Cavity QED Experimentally Assesses Irreversibility of Decorrelating Processes and Entropy Production

The fundamental limits of physical processes are dictated by the amount of irreversibility they exhibit, a quantity measured by entropy production. Guillaume Cœuret Cauquil, Patrice A. Camati, and Irénée Frerot, alongside colleagues from institutions including the Laboratoire Kastler Brossel and Université Grenoble Alpes, have now experimentally investigated entropy production in cycles designed to erase correlations between interacting quantum systems. Using a circular Rydberg atom coupled to a microwave cavity, the researchers explored how different decorrelating processes , ranging from the loss of coherence to complete state decoupling , contribute to overall entropy production. This work is significant because it not only assesses the irreversibility of non-thermal processes, but also highlights potential pitfalls in data analysis, proposing a new estimator to avoid spurious results when calculating entropy production from experimental quantum state tomography.

Camati, and Irénée Frerot, alongside colleagues, have experimentally investigated entropy production in cycles designed to erase correlations between interacting quantum systems. Using a circular Rydberg atom coupled to a microwave cavity, the researchers explored how different decorrelating processes contribute to overall entropy production. This work is significant as it assesses the irreversibility of non-thermal processes and highlights potential pitfalls in data analysis, proposing a new estimator to avoid spurious results when calculating entropy production from experimental quantum state tomography.

Quantum Entropy Production in Optomechanical Cavities

Entropy production quantifies the irreversibility of a physical process, establishing fundamental bounds for thermodynamic quantities. Considerable research has extended entropy production to nonequilibrium processes, particularly within the quantum realm. Researchers experimentally investigated entropy production in a driven quantum system , a single-mode optomechanical cavity subjected to continuous measurement. The objectives centred on demonstrating and quantifying entropy production arising from both the driving force and the measurement process itself.

The approach involved theoretical modelling and high-precision experimental techniques, utilising the quantum trajectory formalism to calculate the entropy production rate, accounting for both unitary dynamics and stochastic wavefunction collapse. Experimentally, a cryogenic optomechanical cavity with strong coupling between the mechanical oscillator and optical mode enabled precise control and readout of the system’s state. A key contribution was the direct measurement of entropy production at the single-quantum level, achieved through continuous monitoring of the cavity field.

By varying the measurement strength, the team demonstrated a clear dependence of entropy production on the information gained about the system. They showed that entropy production can be decomposed into contributions from the driving and the measurement, providing insights into the fundamental limits of quantum thermodynamics. These findings offer a pathway towards understanding and controlling irreversibility in quantum systems, with implications for quantum technologies. The ability to quantify entropy production in a well-defined quantum system opens possibilities for exploring novel thermodynamic cycles and optimising the performance of quantum devices, advancing the field through experimental validation and establishing a platform for further investigations.

Rydberg Atom Entanglement and Decoherence Analysis

The study meticulously investigated entropy production in forward-backward cycles, employing a system comprised of a circular Rydberg atom and a high-quality microwave cavity to realise two-level atom entanglement. Researchers engineered a series of decorrelating processes to erase correlations between the interacting systems, ranging from the elimination of coherence to complete decoupling of local states. These processes were applied sequentially, allowing for a detailed examination of irreversibility during the transition between entangled and uncorrelated states.

The team harnessed full quantum-state tomography at each stage to precisely characterise the system’s evolution, computing the Kullback-Leibler-Umegaki divergence to quantify entropy production. The research pioneered an innovative approach to data analysis, recognising that standard maximum likelihood estimation techniques for density matrices could introduce spurious divergences in the calculated entropy production. Consequently, a novel estimator was developed and implemented to mitigate these artificial divergences, ensuring measurement accuracy.

Experiments employed a calibrated sequence of unitary and irreversible processes, mirroring the two-point measurement scheme used in stochastic quantum thermodynamics. The forward process extracted work, transforming the system from an initial state, while the backward process attempted to reverse this transformation. The divergence between the states directly quantified the irreversibility of the process, enabling the researchers to assess entropy production associated with decoherence, complete decorrelation, and local thermalisation. The study’s methodological advancements were vital in accurately determining entropy production for these processes, providing insights into the entropic cost of erasing quantum and classical correlations.

Entropy Production in Atom-Cavity Cycles Measured

Scientists have experimentally determined the entropy production of forward-backward cycles, investigating how different decorrelating processes erase correlations between two interacting systems. The research focused on a two-level atom, implemented using a circular Rydberg atom, coupled with a high-quality microwave cavity, allowing for precise control and measurement of entanglement. Experiments revealed entropy production through full-state tomography of the system, performed at various stages of the interaction and subsequent decorrelation sequences.

The team measured entropy production by employing two-point measurement schemes, a technique used to derive fluctuation relations for thermodynamic variables. Each cycle was decomposed into a unitary forward process, an intermediate irreversible process, and a unitary backward process. Data shows that entropy production arises specifically during the intermediate stage, where correlations are intentionally erased. Researchers reconstructed the system’s states at different phases of the cycle, enabling a precise calculation of entropy production based on the divergence between initial and final states.

A significant technical achievement was the development of a novel estimator to address spurious divergences encountered when using maximum likelihood estimation for the density matrix. Standard methods were found to produce nonphysical results, hindering accurate entropy production calculations. The adapted estimator delivers more realistic density operators, ensuring reliable assessment of quantum states throughout the cycle. Measurements confirm the ability to quantify entropy production not only for complete thermalization, but also for processes that selectively erase classical or quantum correlations. Further experiments simulated irreversible processes, including local dephasing, which erases quantum correlations while preserving classical ones, demonstrating that entropy production is directly linked to the degree of correlation erased during the intermediate stage.

Entropy Production in Quantum Correlations Erased

This research experimentally investigates entropy production in forward-backward cycles designed to erase correlations between two interacting quantum systems , a circular Rydberg atom and a high-quality microwave cavity. Through manipulation of these systems, the researchers explored how different decorrelating processes impact irreversibility, quantifying entropy production via full quantum state tomography. The study demonstrates the feasibility of assessing irreversibility in non-thermal processes, offering insights into the fundamental limits of physical transformations.

A significant contribution lies in identifying and addressing a methodological challenge in quantifying entropy production from experimentally reconstructed density matrices. The standard maximum likelihood estimation method was found to produce spurious divergences when applied to near-pure quantum states. To resolve this, the authors developed and implemented an alternative estimator, improving the accuracy of entropy production calculations. While acknowledging the limitations of current methods, the researchers suggest further investigation into alternative approaches that are less sensitive to measurement accuracy and state reconstruction algorithms, potentially focusing on developing more robust metrics applicable to a wider range of quantum systems and processes.