In single-particle tracking experiments measuring anomalous diffusion dynamics, understanding ergodicity is crucial, as it ensures that the time average of an observable matches the ensemble average—and can thus be fitted with known ensemble-averaged observables. A commonly used criterion for assessing the ergodicity of a stochastic process is based on the comparison of the mean-squared displacement (MSD) with the time-averaged MSD (TAMSD). This approach has been widely applied and proves effective in cases of weak ergodicity breaking across various systems in both theoretical and experimental studies. However, there is relatively little discussion regarding the theoretical justification and limitations of this definition. Here, we demonstrate that this widely accepted criterion to some extent contradicts the classical definition of ergodicity as well as physical intuition, leading to spurious (non-)ergodicity results when applied to several well-known stochastic models. To address this limitation, we propose using the mean-squared increment (MSI) instead of the MSD for comparison of ensemble- and time-averaged observables. For processes with stationary increments, the results for MSI and MSD coincide. However, if the increments of the process reach stationarity at long times only, they differ, and the MSI-TAMSD comparison still provides the test for ergodicity. Several well-established examples demonstrate, through both trajectory simulations and theoretical analysis, that our MSI-TAMSD criterion provides a more accurate characterization of the genuine (non-)ergodicity of systems where the MSD-TAMSD method fails. Additionally, for systems exhibiting “ultraweak” ergodicity breaking, the MSI can reveal the asymptotic stationarity and ergodic nature of the process’s increments. Our findings emphasize the important role of the MSI observable for SPT experiments and anomalous diffusion studies.

Pyrochlore magnets are a paradigmatic example of three-dimensional frustrated systems and provide an excellent platform for studying a variety of exotic many-body phenomena, including spin liquids, nematic phases, fragmentation, and order by disorder. In recent years, increasing attention has been devoted to bilinear spin models on this lattice, where multiple magnetic phases can be degenerate in energy, often stabilizing unconventional magnetic states. In this work, we focus on one such model, parametrized by the interaction coupling $J_{z\pm }$, which defines a line in parameter space corresponding to the phase boundary between three distinct magnetic phases. Using a combination of analytical and numerical methods, we show that this model exhibits an order-by-disorder mechanism at low temperatures, giving rise to a mixed magnetic phase. This realizes a $\mathbf{q}=0$ long-range-ordered phase in a pyrochlore magnet characterized by two distinct order parameters, which we denote as the $A_2\oplus {\psi }_2$ phase. Furthermore, at $J_{z\pm }=1/\sqrt{2}$, the model acquires a subextensive number of discrete symmetries, which preclude the stabilization of conventional long-range order and instead lead to the emergence of a novel nematic phase. We characterize this nematic phase, describe how its ground-state configurations are constructed, and analyze its stability at higher temperatures and under small deviations from $J_{z\pm }=1/\sqrt{2}$.

Materials hosting flat bands at the vicinity of the Fermi level promote exotic symmetry broken states. Common to many of these are Van Hove singularities at saddle points of the dispersion or even higher-order Van Hove singularities (HOVHSs) where the dispersion is flattened further. The band structure of kagome metals hosts both a flat band and two regular saddle points flanking a Dirac node. We investigate the kagome ferromagnetic metal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ using scanning tunneling spectroscopy. We identify a new mechanism by which a triangular distortion on its kagome ${\mathrm{Co}}_3\mathrm{Sn}$ surface termination considerably flattens the saddle point dispersion and induces an isolated HOVHS with algebraically divergent density of states pinned to the Fermi energy. The distortion-induced HOVHS precipitates a Pomeranchuk instability of the Fermi surface, resulting in the formation of a series of nematic electronic states. We visualize the nematic order across an energy shell of about 100 meV in both real, reciprocal, and momentum spaces, as a cascade of wave function distributions which spontaneously break the remaining rotational symmetry of the underlying distorted kagome lattice, without generating any additional translational symmetry breaking. It signifies the spontaneous removal of a subset of saddle points from the Fermi energy to lower energies. By tracking the electronic wave function structure across the deformed Fermi surface, we further identify a charge pumpinglike evolution of the wave function center of mass. The mechanism we find for the generation of higher-order saddle points under a kagome distortion may be common to other kagome materials, and potentially other lattice structures, suggesting a generic new avenue for inducing unconventional electronic instabilities toward exotic states of matter.

Magnons, the quanta of spin waves, have significant potential for use in modern technologies, especially when they are strongly coupled to another mode for readout and control. Although magnons strongly interact with microwave photons via the magnetic-dipole interaction to form hybrid cavity-magnon polariton modes, the magnon-phonon interaction in micrometer-sized ferromagnetic spheres is typically weak, which is further aggravated by the large polariton linewidths dominated by the magnon dissipation. The material-limited magnon dissipation rate in particular has been regarded as an unavoidable limitation in these systems. Here, we surpass this long-standing limitation by implementing an active microwave feedback loop to suppress the linewidth of cavity-magnon polaritons and reduce their effective decay rate below the magnon-limited linewidth, thereby enhancing the polariton-phonon cooperativity from $C\simeq 1$ to $C\simeq 300$. As a key milestone, we achieved normal-mode splitting between a cavity-magnon polariton and a mechanical mode, providing direct evidence of three-mode hybridization among photons, magnons, and phonons. Our results establish feedback as a general route to accessing strong-coupling regimes in systems previously thought to be limited by material properties and hence open new opportunities for coherent control in hybrid quantum systems.

The dynamics of flexible filaments in flow underpin a broad range of biological and soft-matter phenomena, yet their response to time-dependent forcing remains poorly understood. Here, we investigate the long-time dynamics of Brownian inextensible elastic filaments subjected to strong oscillatory shear by combining microfluidic experiments on actin filaments with numerical simulations based on a fluctuating Euler-Bernoulli elastica model in a viscous fluid. As the oscillation period increases, the interplay of flow-induced buckling and thermal fluctuations drives a transition away from reversible rigid-body dynamics. Remarkably, the time-glide symmetry of oscillatory shear gives rise to two coexisting symmetry-related dynamical attractors, between which stochastic fluctuations induce intermittent switching events. These attractors correspond to cyclic states in which filaments become nearly straight and flow aligned at full periods while adopting buckled conformations at half periods. Individual filaments spontaneously select one attractor but intermittently hop to the other, producing alternating episodes of ordered and chaotic dynamics. Our results establish periodically driven Brownian flexible filaments as an experimentally accessible realization of stochastic symmetry breaking, attractor hopping, and intermittency in a minimal nonequilibrium system, and present novel opportunities for the design and control of soft materials under time-dependent flows.

We demonstrate the emergence of altermagnetism at the surfaces of antiferromagnets, vastly expanding the number of material candidates with altermagnetic characteristics and establishing a route towards two-dimensional altermagnetism through surface-induced symmetry breaking. We do so by developing a surface spin group formalism that fully classifies all surface magnetic states and identifies altermagnetic surface spin groups that can arise at the surfaces of antiferromagnets. We use this formalism to identify over 150 antiferromagnetic entries from the MAGNDATA database with at least one altermagnetic surface, often times with multiple such surfaces in the same material, and clarify the role of surface roughness and terraces. We illustrate this emergent phenomenon in a realistic Lieb lattice-based minimal model and present ab initio calculations on two representative material candidates, $\text{NaMnP}$ and ${\text{FeGe}}_2$, exhibiting $d$-wave and $g$-wave surface altermagnetism, respectively. Our theory naturally resolves the contradiction of recent experimental reports of $d$-wave spin splitting from ARPES measurements on metallic Lieb lattice compounds such as ${\mathrm{KV}}_2{\mathrm{Se}}_2\mathrm{O}$ that have been shown to be antiferromagnetic in the bulk. Hence, we establish a new paradigm for generating effectively two-dimensional altermagnetism by functionalizing the abundant material class of collinear antiferromagnets as viable platforms for controlled surface altermagnetism.

As an instance of geometrical frustration with nonmagnetic degrees of freedom, we explore the physics of local spin $S\ge 1$ moments on the pyrochlore lattice that interact via pure quadrupolar couplings. In the presence of spin-orbit coupling, there are nine allowed couplings between nearest-neighbor quadrupoles. We determine the semiclassical phases and survey the phase diagram of the model. One may view the Hamiltonian as being composed of two copies of the well-studied dipolar model with couplings between the copies, and we find that each easy-plane dipolar phase has two quadrupolar counterparts. As geometrical frustration is important over broad swathes of the parameter space, there are many classical quadrupolar liquids and regions with order-by-disorder selection of discrete states. Order by disorder with quadrupoles admits cubic terms in the Landau theory whose effects appear in Monte Carlo simulations and flavor-wave calculations for fixed spin $S$. We showcase a number of examples of quadrupolar spin liquids, including one realizing a rank-3 symmetric tensor gauge theory exhibiting sixfold pinch point singularities. We discuss remarkable differences between the quadrupolar physics of different spin quantum number. We also discuss connections to the non-Kramers rare earth pyrochlore materials.

In order to understand the unexpected similarity and the correlated behavior in kagome superconductor families $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K},\mathrm{Rb},\mathrm{Cs}$) and $A{\mathrm{Ti}}_3{\mathrm{Bi}}_5$ ($A=\mathrm{Rb},\mathrm{Cs}$), we investigate the Hartree-scale local electronic structure of these systems. Our result indicates that V and Ti ions are both of $2+$ valence such that the corresponding itinerant carrier densities are similar, and the difference in electron count is instead reflected in their quantum fluctuating ionic magnetic moments. However, due to the frustrated lattice geometry of these materials, such local moments are difficult to experimentally observe via standard probes. For verification, we systematically introduce nonmagnetic Sn impurities to locally relieve the geometric frustration and experimentally demonstrate the existence of well-defined local magnetic moments via magnetic susceptibility and muon spin rotation or relaxation ($\mu \mathrm{SR}$) measurements. All experiments discover a systematic increase of magnetic susceptibility upon increasing nonmagnetic impurity level. Our discovered ionic moments suggest a paradigm shift from the existing itinerant carrier-only picture to one incorporating strong correlation from local ionic spins. The associated interatomic and local-itinerant correlations offer a solid ground for the emergence of the observed rich correlated behavior in this new family of superconducting materials.

A hallmark of classical ergodicity is the complete loss of memory of the initial conditions due to eventual uniform covering of a priori available phase space. In quantum counterparts of such systems, however, this classical ergodic ideal is fundamentally limited: Here, we introduce the concept of a quantum birthmark, a permanent signature left by the initial state and its early-time evolution in a general quantum system, which gives rise to nonergodic behavior persisting even in the infinite-time limit. We present a birthmark framework outlining a ubiquitous memory effect for an arbitrary, nonstationary state composed of two factors conspiring together: the universal and the revival enhancement. The former sets the minimal amplification carried by the time evolution of a quantum state based on global symmetries, whereas the latter incorporates the further enhancement stemming from the early dynamics, particularly prominent in the presence of recurrences that occur before the Heisenberg time. As a concrete example, we identify quantum birthmarks in the venerable stadium billiard, where they can be significantly enhanced by quantum scars. Finally, we discuss the broader implications of quantum birthmarks, including their role as a natural extension of all types of scarring theories to generic nonstationary quantum systems and prospects for experimental observation. Generally, our work opens an unexplored avenue for understanding the elusive quantum nature of ergodicity.

Extended Wigner’s friend scenarios (EWFSs) model agents themselves as unitarily evolving quantum systems, going beyond the standard use of quantum theory where agents are treated classically. Several no-go results suggest deep challenges in this setting: Frauchiger and Renner argued that quantum agents reasoning using quantum theory would arrive at logical paradoxes, while the local-friendliness theorem raises difficulties for maintaining objective notions of measurement events and causal reasoning in EWFSs. This prompts a key question: Can the ability to make and test scientific predictions and reason consistently still be maintained if quantum theory applies universally, even to agents and their memories? We answer this positively by developing a general quantum circuit framework for EWFSs without assuming absolute measurement outcomes. We formalize Heisenberg cuts in terms of distinct choices of channels in the circuit, labeled by “settings,” and show that Frauchiger-Renner-type paradoxes are fully resolved by making explicit the conditioning on these channels or settings. We provide concrete reasoning rules for quantum agents that ensure logical, probabilistic, and causal consistency. Our framework captures all perspectives in an EWFS within a single causal structure while allowing events to be fundamentally subjective, and we show how objective events and predictions nevertheless emerge in current real-world quantum experiments. Thus, we demonstrate a relational yet operational framework that overcomes key challenges in EWFSs without altering the Born rule, unitarity, or classical logic and probability for outcomes. This establishes a formal platform to analyze and compare different EWFS arguments and extend quantum information methods consistently into the Wigner’s friend domain.

Nonreciprocal interactions—where forces between entities are asymmetric—govern a wide range of nonequilibrium phenomena, yet their role in structural transitions in living and active systems remains elusive. Here, we demonstrate a transition between nonreciprocal states using starfish embryos at different stages of development, where interactions are inherently asymmetric and tunable. Experiments, interaction inference, and topological analysis yield a nonreciprocal state diagram spanning crystalline, flock-like, and fragmented states, revealing that weak nonreciprocity promotes structural order while stronger asymmetry disrupts it. To capture these transitions, we introduce topological landscapes, mapping the distribution of structural motifs across state space. We further develop topological flowscapes, a dynamic framework that quantifies transitions between collective states and detects an informational rate shift and an emergent proofreading from the experimental state transition. Together, these results establish a general approach for decoding nonequilibrium transitions and uncover how asymmetric interactions sculpt the dynamical and structural architecture of active and living matter.

The study of the charmed baryons is crucial for investigating the strong and weak interactions in the standard model and for gaining insights into the internal structure of baryons. In an $e^{+}{e}^{-}$ experiment, the lightest charmed baryon, ${\mathrm{\Lambda }}_c^{+}$, can be produced in pairs through the single-photon annihilation process. This process can be described by two complex electromagnetic form factors. The presence of a nonzero relative phase between these form factors gives rise to a transverse polarization of the charmed baryon and provides additional constraints on the dynamic parameters in the decays. In this article, we present the first observation of the transverse polarization of ${\mathrm{\Lambda }}_c^{+}$ in the reaction $e^{+}{e}^{-}\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}$, based on $6.4{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ annihilation data collected at center-of-mass energies between 4600 MeV and 4951 MeV with the BESIII detector. The decay asymmetry parameters in the decays ${\mathrm{\Lambda }}_c^{+}\to p{K}_S^0$, $\mathrm{\Lambda }{\pi }^{+}$, ${\mathrm{\Sigma }}^0{\pi }^{+}$, and ${\mathrm{\Sigma }}^{+}{\pi }^0$ are simultaneously extracted from the joint angular distributions. By independently extracting the above parameters for the process and its $CP$-conjugate counterpart, we derive both the strong and weak phase differences between different partial waves and test several $CP$ observables. The obtained results fully demonstrate the feasibility of exploiting transverse polarization at electron-positron colliders to search for $CP$-violating effects, opening up a new avenue for exploring $CP$ violation in the charmed baryon sector.

Spin-orbitronics harnesses spin-orbit coupling to generate pure spin currents for energy-efficient information processing, particularly through the spin-orbit torque (SOT) that drives magnetization switching. However, achieving efficient SOT switching of perpendicular magnetization, which is crucial for high-density applications, remains challenging due to the difficulty in generating spin currents with both high conductivity and out-of-plane polarization in conventional, industry-compatible spin sources, such as heavy metals. Here, we overcome this limitation by demonstrating a noncollinear spin-orbit filtering effect at the surface termination of conventional spin sources. This effect selectively transmits electrons based on the relative alignment between their spin vectors and the interfacial Rashba-Edelstein field. By implementing this strategy in platinum (Pt) epitaxial films with ($n10$) orientations ($n=2–4$), we achieve exceptionally strong $z$-polarized spin currents. The resulting out-of-plane spin Hall conductivity reaches a record value of $0.75\times {10}^5(ħ/2e){\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$, surpassing all previous approaches, and enables robust field-free switching of perpendicular magnetization. Notably, the field-free switching is precisely controlled by engineering the crystal point group symmetry from $C_{1\mathrm{v}}$ to $C_{4\mathrm{v}}$. Our work establishes a general framework for transforming conventional high-symmetry materials into out-of-plane spin sources, paving the way for highly efficient and scalable spintronic memory technology.

Variational Monte Carlo (VMC) is a powerful and fast-growing method for optimizing and evolving parametrized many-body wave functions, especially with modern neural-network quantum states. In practice, however, the stochastic estimators that form the backbone of the method can become unstable or biased due to the presence of nodes, a ubiquitous feature of quantum wave functions. In the continuum, this results in heavy-tailed estimators with potentially divergent variances, while in discrete Hilbert spaces the sampling distribution can miss parts of the support needed to form unbiased estimators. These statistical pathologies lead to unreliable optimization trajectories in stochastic reconfiguration or incorrect variational dynamics in time-dependent variational Monte Carlo (t-VMC) and severely limit the power of the numerical simulations. We introduce blurred sampling to address these difficulties. The method has a number of rigorous properties that make it well behaved, effective, and computationally efficient. Additionally, it is a postprocessing approach that can be used without modifying the underlying sampler and incurs only minimal overhead. We demonstrate its effectiveness in a broad range of problems—including ground-state optimization in both continuum and discrete settings as well as in time-dependent variational Monte Carlo, where it resolves previously identified failure modes—and further apply it to large-scale problems in spin dynamics. This work establishes a broadly applicable framework for robust VMC and t-VMC calculations.

By combining the assumptions of Bell locality with those of generalized noncontextuality, we define classes of noncontextuality inequalities for correlations arising in a bipartite Bell circuit. These classes are distinguished by which subsets of the full set of operational identities are taken as input to the principle of noncontextuality; certain natural subsets form a hierarchy that provides a way of understanding and classifying different forms of quantum correlations, including entanglement, steering, and nonlocality. Each level of this hierarchy gives rise to a corresponding class of noncontextuality inequalities whose violation witnesses one of these forms of bipartite quantum resourcefulness, thereby yielding different sufficient conditions for entanglement. The resulting entanglement certification paradigm requires no prior characterization of the measurements, delivers a verdict independent of tomographic gauge freedom, and can certify any entangled state without auxiliary entangled sources. To illustrate the power of this paradigm, we show that noncontextuality inequalities can certify entanglement for families of two-qubit isotropic states for which certification by Bell or steering inequalities is known to be impossible. We also show that, compared with the Bell test, this approach certifies a much larger fraction of entangled states, while the associated membership problem is computationally more tractable. On the experimental side, we describe techniques to ensure nontrivial operational identities in the presence of noisy and imperfect implementations. We also identify the sufficient condition under which these techniques are valid, namely, a particular notion of tomographic completeness of the implemented Bell circuit, which ensures that the operational identities are independent of tomographic gauge. Finally, we provide an experimental demonstration of the superior performance of this entanglement certification technique using polarization-entangled photons.

Quantum symmetrization is the task of transforming a nonstrictly increasing list of $n$ integers into an equal superposition of all permutations of the list (or more generally, performing this operation coherently on a superposition of such lists). This task plays a key role in initial state preparation for first-quantized simulations. Motivated by an application to fermionic systems, various algorithms have been proposed to solve a weaker version of symmetrization in which the input list is strictly increasing, but the general symmetrization problem with repetitions in the input list has not been well studied. We present the first efficient quantum algorithms for the general symmetrization problem. If $m$ is the greatest possible value of the input list, our first algorithm symmetrizes any single classical input list using $\tilde{O}(\log n)$ depth and $O(n\log n+\log m)$ ancilla qubits, and our second algorithm symmetrizes an arbitrary superposition of input lists using $\tilde{O}({\log }^{3}n)$ depth and $O(n\log n)$ ancilla qubits. Our algorithms enable efficient simulation of bosonic quantum systems in first quantization (in particular, we describe a simulation of the Bose-Hubbard model with complexity $\mathrm{poly}(n,\log m)$) and can prepare (superpositions of) Dicke states of any Hamming weight in $\tilde{O}(\log n)$ depth (respectively, $\tilde{O}({\log }^{3}n)$ depth) using $O(n\log n)$ ancilla qubits. We also propose an $\tilde{O}({\log }^{3}n)$-depth quantum algorithm to transform second-quantized states to first-quantized states. Using this algorithm, QFT-based quantum telescope arrays can image brighter photon sources, extending quantum interferometric imaging systems to a new regime.

In the past decade, photoinduced superconductinglike behaviors have been reported in a number of materials driven by intense pump fields. Of particular interest is the high-$T_c$ cuprate ${\mathrm{YBa}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6+\mathrm{x}}$ (YBCO), where such effect has been reported up to the so-called pseudogap temperature $T^{*}\sim 300–400\mathrm{K}$. In a recent tour-de-force experiment, a transient magnetic field which is proportional to and in the same direction of an applied field has been observed outside the sample, suggestive of flux exclusion due to the Meissner effect. In this paper, we present an alternative interpretation of these experiments based on a mechanism that we term the flux-Floquet instability of the sine-Gordon (SG) model. We take as our premise the model of preformed Cooper pairs in the pseudogap phase. Starting from the local superconducting order parameter in equilibrium, we introduce an extended SG model to describe the dynamics of the relative phase between the layers of a bilayer. We demonstrate that a combination of external magnetic field and strong terahertz drive used in experiments by Fava et al. results in a novel Floquet instability in the model. This instability leads to currents at the edges of the bilayer formed by defects or grain boundaries, with the current flowing in the opposite direction of the equilibrium screening current, producing a giant paramagnetic magnetization in the same direction as the applied field. We show how this scenario can fit most of the available data. To the extent that this model can account for the data, we conclude that the experiments have the important consequence of revealing the presence of local pairing in the pseudogap phase. While the bulk of this paper addresses the experiment on YBCO, our results reveal a new instability in the SG equation that is of fundamental interest, with potential applications such as providing a mechanism for generating large magnetic fields at ultrafast timescales in Josephson devices.

Excitons, the correlated electron-hole pairs governing optical and transport properties in organic semiconductors, have long resisted direct experimental access to their full quantum-mechanical wave functions. Here, we use femtosecond time-resolved photoemission orbital tomography (trPOT) combining high-harmonic probe pulses with time- and momentum-resolved photoelectron spectroscopy to directly image the momentum-space distribution and ultrafast dynamics of excitons in $\alpha$-sexithiophene thin films. We introduce a model that enables reconstruction of the exciton wave function in real space, including both its spatial extent and its internal phase structure. The reconstructed wave function reveals coherent delocalization across approximately three molecular units and exhibits a characteristic phase modulation, consistent with ab initio calculations within the framework of many-body perturbation theory. Time-resolved measurements further indicate an approximately 25% contraction of the exciton radius within 400 fs, suggesting self-trapping driven by exciton-phonon coupling. These results establish trPOT as a general and experimentally accessible approach for resolving exciton wave functions—with spatial, phase, and temporal sensitivity—in a broad class of molecular and low-dimensional materials.

Preparation of quantum thermal states of many-body systems is a key computational challenge for quantum processors, with applications in physics, chemistry, and classical optimization. We provide a simple and efficient algorithm for thermal state preparation, combining engineered bath resetting and modulated system-bath coupling to derive a quantum channel approximately satisfying quantum detailed balance relations. We show that the fixed point $\widehat{\sigma }$ of the channel approximates the Gibbs state as $\parallel \widehat{\sigma }-{\widehat{\sigma }}_{\beta }\parallel \sim {\theta }^2$, where $\theta$ is the system-bath coupling and ${\widehat{\sigma }}_{\beta }\propto e^{-\beta {\widehat{H}}_S}$. We provide extensive numerics, for the example of the 2D quantum Ising model, confirming that the protocol successfully prepares the thermal state throughout the finite-temperature phase diagram, including near the quantum phase transition. Simulations for free-fermion systems provide further evidence for the accuracy of the protocol for large system sizes in the weak-coupling limit. Our algorithm provides a path to efficient quantum simulation of quantum-correlated states at finite temperature with current and near-term quantum processors.

Quantifying the irreversibility and dissipation of nonequilibrium processes is crucial to understanding their behavior, assessing their possible capabilities, and characterizing their efficiency. We introduce a physical quantity that quantifies the irreversibility of stochastic Langevin systems from the observation of individual molecules’ displacements. Categorizing these displacements into a few groups based on their initial and final positions allows us to measure irreversibility precisely without the need to know the forces and magnitude of the fluctuations acting on the system. For short times, our model-free estimate of irreversibility is related to entropy production by a conditional fluctuation theorem. For short times and, in general, for stationary protocols, our estimate provides a lower bound to the average entropy production. We validate the method on single-molecule force spectroscopy experiments of proteins subject to force ramps. We show that irreversibility is sensitive to detailed features of the energy landscape underlying the protein folding dynamics and suggest how our methods can be employed to unveil key properties of protein folding processes.