Experimental Quantum Thermodynamics Using Nuclear Magnetic Resonance

Abstract

Quantum thermodynamics, an interdisciplinary field bridging quantum mechanics and thermodynamics, explores how fundamental thermodynamic laws emerge from quantum mechanical principles. While classical thermodynamics loses applicability at microscopic scales, quantum thermodynamics effectively addresses this limitation at quantum levels. Nuclear magnetic resonance (NMR), a mature and precise quantum computing platform leveraging atomic nuclei's intrinsic magnetic properties for quantum information encoding, serves as an ideal experimental setting for such investigations. Especially, NMR naturally facilitates in-depth studies of thermodynamic concepts like heat transfer and entropy management at the quantum scale, making it exceptionally well-suited for quantum thermodynamics research.

Leveraging this advantageous system, we experimentally realize two distinct quantum refrigeration models. The first is a self-contained quantum refrigerator implemented within a three-qubit NMR processor, operating entirely without external work input, thus fundamentally differing from classical refrigerators. The second model explores quantum refrigeration powered by indefinite causal order (ICO) using the quantum switch. Remarkably, this quantum refrigerator demonstrates a finite coefficient of performance (COP) even when the temperature difference between the hot and cold reservoirs vanishes—a scenario under which classical refrigerators typically exhibit infinite COP. These quantum refrigerators, however, appear to challenge conventional thermodynamic laws: the self-contained refrigerator seems to defy the principle of energy conservation by requiring no external work, and the ICO-based refrigerator only utilizes a purely energy-preserving operation. For the former, the paradox is resolved straightforwardly by identifying that energy is internally transferred from the third qubit. In contrast, the operational mechanism underlying the latter scenario, driven by ICO, necessitates further rigorous exploration.

To elucidate this issue, we conduct comprehensive experiments investigating the quantum switch under various operational conditions, thus clarifying its compatibility with established thermodynamic laws. By explicitly linking the quantum switch to enhancements in communication channel capacities, we demonstrate experimentally that the switch is a non-free operation, consuming free energy associated specifically with quantum coherence present in the control system. Furthermore, our results provide experimental validation for an analytical upper bound on information capacity enhancements and illustrate circumstances under which this theoretical bound can be surpassed when energy-altering quantum channels are employed.

These experimental studies advance the forefront of quantum thermodynamics. The demonstrated quantum refrigerators serve as robust prototypes that can be generalized to explore diverse thermodynamic regimes and extended to practical applications, such as algorithmic cooling techniques. The investigation into the quantum switch its potential as valuable resources in quantum thermodynamics, paving the way toward a broader and more comprehensive quantum thermodynamic resource theory. Collectively, this work not only deepens our theoretical understanding but also opens avenues for future research and technological innovation in the rapidly evolving field of quantum thermodynamics.

Date of Award	9 Sept 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Xin Sunny WANG (Supervisor)