performance and efficiency. This work, building on prior investigations into quantum thermodynamics, establishes a pathway towards harnessing coherence as a practical resource for quantum information processing and energy management. The findings demonstrate that initial-state design represents a viable and scalable strategy for coherence control, offering practical guidance for developing more efficient quantum technologies.