arXiv subjects

Ruicheng Bao

Publications and source records attributed to Ruicheng Bao.

At least 19 recordsLinked to original sources

arXiv2026

Thermodynamics of Speed and Efficiency with Quantum Feedback

Rapid thermodynamic operations generally entail irreversible dissipation, giving rise to trade-offs between operating speed and thermodynamic cost. Yet, how quantum feedback fundamentally reshapes these trade-offs remains elusive. Here, we derive a finite-time current--dissipation relation for continuously monitored open quantum dynamics with jump feedback. From this fundamental relation, we establish a quantum speed limit and a power-efficiency trade-off for stationary information engines, identifying how feedback affects state transformations and the performance of engines through both informational entropy and direct dynamical contributions. Our results provide a quantitative foundation for fast, low-dissipation quantum thermodynamics in the presence of feedback control, exemplified by attaining the generalized Landauer bound in finite-time information erasure and approaching unit information efficiency at finite power in the large-system limit of a collective engine.

quant-ph↗

arXiv2026

Finite-time minimum-work protocols for underdamped traps driven by active forces

Active systems provide non-equilibrium energy that can, in principle, be harvested using control protocols, making the optimization and reliability of work extraction a fundamental problem in microscopic thermodynamics. Yet how inertia and activity jointly modify the optimal work extraction protocol, the maximum extractable work its fluctuations, and implementability of protocols remains unclear. Here, we design minimum-work protocols, which maximize work extraction whenever extraction is possible, for an underdamped active particle in a translated harmonic trap. Remarkably, both the minimum mean work and its fluctuations are independent of mass, although inertia modifies the protocol. The results are then applied to an underdamped active Ornstein-Uhlenbeck particle (AOUP), designing an explicit work-extraction criterion and revealing extracted work's dependence on rotational noise and initial conditions. We further show that inertia strongly affects the difficulty of implementing the optimal protocol, producing a transition-like change between underdamped and overdamped optimal protocols. These findings uncover work extraction mechanisms at the microscale and guide the design of reliable energy-harvesting microscopic devices and robots.

cond-mat.stat-mech↗

arXiv2026

Reset-Enhanced Dissipative Quantum Metrology

Quantum metrology is ultimately shaped not only by measurements, but also by the dissipative dynamics that prepare and maintain the probe. Nevertheless, a general mechanism for enhancing both steady-state and transient precision in open quantum systems has been lacking. Here we show that adding a simple reset channel provides such a mechanism: reset acts as a minimal dissipative control that can enhance metrological precision without modifying the intrinsic Hamiltonian or applying continuous coherent driving. We derive a unified quantum-Fisher-information (QFI) reset-rate response identity, showing how reset enhances steady-state and transient estimation while revealing a sharp transient distinction. For static-parameter estimation in diagonal dynamics, reset leaves the leading short-time QFI unchanged; for the separate task of elapsed-time estimation, it generates an immediate drift contribution that can scale quadratically with the reset rate in the short-time limit. For frequency estimation, the enhancement is instead dominated by a genuinely quantum eigenvector contribution. These results establish reset as a simple dissipative resource for open-system quantum metrology.

quant-ph↗

arXiv2026

Extreme First-Passage Time of Many Interacting Particles

Extreme first-passage events are broadly relevant to biological, chemical, and physical processes in which the first successful arrival determines the outcome. Existing theories are largely confined to noninteracting searchers. Interacting extreme-statistics problems are notoriously difficult because correlations destroy probability factorization. We establish a general framework for interacting extreme search. A no-go theorem shows that broad classes of short-range interactions cannot beat the $1/\ln N$ extreme timescale of $N$ independent Brownian searchers, and complementary upper bounds prove that this scale is exact for repulsive interactions. We then identify three sharp mechanisms beyond the logarithmic class and derive a unified interaction-driven acceleration limit. In particular, deterministic pairwise interaction can at most reduce the extreme search time to order $1/N$, while stochastic pairwise forcing attains $1/(N\ln N)$. Our results separate acceleration due to statistical redundancy from that generated by coherent many-body transport or amplified fluctuations, deepening our understanding of interacting many-body systems.

cond-mat.stat-mech↗

arXiv2026

Exact Fluctuation-Response Relations for Underdamped Langevin Dynamics

Thermodynamic uncertainty relations connect current fluctuations to dissipation and, in certain settings, can emerge from underlying fluctuation-response relations. However, in underdamped dynamics, conventional mean-current uncertainty relations can fail, and the underlying connection between fluctuations, response, and dissipation remains elusive. Here, we uncover this connection by deriving an exact finite-time fluctuation-response equality for underdamped Langevin dynamics, which is valid for arbitrary time-dependent driving and general additive observables. The equality yields sharp response bounds and a variational characterization of the dynamically generated variance. Choosing the perturbation along the irreversible probability flow, we obtain an experimentally accessible friction-response thermodynamic uncertainty relation that is saturable at any finite observation time. It provides both an exact variational principle for entropy production and a weak-noise coherence-dissipation bound for underdamped limit cycles. We further show that the conventional uncertainty factor of velocity-resolved currents can decay exponentially with dissipation, even in driven free diffusion, whereas the friction-response factor retains its universal lower bound. These results establish response, rather than the mean current itself, as the quantity directly linking fluctuations and dissipation in underdamped dynamics.

cond-mat.stat-mech↗

arXiv2026

Hierarchical Reconstruction of Time-arrow from Multi-time Correlations

The entropy production rate (EPR), a key measure of thermodynamic irreversibility in thermodynamics of small systems, is difficult to determine directly in experiments, motivating lower-bound-based estimation from observations. However, a systematic framework for organizing increasing amounts of the irreversibility information in experimental state observables into progressively tighter bounds remains lacking. Here, we show that multi-time correlations of a class of state observations naturally encode this information to provide a hierarchy. By defining a reconstruction operation as a combination of correlations, we obtain a sequence of lower bounds on the EPR. Correlations of higher order capture the thermodynamic information at greater temporal depth, thereby capturing more irreversibility and yielding tighter bounds. Under ideal conditions, this hierarchy converges to the full EPR in the limit of infinitely dense observations over a finite time window.

cond-mat.stat-mech↗

arXiv2026

Random Coarse-Graining with Applications to Measuring Irreversibility

Thermodynamic irreversibility is a fundamental concept in statistical physics, yet its experimental measurement remains challenging, especially for complex systems. At the same time, imprecise readout is ubiquitous in experiments, but its effect on thermodynamic irreversibility has not been formulated in a general way. We introduce a novel random coarse-graining framework that incorporates a probabilistic mapping from fine-grained to coarse-grained states, and we use it both to model imprecise measurements and to identify model-free measures of irreversibility in complex many-body systems. These measures are constructed from the asymmetry of cross-correlation functions between suitably chosen observables, providing rigorous lower bounds on the entropy production. For many-particle systems, we propose a particularly practical implementation that divides real space into virtual boxes and monitors particle number densities within them, requiring only simple counting from video microscopy, without single-particle tracking, trajectory reconstruction, or prior knowledge of interactions. Owing to its generality and limited data requirements, the random coarse-graining framework offers broad applicability across diverse nonequilibrium systems.

cond-mat.stat-mech↗

arXiv2026

A Universal Control Budget for First-Passage Kinetics

The first-passage time is the natural observable of reaction completion, yet how its mean responds to a rate change has lacked a general constraint. We show that the logarithmic sensitivity of the mean first-passage time of any finite Markov chain to any rate is bounded by one in magnitude, and that these sensitivities sum to -1. Together the two laws form a conserved control budget: speeding completion through some transitions must be paid for by others, and a coordinated change shifts the completion time only as far as the budget allows. Raising an activation barrier or shifting the depth of a well moves many rates at once, yet neither can shift the completion time further than a single rate could. The budget caps kinetic-proofreading discrimination at the checkpoint count, and prices it in sensitivity to substrate concentration.

cond-mat.stat-mech↗

arXiv2026

Exact First-Passage Time Response Theory from Steady-State Response

The mean first-passage time (MFPT) provides a universal temporal measure of transport, reaction, search, and switching processes in physical, chemical, and biological systems. Understanding how MFPTs respond to perturbations is therefore crucial for prediction and control, yet a systematic theory has been lacking. We establish a compact theoretical framework for linear and nonlinear MFPT response in continuous-time Markov processes. The key tool is an exact correspondence that maps the intrinsically transient response of MFPTs onto the steady-state response of an auxiliary system. This correspondence yields exact and universal response relations for MFPTs between arbitrary state pairs, expressed entirely in terms of unperturbed MFPTs and steady-state probabilities. We then obtain a factorized physical decomposition of the MFPT response into linear upstream, linear downstream, and nonlinear contributions. Further corollaries include response-curve inference rules, fundamental bounds on MFPT responses, analytical expressions for higher-order responses of MFPTs and steady-state probabilities, and multi-rate response formulas. Additionally, our result offers computational advantages in calculating both MFPTs and steady-state distributions. Finally, a biologically motivated folding network is analyzed, and a recently reported paradox on MFPT is clarified.

cond-mat.stat-mech↗

arXiv2026

Entanglement Mpemba Effect

Generating entanglement rapidly and reliably is essential for quantum information processing, communication, and metrology. Dissipative preparation is attractive because engineered reservoirs robustly drive a system toward an entangled target, yet relaxation can carry a substantial time cost. Here we formulate the entanglement Mpemba effect, whereby an initially less entangled state overtakes a more entangled state under the same open-system dynamics. This effect turns initial-state engineering into a route for faster preparation without altering the dissipative protocol. We derive a general criterion for the reversal from the relaxation spectrum, applicable even when entanglement evolves nonmonotonically. A reversal of deterministic local operations and classical communication (LOCC)-reachability preorder provides a measure-independent certificate of reversed entanglement order. Exactly solvable models show that initial-state selection can substantially shorten the time required to reach high entanglement. We further propose an experimentally relevant trapped-ion protocol that can realize the entanglement Mpemba effect.

cond-mat.stat-mech↗

arXiv2026

Optimal Finite-Time Control of Nonreciprocal Brownian Dimers: Thermodynamic Anomaly and Multiple Transitions

We solve exactly a finite-time thermodynamic optimal control problem for two nonreciprocally interacting Brownian particles translated by two harmonic traps. The controller manipulates both the center and separation of the pair. Nonreciprocal interactions generate an internal active force that couples these two channels. The optimal protocol is oscillatory, deliberately opens the dimer even when the target separation is unchanged, and can extract work during transport. A central finding is a finite critical time beyond which the external-work infimum is $-\infty$: at any prescribed duration beyond this threshold, both extractable work and output power are unbounded. Physical regularizations such as finite trap range and force saturation restore a finite optimum and convert the anomaly into optimal-protocol transitions: in the zero-target case, a hard finite range produces a first-order-like jump from the zero protocol to a maximum-range protocol, whereas smooth force saturation gives a continuous, second-order-like onset. Under finite-range constraints, the optimal protocol can further undergo multiple finite-time transitions, producing multiple work-duration kinks with no qualitative analog in prior studies.

cond-mat.stat-mech↗

arXiv2026

Typical Mixing and Rare-State Bottlenecks in Open Quantum Systems

Mixing in open quantum systems is often summarized by a single worst-case time, even though that benchmark can be set by exponentially rare initial states. We show that for broad unstructured ensembles the nonlinear trace-distance relaxation curve itself concentrates around a deterministic mean. For Haar-random pure states this yields fixed-time concentration of the instantaneous trace distance to the steady state, which we term vertical concentration since typical relaxation curves bundle along the distance axis. Whenever the mean curve crosses the distance threshold with a finite slope, it converts this vertical concentration into a horizontal concentration of the mixing time, extending typicality from standard physical observables to a fundamentally non-observable dynamical quantity. This sharp concentration naturally raises the question of how the typical mixing timescale compares to the worst-case benchmark. We show that in a one-mode tail regime, this separation is controlled by the logarithmic ratio of extremal to typical initial-state overlaps for the slow left eigenoperator. This rare-state bottleneck law yields a hierarchy that is logarithmic in skin-effect settings, linear for boundary-supported many-body slow modes, and exponential in a protected-sector family where generic states mix rapidly while rare states stagnate. The framework also extends beyond Haar to exact and approximate unitary 2-designs and Hilbert-Schmidt/induced ensembles.

quant-ph↗

arXiv2026

Nonlinear Response Identities and Bounds for Nonequilibrium Steady States

Understanding how systems respond to external perturbations is fundamental to statistical physics. For systems far from equilibrium, a general framework for response remains elusive. While progress has been made on the linear response of nonequilibrium systems, a theory for the nonlinear regime under finite perturbations has been lacking. Here, building on a novel connection between response and mean first-passage times in continuous-time Markov chains, we derive a comprehensive theory for the nonlinear response to archetypal local perturbations. We establish an exact identity that universally connects the nonlinear response of any observable to its linear counterpart via a simple scaling factor. This identity directly yields universal bounds on the response magnitude. Furthermore, we establish a universal bound on response resolution -- an inequality constraining an observable's change by its intrinsic fluctuations -- thereby setting a fundamental limit on signal-to-noise ratio. These results provide a rigorous and general framework for analyzing nonlinear response far from equilibrium, which we illustrate with an application to transcriptional regulation.

cond-mat.stat-mech↗

arXiv2026

Characteristic oscillations in frequency-resolved heat dissipation of linear time-delayed Langevin systems: Approach from the violation of the fluctuation-response relation

Time-delayed effects are widely present in nature, often accompanied by distinctive nonequilibrium features, such as negative apparent heat dissipation. To elucidate detailed structures of the dissipation, we study the frequency decomposition of the heat dissipation in linear time-delayed Langevin systems. We decompose the heat dissipation into frequency spectrum using the Harada-Sasa equality, which relates the heat dissipation to the violation of the fluctuation-response relation (FRR). We find a characteristic oscillatory behavior in the spectrum, and the oscillation asymptotically decays with an envelope inversely proportional to the frequency in the high-frequency region. Furthermore, the oscillation over the low-frequency region reflects the magnitude and sign of the heat dissipation. We confirm the generality of the results by extending our analysis to systems with multiple delay times. Since the violation of FRR is experimentally accessible, our results suggest an experimental direction for detecting and analyzing detailed characteristics of dissipation in time-delayed systems.

cond-mat.stat-mech↗

arXiv2026

Initial-State Typicality in Quantum Relaxation

Relaxation in open quantum systems is fundamental to quantum science and technologies. Yet, the influence of the initial state on relaxation remains a central, largely unanswered question. Here, by systematically characterizing the relaxation behavior of generic initial states, we uncover a typicality phenomenon in high-dimensional open quantum systems: relaxation becomes nearly initial-state-independent as system size increases under verifiable conditions. Crucially, we prove this typicality for thermalization processes above a size-independent temperature. Our findings extend the typicality to open quantum dynamics, in turn identifying a class of systems where two widely used quantities -- the Liouvillian gap and the maximal relaxation time -- merit re-examination. We formalize this with two new concepts: the 'typical strong Mpemba effect' and the 'typical relaxation time'. Beyond these conceptual advances, our results provide practical implications: a scalable route to accelerating relaxation and a typical mixing-time benchmark that complements conventional worst-case metrics for quantum simulations and state preparation.

quant-ph↗

arXiv2026

Physically natural metric-measure Lindbladian ensembles and their learning hardness

In open quantum systems, a basic question at the interface of quantum information, statistical physics, and many-body dynamics is how well can one infer the structure of noise and dissipation generators from finite-time measurement statistics alone. Motivated by this question, we study the learnability and cryptographic applications of random open-system dynamics generated by Lindblad-Gorini-Kossakowski-Sudarshan (GKSL) master equations. Working in the affine hull of the GKSL cone, we introduce physically motivated ensembles of random local Lindbladians via a linear parametrisation around a reference generator. On top of this geometric structure, we extend statistical query (SQ) and quantum-process statistical query (QPStat) frameworks to the open-system setting and prove exponential (in the parameter dimension $M$) lower bounds on the number of queries required to learn random Lindbladian dynamics. In particular, we establish average-case SQ-hardness for learning output distributions in total variation distance and average-case QPStat-hardness for learning Lindbladian channels in diamond norm. To support these results physically, we derive a linear-response expression for the ensemble-averaged total variation distance and verify the required nonvanishing scaling in a random local amplitude-damping chain. Finally, we design two Lindbladian physically unclonable function (Lindbladian-PUF) protocols based on random Lindbladian ensembles with distribution-level and tomography-based verification, thereby providing open-system examples where learning hardness can be translated into cryptographic security guarantees.

quant-ph↗

arXiv2025

Universal trade-off between irreversibility and intrinsic timescale in thermal relaxation with applications to thermodynamic inference

We establish a general lower bound for the entropy production rate (EPR) based on the Kullback-Leibler divergence and the Logarithmic-Sobolev constant that characterizes the time-scale of relaxation. This bound can be considered as an enhanced second law of thermodynamics. When applied to thermal relaxation, it reveals a universal trade-off relation between the dissipation rate and the intrinsic relaxation timescale. From this relation, a thermodynamic upper bound on the relaxation time between two given states emerges, acting as an inverse speed limit over the entire time region. We also obtain a quantum version of this upper bound, which is always tighter than its classical counterpart, incorporating an additional term due to decoherence. Remarkably, we further demonstrate that the trade-off relation remains valid for any generally non-Markovian coarse-grained relaxation dynamics, highlighting its significant applications in thermodynamic inference. This trade-off relation is a new tool in inferring EPRs in molecular dynamics simulations and practical experiments.

cond-mat.stat-mech↗

arXiv2025

Accelerating Quantum Relaxation via Temporary Reset: A Mpemba-Inspired Approach

Slow relaxation processes spanning widely separated timescales pose fundamental challenges for probing steady-state properties and engineering functional quantum systems, such as quantum heat engines and quantum computing devices. We introduce a protocol that enables significant acceleration of relaxation in general Markovian open quantum systems by temporarily coupling the system to a reset channel, inspired by the Mpemba effect. Crucially, this acceleration persists even when the slowest decaying Lindbladian modes form complex-conjugate pairs. Unlike previous approaches, which typically target a single mode, our protocol may suppress multiple relaxation modes simultaneously. This framework provides a versatile and experimentally feasible tool for controlling relaxation timescales, with broad implications for quantum thermodynamics, computation, and state preparation.

quant-ph↗