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arXiv:2503.20729 · Physical review. B./Physical review. B

Measuring work in quantum many-body systems using a dynamical work agent

Cheolhee Han, Nadav Y. Katz, Eran Sela

Published Jul 2, 2025• 2 citations
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Abstract

We consider a generic quantum many-body system initiated at thermal equilibrium and driven by an external parameter, and discuss the prospect for measuring the work done by the varying parameter on the system. While existing methods are based on a full control of the system's Hamiltonian and are thus limited to few-level quantum systems, measuring work in many-body quantum systems remains challenging. Our approach relies on transforming the external parameter into a dynamical ``work agent", for which we consider an harmonic oscillator in a semiclassical coherent state with a large photon number. We define a work generating function which coincides with the standard two-point measurement protocol for work measurement in the limit of a large photon number. While \emph{in principle} it allows to relate the moments of work $\langle W^n \rangle$ to observables of the work agent, we focus on the average work, which is obtained from energy conservation by the change of the energy of the agent, which can be measured using photon number detection. We illustrate this concept on a transmon-microcavity system, which displays various quantum coherent effects including Landau-Zener Stükelberg interference and collapse and revival of Rabi oscillations. We discuss how our setup allows to measure work in a variety of quantum many-body systems.

DOI
10.1103/2p33-wy8h

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We consider a generic quantum many-body system initiated at thermal equilibrium and driven by an external parameter, and discuss the prospect for measuring the work done by the varying parameter on the system. While existing methods are based on a full control of the system's Hamiltonian and are thus limited to few-level quantum systems, measuring work in many-body quantum systems remains challenging. Our approach relies on transforming the external parameter into a dynamical ``work agent", for which we consider an harmonic oscillator in a semiclassical coherent state with a large photon number. We define a work generating function which coincides with the standard two-point measurement protocol for work measurement in the limit of a large photon number. While \emph{in principle} it allows to relate the moments of work ⟨Wn⟩\langle W^n \rangle⟨Wn⟩ to observables of the work agent, we focus on the average work, which is obtained from energy conservation by the change of the energy of the agent, which can be measured using photon number detection. We illustrate this concept on a transmon-microcavity system, which displays various quantum coherent effects including Landau-Zener Stükelberg interference and collapse and revival of Rabi oscillations. We discuss how our setup allows to measure work in a variety of quantum many-body systems.
Abstract Heat and work in thermodynamics refer to the measurement of changes in energy content of external bodies (baths and agents). We discuss the implications of quantum mechanics on the possibility to measure work in the mesoscopic context. The agent is a quantum entity (say an oscillator) that is used to drive the system. An inherent limitation is related to back-reaction, leading to a classical-like restriction. We find that in order to resolve fingerprints of interference, an additional quantum uncertainty limitation should be taken into account in the design of the agent. This quantum limitation is fundamental, not merely noise-like blurring.
We consider work fluctuation theorems for isolated driven systems and the possibility to probe them in mesoscopic systems. In this context nonequilibrium fluctuation theorems (NFTs) relate work performed on a system as its Hamiltonian varies with time, to equilibrium data of the initial and final states. In a classical context the system energy can be directly measured, while a quantum implementation requires the incorporation of a work agent. Here, as a work agent we consider a dynamical single-coordinate object, which exchanges energy with the system. The work done on the system is defined as the energy reduction of the work agent, which requires an energy measurement of the agent (only) at the end of the process. To justify the applicability of the NFT we require the agent's trajectory to be weakly affected by the energy exchange with the system. We furthermore argue that the uncertainty in the energy measurements imposes an inherent quantum limitation on the validity of the NFT. We demonstrate our findings for a two-level system, and discuss applications to more complex mesoscopic systems.
Abstract To investigate the role of initial quantum coherence in work-probability distribution, it is necessary to consider an incomplete or partial measurement, in which the energy cannot be fully discriminated by the detector. In this paper, we use a harmonic oscillator with a coherent or squeezing state to realize this incomplete or partial measurement, and propose a unified framework of quantum work statistics for a closed system with an arbitrary initial state. We find that work is proportional to the change of the real part of the coherent state parameter, i.e., quantum work can be estimated by the coherent state parameter. The resulting work-probability distribution includes the initial quantum coherence, and can be reduced to the result of the traditional two projective energy measurement scheme (TPM) by squeezing the state of the harmonic oscillator. Our measurement scheme reveals the fundamental connections between measurement error and coherent work. By introducing a ‘coherent work-to-noise ratio’, we find the optimal measurement error, which is determined by the energy difference between the superposed energy levels. As an application, we consider a driven two-level system and investigate the effects of driving velocity on work statistics. We find that only when the driving velocity matches the transition frequency of the system can initial quantum coherence play an important role.
The collapse and revival of localized states in many-body systems is a quintessential example of a quantum interference phenomenon that cannot be explained purely within mean-field theories. Historically, the majority of previous work on quantum revivals has been in a Schrödinger mechanics context, typically for effectively 1D systems with a focus on the autocorrelation function as the key detection observable. The general revival subject acquires a far greater complexity in the bosonic many-body context that offers a larger variety of observables and intrinsically implies high dimensionality. Here, the focus is on the simplest nontrivial (i.e., not 1D) bosonic many-body system, namely a symmetric two-site Bose-Hubbard model. The non-equilibrium situation of a coherent initial state prepared on one of the two sites of this integrable 2D system in the strongly self-trapping regime is considered. The revival behaviors emerging from the resulting dynamics within expectation values, coherences, and the autocorrelation function are theoretically derived, and the entanglement behavior is illustrated. Several new features, unknown so far in the Schrödinger context and identified here in terms of five ``puzzles'', are encountered and discussed, among them, a synchronized resurgence behavior, differing revivals times depending on the specific observable, and a manifestation of a precession that appears in the underlying mean field dynamics. These new phenomena can be entirely understood through a comprehensive semiclassical theory, building upon the underlying classical (mean-field) phase-space structure. A quantitative comparison of the latter with semiclassical predictions and quantum perturbative calculations yields an overall excellent agreement and confirms that a full semiclassical theory is pivotal for the understanding of these genuinely quantum many-body interference phenomena.
We study the production of photons in a model of three bosonic atomic modes nonlinearly coupled to a cavity mode. In the absence of external driving and dissipation, the energy levels at different photon numbers assemble into the steps of an energy staircase which can be employed as guidance for preparing multi-photon states. We consider adiabatic photon production, driving the system through a sequence of Landau–Zener transitions in the presence of external coherent light pumping. We also analyse the non-equilibrium dynamics of the system resulting from the competition of the sudden switch of coherent photon pumping and cavity photon losses, and we find that the system approaches a plateau with a given number of photons, which becomes metastable upon increasing the rate of photon pumping. We discuss the sensitivity of the time scales for the onset of this metastable behavior to system parameters and predict the value of photons attained, solving the driven-dissipative dynamics including three-body correlations between light and matter degrees of freedom.
We present a new method to measure the work w performed on a driven quantum system and to sample its probability distribution P(w). The method is based on a simple fact that remained unnoticed until now: Work on a quantum system can be measured by performing a generalized quantum measurement at a single time. Such measurement, which technically speaking is denoted as a positive operator valued measure reduces to an ordinary projective measurement on an enlarged system. This observation not only demystifies work measurement but also suggests a new quantum algorithm to efficiently sample the distribution P(w). This can be used, in combination with fluctuation theorems, to estimate free energies of quantum states on a quantum computer.