REVIEW 3 major objections 4 minor 39 references
Mechanical work extraction from an error-prone active dynamic Szilard engine
T0 review · 3 major / 4 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read This paper claims a realistic, mechanically coupled Szilard engine can extract positive work from one actively propelled particle despite noisy measurements, and can convert measurement information into work at efficiency above Landauer's…
desk verdict The one-shot engine analysis is careful and worth engaging, but the headline cyclic efficiency gain rests on a free-dynamics approximation the authors admit is inexact at the operating point, so that part is not yet established. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the one-shot average work expression $W_{\rm os} = (1-\varepsilon)p_{\rm fnt}W^+ + [\varepsilon + (1-\varepsilon)(1-p_{\rm fnt})]W^-$, assembled from the probability of placing the piston on the correct side, the distance-dependent contact delay $\tau_\delta$, and the exponential tumbling statistics of the run-and-tumble particle. The piston is a local steric element subject to an external force set at $F_w=1/2$ in units of the stall force, so misses and dissipation during free piston motion are intrinsic to the design. On the information side, the one-shot cost is $\Delta S_{\rm os} = -\ln(\sigma_x\sqrt{2\pi e}/L) + \ln 2 - s_\varepsilon$, and the cyclic cost is carried by the residual-information formula $\Delta S_\tau \simeq D_p\tau/(2\sigma_x^2) + \tau(1-2\varepsilon)\ln\big((1-\varepsilon)/\varepsilon\big)$, derived under free run-and-tumble dynamics between measurements. These pieces combine into the efficiency $\eta = W/(\mathrm{Pe}^{-1}\Delta S)$, and the claim that $\eta$ can exceed 1 follows from the scaling $\eta^*_{\rm os} \sim \mathrm{Pe}$ at fixed measurement precision.
What would settle it
Measure the information gain per cycle in a feedback experiment on a single tracked active particle: Eq. (28) predicts $\Delta S_\tau$ grows linearly in $\tau$ with slope $D_p/(2\sigma_x^2)+(1-2\varepsilon)\ln((1-\varepsilon)/\varepsilon)$, so repeating the measurement at two different positional-noise levels $\sigma_x$ and checking the predicted ratio of slopes would settle whether the cyclic advantage is real or an artifact of the free-dynamics approximation.
Extended reading notes
Core claim
The paper aims to establish that an active Szilard engine whose work extraction is mediated by a finite mechanical piston, rather than a time-dependent virtual potential, can convert measurement information into mechanical work with an information efficiency $\eta = W_{\rm os}/(\mathrm{Pe}^{-1}\Delta S_{\rm os})$ exceeding the Landauer value of 1 for sufficiently large Péclet number. It identifies nontrivial optima in the protocol duration $\tau$ and piston offset $\delta_m$: the best average work occurs near $\tau \simeq 0.3\,\tau_M$ and $\delta_m \simeq 2\sigma_x$, reaching about 15% of the ideal error-free power, and positive one-shot work is confined to a neighborhood of this optimum. Measurement imperfections act asymmetrically: the power optimum is independent of the direction-measurement error $\varepsilon$, while both work and power optima respond to the positional error $\sigma_x$. In cyclic operation, the mutual information from the previous measurement is not fully erased, so refreshing the estimate costs less than a fresh one-shot measurement, and the efficiency optimum is higher by a factor of 4–5 while also acquiring a negative-efficiency regime at large errors.
Load-bearing premise
The cyclic efficiency result depends on treating the particle's motion between measurements as free run-and-tumble dynamics, ignoring collisions with the piston and walls; if those collisions scramble the particle's state enough, the residual information that lowers the next measurement's cost would be smaller than estimated.
Editorial extensions
If this is right
- At fixed measurement precision, optimal protocols are robust: the locations of the work and power maxima shift only weakly with $\varepsilon$ and $\sigma_x$, so no fine-tuning is needed.
- The one-shot information efficiency exhibits a continuous transition: below a critical information gain (about 5 nats in the example), the nontrivial optimum has zero efficiency, then efficiency rises, peaks, and eventually falls as measurement precision is pushed further.
- Cyclic operation dominates one-shot operation: residual mutual information between successive measurements reduces the effective information cost, giving a factor 4–5 higher efficiency, and any refractory pause between cycles strictly lowers efficiency.
- The Landauer bound $\eta=1$ is crossed at $\mathrm{Pe}\simeq 10^3$ for the example parameters, and $\eta^*_{\rm os}\sim\mathrm{Pe}$ overall.
- The independence of the power optimum from the directional error $\varepsilon$ is exact, because the factor $1-\varepsilon$ factors out of the extremization condition for the power.
Reading between the lines
- The predicted work peak near $\delta_m \simeq 2\sigma_x$ and $\tau \simeq 0.3\,\tau_M$ suggests a concrete experimental search: scan piston offset and protocol duration under video-microscopy feedback on a single active colloid, and check whether the work contour and the zero-work boundary match the model.
- The cyclic efficiency gain is computed with the piston and boundaries omitted from the inter-measurement dynamics; modifying the Green's function to include reflection off the piston is the natural next calculation, and it could shrink the factor 4–5 advantage.
- The efficiency metric used here deliberately omits the steady dissipation needed to sustain the active self-propulsion; counting that cost as an additional operational expense would define a smaller efficiency, so the Landauer-violation result is specific to information-to-work accounting rather than total-budget accounting.
- In two or three dimensions, a tumble does not immediately detach the particle from the piston, so the same mechanical design is likely to tolerate directional measurement errors better than the one-dimensional version treated here.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies a one-dimensional run-and-tumble active Szilard engine in which a demon measures the particle position and direction with finite errors and places a mechanical piston ahead of the particle. The piston is pushed by the particle against an external force, extracting work over a protocol of duration τ. The authors derive an explicit one-shot average-work expression, locate nontrivial optima in (τ, δ_m), and analyze how these optima depend on the position error σ_x and direction error ε. They then define an information efficiency η = W_os/(Pe^{-1}ΔS_os), report that it can exceed 1 at large Péclet number, and interpret this as a violation of Landauer's bound. Finally, for cyclic operation they approximate the refreshed mutual information ΔS_τ using free run-and-tumble dynamics between measurements, obtaining a factor-of-4–5 improvement in efficiency at optimal protocol parameters.
Significance. If the results are fully established, the paper would be a useful contribution to the active-information-engine literature: it models a genuinely steric, local mechanical coupling rather than a globally imposed virtual potential, includes finite measurement errors in both position and direction, and provides closed-form one-shot work statistics in Appendix A. The one-shot derivation in Sec. III is internally consistent given the stated one-tumble and high-Péclet assumptions, and the authors are transparent about several limitations, including the inexactness of the cyclic information calculation at the operating force F_w = 1/2 and the choice not to count active dissipation in the efficiency. However, the headline cyclic-efficiency gain and the Landauer-violation claim rest on assumptions that are either explicitly invalid at the operating point or sensitive to the efficiency normalization, so the paper needs revision before the central claims can be accepted.
major comments (3)
- [Sec. IV.B, Eqs. (24)–(29) and Fig. 5] The cyclic information gain ΔS_τ, which drives the advertised factor-of-4–5 efficiency improvement, is computed under approximation (i): free run-and-tumble dynamics with no piston interaction and no domain boundaries. The authors explicitly state that this is accurate for L/d_M ≫ 1 and F_w ≪ 1, but the relevant operating force is F_w = 1/2, where they say the results “won’t be exact.” This is load-bearing, not a minor caveat: at F_w = 1/2 the piston is a hard steric constraint that blocks the particle during contact and shifts its trajectory after collisions, so the unconstrained Green’s function in Eq. (27) can mis-estimate the posterior width and the residual mutual information by O(1). The factor-4–5 improvement in Fig. 5(b) is therefore not established for the work-optimal regime. I ask the authors to compute or bound ΔS_τ under the piston-constrained dynamics for F_w = 1/2 (for instance by numerical simulation of the constrained Langevin dynamics) or to restrict the cyclic-efficiency claim to the parameter regime where approximation (i) is controlled.
- [Sec. IV.A, Eq. (22), and abstract] The claim of Landauer-bound violation is based on the efficiency η_os = W_os/(Pe^{-1} ΔS_os), where Pe = γ_p v_0 d_M/k_B T. Because W_os is expressed in units of the active energy scale γ_p v_0 d_M, this definition makes η_os ∼ Pe whenever the reduced work and information gain are O(1) and independent of Pe. The statement that η_os can be made arbitrarily large by increasing Pe is therefore a consequence of excluding the active dissipation from the cost, rather than a violation of the equilibrium Landauer bound. The authors do mention an alternative efficiency η̃_os that includes the active dissipation, but they do not use it in the main claim. The abstract should be reworded to say that the engine can exceed the equilibrium information-to-work bound for an information efficiency that excludes the active maintenance cost, not that it violates Landauer’s bound in the usual thermodynamic sense.
- [Sec. IV.B, Eq. (28)] The small-τ expansion for ΔS_τ retains only the diffusive term D_p τ/(2σ_x^2) and the binary-state term, while neglecting tumbling contributions at O(τ^2). For the parameter values used in Fig. 5, τ* is indeed small, so the truncation is plausible. However, the derivation also uses the free-dynamics posterior from Eq. (27), which permits the particle to occupy both sides of the origin after a tumble and ignores the piston-induced shift of no-tumble trajectories. Since ΔS_τ is an entropy difference between this free-evolved posterior and the post-measurement Gaussian, the structural difference with the constrained process can change the result by more than the stated O(τ^2) error. The numerical verification in Fig. 5(a) checks only the approximation against the full free-dynamics expression, not against the constrained dynamics, so it does not resolve this concern.
minor comments (4)
- [Eq. (27)] The delta function δ(x_τ + t) contains an undefined variable t; it should presumably be δ(x_τ + τ), consistent with the other terms in that equation.
- [Eq. (A2)] The symbols γ2 and 2γ2 in C1 and C2 are undefined; they should presumably be γ_w and 2γ_w, respectively.
- [Sec. II.A and Eq. (22)] The phrase “the natural unit of efficiency is the active Péclet number Pe” is confusing, since Pe is a dimensionless parameter, not a unit; the sentence could be rephrased to say that the reduced thermal energy scale is Pe^{-1}, which makes η naturally of order Pe in reduced units.
- [Sec. IV.B, Eq. (26)] The notation G_τ^B(x′ − τ′) mixes a physical time τ′ with a Green’s-function duration τ′ in a way that is hard to follow; using separate symbols for the tumble time and the propagation time would improve readability.
Circularity Check
No significant circularity: the work and efficiency calculations are self-contained, and the flagged items are an acknowledged approximation and a transparent normalization convention, not reductions of the derivation to its inputs.
full rationale
The derivation chain is self-contained. The one-shot work W_os is computed from the model probabilities (Eqs. (7)-(17)) with no parameter fitted to the claimed predictions; the optima in Figs. 2-3 follow from the analytic expression in Appendix A. The information cost Delta S_os is computed directly from the Gaussian measurement model (Eqs. (19)-(20)), and the cyclic cost Delta S_tau is derived from a stated free run-and-tumble Green's function approximation (Eqs. (23)-(28)), with the approximation and its F_w = 1/2 caveat explicitly acknowledged. The linear-in-Pe behavior of eta_os is a transparent consequence of the normalization in Eq. (22), stated as such by the authors ('as suggested by Eq. (22)'), and the Landauer comparison uses the independently computed W_os > 0; this is a modeling convention, not a reduction of the result to its inputs. Self-citations (Refs. [13,14,16]) support context and prior results, but the central work and efficiency calculations are re-derived in this paper. The free-dynamics approximation for Delta S_tau is a robustness limitation, not circularity.
Assumptions & free parameters
free parameters (6)
- Position measurement error sigma_x =
varied; e.g. sigma_x/d_M = 0.01 in Fig. 2, 10^-2 in Fig. 5
- Direction measurement error epsilon =
e.g. 0.1 in Fig. 2 and Fig. 5
- Piston force F_w =
1/2 in units of gamma_p v_0
- Piston friction gamma_w =
1 in numerical figures
- Peclet number Pe =
large; eta proportional to Pe
- Domain length L =
unspecified, with sigma_x much less than L
assumptions (8)
- domain assumption Run-and-tumble dynamics with dichotomous noise and Gaussian translational noise, Eqs. (1)-(2)
- domain assumption High Peclet number, Pe >> 1, so translational diffusion is neglected in the work calculation
- domain assumption At most one tumbling event per protocol duration tau
- domain assumption Free run-and-tumble dynamics between successive measurements, ignoring the piston interaction and boundaries
- domain assumption Uniform pre-measurement distribution P(x,w) = 1/(2L)
- domain assumption Gaussian measurement posterior, Eq. (4)
- domain assumption Landauer cost of measurement k_B T_m Delta S
- domain assumption For short protocols, theta-function terms in Eq. (27) are negligible
Cite this review
Pith. "Pith review of Mechanical work extraction from an error-prone active dynamic Szilard engine." pith.science (2026). https://pith.science/paper/YCOCFGL7
@misc{pith2026250509523,
author = {Pith},
title = {Pith review of: Mechanical work extraction from an error-prone active dynamic Szilard engine},
year = {2026},
howpublished = {\url{https://pith.science/paper/YCOCFGL7}},
note = {Machine review of arXiv:2505.09523}
}
read the original abstract
Isothermal information engines operate by extracting net work from a single heat bath through measurement and feedback control. In this work, we analyze a realistic active Szilard engine operating on a single active particle by means of steric interaction with an externally controlled mechanical element. In particular, we provide a comprehensive study of how finite measurement accuracy affects the engine's work and power output, as well as the cost of operation. Having established the existence of non-trivial optima for work and power output, we study the dependence of their loci on the measurement error parameters and identify conditions for their positivity under one-shot and cyclic engine operation. By computing a suitably defined information efficiency, we also demonstrate that this engine design allows for the violation of Landauer's bound on the efficiency of information-to-work conversion. Notably, the information efficiency for one-shot operation exhibits a discontinuous transition and a non-monotonic dependence on the measurement precision. Finally, we show that cyclic operation improves information efficiency by harvesting residual mutual information between successive measurements.
Figures
Figures from the paper (3 more)
Reference graph
Works this paper leans on
-
[1]
C. G. Knott, Quote from undated letter from Maxwell to Tait, Life and Scientific Work of Peter Guthrie Tait. Cambridge University Press , 215 (1911)
work page 1911
-
[2]
L. Szilard, On the decrease of entropy in a thermody- namic system by the intervention of intelligent beings, Behav. Sci9, 301 (1964)
work page 1964
-
[3]
K. J. Ray and J. P. Crutchfield, Variations on a demonic theme: Szilard’s other engines, Chaos: An Interdisciplinary Journal of Nonlinear Science30, 10.1063/5.0012052 (2020)
-
[4]
A. B. Boyd and J. P. Crutchfield, Maxwell demon dy- namics: Deterministic chaos, the szilard map, and the intelligence of thermodynamic systems, Physical review letters116, 190601 (2016)
work page 2016
-
[5]
Landauer, Irreversibility and heat generation in the computing process, IBM J
R. Landauer, Irreversibility and heat generation in the computing process, IBM J. Res. Dev.5, 183 (1961)
1961
-
[6]
C. H. Bennett, Notes on landauer’s principle, reversible computation, and maxwell’s demon, Stud. Hist. Philos. Sci. B34, 501 (2003)
work page 2003
-
[7]
Strasberg, G
P. Strasberg, G. Schaller, T. Brandes, and M. Espos- ito, Thermodynamics of a physical model implementing a maxwell demon, Phys. Rev. Lett.110, 040601 (2013)
2013
-
[8]
N. Shiraishi, S. Ito, K. Kawaguchi, and T. Sagawa, Role of measurement-feedback separation in autonomous maxwell’s demons, New J. Phys.17, 045012 (2015). 13
work page 2015
Show all 39 references
-
[9]
Mandal and C
D. Mandal and C. Jarzynski, Work and information pro- cessing in a solvable model of maxwell’s demon, Proc. Natl. Acad. Sci.109, 11641 (2012)
2012
-
[10]
J. M. Horowitz, T. Sagawa, and J. M. Parrondo, Imitat- ing chemical motors with optimal information motors, Phys. Rev. Lett.111, 010602 (2013)
2013
-
[11]
Maruyama, F
K. Maruyama, F. Nori, and V. Vedral, Colloquium: The physics of maxwell’s demon and information, Rev. Mod. Phys.81, 1 (2009)
2009
-
[12]
J. M. R. Parrondo, J. M. Horowitz, and T. Sagawa, Ther- modynamics of information, Nat. Phys.11, 131 (2015)
2015
-
[13]
Malgaretti, P
P. Malgaretti, P. Nowakowski, and H. Stark, Mechani- cal pressure and work cycle of confined active brownian particles, Europhysics Letters134, 20002 (2021)
2021
-
[14]
Malgaretti and H
P. Malgaretti and H. Stark, Szilard engines and information-based work extraction for active systems, Physical review letters129, 228005 (2022)
2022
-
[15]
T. K. Saha, J. Ehrich, M. c. v. Gavrilov, S. Still, D. A. Sivak, and J. Bechhoefer, Information engine in a nonequilibrium bath, Phys. Rev. Lett.131, 057101 (2023)
2023
-
[16]
Cocconi and L
L. Cocconi and L. Chen, Efficiency of an autonomous, dynamic information engine operating on a single active particle, Physical Review E110, 014602 (2024)
2024
-
[17]
Paneru, S
G. Paneru, S. Dutta, and H. K. Pak, Colossal power extraction from active cyclic brownian information en- gines, The Journal of Physical Chemistry Letters13, 6912 (2022)
2022
- [18]
-
[19]
L. K. Davis, K. Proesmans, and E. Fodor, Active matter under control: Insights from response theory, Phys. Rev. X14, 011012 (2024)
2024
-
[20]
J. M. Parrondo, J. M. Horowitz, and T. Sagawa, Thermo- dynamics of information, Nature physics11, 131 (2015)
2015
-
[21]
Abreu and U
D. Abreu and U. Seifert, Extracting work from a sin- gle heat bath through feedback, Europhysics Letters94, 10001 (2011)
2011
-
[22]
J. M. Horowitz and S. Vaikuntanathan, Nonequilibrium detailed fluctuation theorem for repeated discrete feed- back, Physical Review E—Statistical, Nonlinear, and Soft Matter Physics82, 061120 (2010)
2010
-
[23]
Proesmans, J
K. Proesmans, J. Ehrich, and J. Bechhoefer, Finite-time landauer principle, Phys. Rev. Lett.125, 100602 (2020)
2020
-
[24]
K. V and T. Joseph, Imprecise maxwell’s demon with feedback delay: An exactly solvable information engine model, Phys. Rev. E111, L042104 (2025)
2025
-
[25]
Kumar and J
A. Kumar and J. Bechhoefer, Nanoscale virtual poten- tials using optical tweezers, Applied Physics Letters113, 10.1063/1.5055580 (2018)
2018 doi
-
[26]
Di Leonardo, L
R. Di Leonardo, L. Angelani, D. Dell’Arciprete, G. Ruocco, V. Iebba, S. Schippa, M. P. Conte, F. Mecarini, F. De Angelis, and E. Di Fabrizio, Bacterial ratchet motors, Proc. Natl. Acad. Sci. U.S.A.107, 9541 (2010)
2010
-
[27]
S. P. Thampi, A. Doostmohammadi, T. N. Shendruk, R. Golestanian, and J. M. Yeomans, Active microma- chines: Microfluidics powered by mesoscale turbulence, Sci. Adv.2, e1501854 (2016)
2016
-
[28]
P¨ onisch and V
W. P¨ onisch and V. Zaburdaev, A pili-driven bacterial turbine, Front. Phys.10, 875687 (2022)
2022
-
[29]
C. J. Bustamante, Y. R. Chemla, S. Liu, and M. D. Wang, Optical tweezers in single-molecule biophysics, Nature Reviews Methods Primers1, 25 (2021)
2021
-
[30]
Bechinger, R
C. Bechinger, R. Di Leonardo, H. L¨ owen, C. Reichhardt, G. Volpe, and G. Volpe, Active particles in complex and crowded environments, Reviews of modern physics88, 045006 (2016)
2016
-
[31]
Garcia-Millan and G
R. Garcia-Millan and G. Pruessner, Run-and-tumble mo- tion in a harmonic potential: field theory and entropy production, J. Stat. Mech. Theory Exp.2021, 063203 (2021)
2021
-
[32]
Cocconi, J
L. Cocconi, J. Knight, and C. Roberts, Optimal power extraction from active particles with hidden states, Phys. Rev. Lett.131, 188301 (2023)
2023
-
[33]
Chatzittofi, J
M. Chatzittofi, J. Agudo-Canalejo, and R. Golesta- nian, Entropy production and thermodynamic inference for stochastic microswimmers, (2023), arXiv:2310.15311 [cond-mat.stat-mech]
2023 arXiv
-
[34]
Speck, Stochastic thermodynamics for active matter, Europhys
T. Speck, Stochastic thermodynamics for active matter, Europhys. Lett.114, 30006 (2016)
2016
-
[35]
J. H. Fritz and U. Seifert, Thermodynamically consistent model of an active ornstein–uhlenbeck particle, J. Stat. Mech. Theory Exp. , 093204 (2023)
2023
-
[36]
F. J. Cao and M. Feito, Thermodynamics of feedback controlled systems, Phys. Rev. E79, 041118 (2009)
2009
-
[37]
Zhang and G
Z. Zhang and G. Pruessner, Field theory of free run and tumble particles in d dimensions, Journal of Physics A: Mathematical and Theoretical55, 045204 (2022)
2022
-
[38]
Bothe and G
M. Bothe and G. Pruessner, Doi-peliti field theory of free active ornstein-uhlenbeck particles, Physical Review E 103, 062105 (2021)
2021
-
[39]
B¨ auerle, R
T. B¨ auerle, R. C. L¨ offler, and C. Bechinger, Formation of stable and responsive collective states in suspensions of active colloids, Nature Communications11, 2547 (2020)
2020
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