Elucidating the emergence of irreversible macroscopic laws from reversible quantum many-body dynamics remains challenging, particularly in disordered media. Here, in a doubly disordered electron-nuclear spin network in nitrogen-doped diamond, we uncover an emergent decoherence law for ${}^{13}\mathrm{C}$ polarization, $M(t)=e^{-\sqrt{R_{p}t}}{e}^{-R_{d}t}$, that persists over $>500\mathrm{s}$ and across broad Hamiltonian regimes. We trace the law to two interdependent, electron-mediated channels: direct depolarization by a fluctuating electron bath and an indirect pathway governed by polarization transport within the dilute nuclear network, which is anomalous and subdiffusive. Combining Floquet engineering with all-optical modulation of the electron environment, we demonstrate deterministic control of the channels and the ability to selectively eliminate either one. Disorder, typically viewed as detrimental, here proves protective by generating rare electron-free clusters that trap polarization and dominate late-time decay.

As the largest gravitationally bound objects in the Universe, galaxy clusters have provided the first piece of evidence for the presence of dark matter and may be suitable targets for indirect dark matter searches. Among various signals, the GeV-TeV $\gamma$-ray line has been taken as the smoking-gun signal of the dark matter annihilation and decay, since no known astrophysical or physical process(es) could generate such a peculiar spectrum. With 15.5 years of Fermi-LAT P8R3 publicly available data, we search for the $\gamma$-ray line emission in the directions of 13 massive galaxy clusters at redshifts $z\le 0.028$ with an unbinned likelihood analysis. A $\gamma$-ray line signal at $\sim 43.2\mathrm{GeV}$ has a net test statistic (TS) value of $\approx 30$ if we take into account only the data in the directions of the Virgo, Fornax, and Ophiuchus clusters, three massive clusters with the highest J-factors expected to generate the dark matter annihilation signal. The signal still presents when the data of 10 other nearby massive clusters are also included, though the TS value decreases to $\approx 21$, likely because of their lower signal-to-noise ratios. The absence of this signal in the inner Galaxy disfavors both the instrumental effect and the canonical dark matter annihilation interpretation, and a more sophisticated dark matter model or very peculiar astrophysical scenario might be needed. This $\gamma$-ray line signal, if intrinsic, could be unambiguously verified by the Very Large Area $\gamma$-ray Space Telescope in its first two years of performance.

We show that the tree-level spectrum of heavy particles can be directly extracted from the Wilson coefficients of the corresponding effective field theory at low energies. This procedure is exact when the number of resonances is finite and otherwise approximate. Our results are derived from a new class of analytic dispersion relations that depend nonlinearly on the scattering amplitude and apply to an exceedingly broad class of theories and kinematics.

We apply a tag-and-probe method to precisely measure the absolute branching fraction of the decay ${\eta }_c\to \gamma \gamma$ with the BESIII experiment at BEPCII. Starting with a large initial sample of $(2712.4\pm 14.3)\times {10}^6\psi (3686)$ events, a sample of $0.16\times {10}^6{\eta }_c$ events is tagged via the golden channel $\psi (3686)\to {\pi }^0{h}_c$, $h_c\to \gamma {\eta }_c$, effectively avoiding interference effects. The absolute branching fraction of ${\eta }_c\to \gamma \gamma$ is measured for the first time to be $\mathcal{B}({\eta }_c\to \gamma \gamma )=(2.45\pm 0.48_{\mathrm{stat}}\pm 0.09_{\mathrm{syst}})\times {10}^{-4}$. Using the world average value of the total width of the ${\eta }_c$, the partial decay width of ${\eta }_c\to \gamma \gamma$ is calculated to be $\mathrm{\Gamma }({\eta }_c\to \gamma \gamma )=(7.48\pm 1.48_{\mathrm{stat}}\pm 0.30_{\mathrm{syst}})\mathrm{keV}$.

We report the first searches for charged-lepton-flavor violation in decays of ${\chi }_{bJ}(1P)$ ($J=0$, 1, and 2) to a pair of charged leptons using 158 million $\mathrm{\Upsilon }(2S)$ decays collected with the Belle detector in $e^{+}{e}^{-}$ collisions at the KEKB collider. No significant signal is observed, and we set upper limits on the branching fractions for ${\chi }_{bJ}(1P)$ decays to $e^{\pm }{\mu }^{\mp }$ at the level of ${10}^{-6}$ and to $e^{\pm }{\tau }^{\mp }$ or ${\mu }^{\pm }{\tau }^{\mp }$ at the level of ${10}^{-5}$. Limits on ${\chi }_{b0}(1P)$ decays are translated into bounds on the corresponding Wilson coefficients of scalar operators that mediate charged-lepton-flavor violation.

Decay of a four-level diamond scheme via a cascade is a potential source of entangled photon pairs. A solid-state implementation is the biexciton cascade in a semiconductor quantum dot. While high entanglement fidelities have been demonstrated, the two photons, XX and X, are temporally correlated, typically resulting in poor photon coherence. Here, we demonstrate a high two-photon interference visibility (a measure of the photon coherence) for both XX ($90\pm 2\%$) and X ($80\pm 6\%$) photons. This is achieved by Purcell enhancing the biexciton transition in a low-noise device. We find that the interference visibility follows the well-known quantum optics result upon tuning the XX:X lifetime ratio over 2 orders of magnitude. In addition, we show that the XX purity, 98%, is limited not by quantum optics but by the phonons in the semiconductor matrix via cavity feeding.

We propose a multiphoton interference protocol that characterizes the pairwise overlaps of the internal modes of single photons more efficiently than pairwise Hong-Ou-Mandel characterization experiments. We experimentally implement this protocol to characterize three photons. We show that our implementation of the characterization protocol outperforms the pairwise Hong-Ou-Mandel characterization, even if the Hong-Ou-Mandel characterization would have been performed in a noiseless, perfect experiment. We demonstrate this via the Fisher information matrix. Surprisingly, this advantage persists at arbitrary nonzero transmission, demonstrating the viability of this protocol for real-world characterization of single photons.

With the achievement of thermonuclear ignition, there is interest to increase the performance of inertial-confinement-fusion (ICF) systems to reach high fusion yields and high-energy gain for energy-production purposes. One approach to achieve this is magnetic-direct-drive (MDD) ICF, where magnetic fields are used to compress ICF targets to thermonuclear conditions. To study the scaling of MDD ICF to high yields, we performed experiments on the 20-MA, 100-ns Z machine to study the invariant behavior of the magneto-Rayleigh-Taylor instability when changing the magnetic-field drive. Similarity-scaled aluminum cylindrical shells with preseeded axial perturbations were imploded at two different maximum currents: 14.7 and 19.2 MA. Consistent with dynamical similarity within experimental uncertainties, the magneto-Rayleigh-Taylor growth history measured via x-ray radiography exhibits agreement at the 16% level or better for late-time growth factors of 10x to 50x. However, the early instability development is not dynamically similar due to Ohmic-heating effects, which are not conserved by the proposed scaling prescriptions. This Letter exemplifies the application of similarity scaling to rigorously investigate the scaling of MDD ICF to high yields.

Symmetry-protected ideal flat bands in one-dimensional Hermitian lattices are populated by compact localized states—a special class of localization with wave functions confined within a small region. In this Letter, we demonstrate that the non-Hermitian skin effect can appear in a flat band. Unlike conventional non-Hermitian skin effects for dispersive bands that are protected by nontrivial point-gap topology, the flat-band skin effect (FBSE) emerges even though the flat band remains a point on the complex-energy plane with trivial spectral topology. The FBSE is intriguingly associated with the nontrivial spectral topology of the dispersive bands enclosing the flat band, so it only emerges within a finite range of non-Hermitian parameters and can counterintuitively disappear at large non-Hermiticity. We experimentally observe the FBSE and its parameter dependence in a non-Hermitian mechanical lattice. Our theoretical analyses further reveal that the gap between the flat and the dispersive bands can close at higher-order exceptional points under both periodic and open boundary conditions. The flat-band wave functions are discontinuous in quantum distance across these exceptional points, a property only found in singular flat bands in Hermitian systems. Our Letter reveals flat-band phenomena unique to non-Hermitian systems and highlights new possibilities in quantum geometry and localization control.

The topological magnetoelectric effect (TME) in three-dimensional topological insulators (TIs), described by $\mathrm{\Delta }P=(e^2/2h)N_{\text{Ch}}^{(2)}\mathrm{\Delta }B$, serves as a condensed-matter realization of the four-dimensional quantum Hall effect (4D QHE). In dual-gate axion insulator devices, the TME-induced polarization yields a current $I_{\mathrm{TME}}\propto (C_{\text{total}}/C_{\mathrm{S}})Q_{4\mathrm{D}-\mathrm{QHE}}$, where the signal is suppressed by the capacitance ratio $C_{\text{total}}/C_{\mathrm{S}}$. Here we propose an active compensation scheme that introduces a tunable negative capacitance $C_{\mathrm{comp}}\approx -C_{\mathrm{gate}}$ into the gate line, effectively canceling the gate dielectric capacitance and driving $C_{\text{total}}/C_{\mathrm{S}}\to 1$. We validate the method using a quantum anomalous Hall (QAH) device, which shares the same surface-state physics as the axion insulator but permits direct charge measurement via a single gate, recovering over 95% of the quantized charge signal from an initially half-attenuated state. This compensation method provides a robust means of resolving minute TME signals, offering a pathway toward direct measurements of the 4D QHE.

We report mean-field critical behavior in ${\mathrm{Eu}}_2{\text{MnSi}}_2{\mathrm{O}}_7$, a melilite-type ferrimagnet with spin-only ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Mn}}^{2+}$ moments and negligible orbital contributions. Magnetization measurements combined with neutron powder diffraction reveal critical exponents close to the mean-field values, indicating that long-range dipolar interactions govern the asymptotic critical behavior in this insulating ferrimagnet. The refined magnetic structure, described by the magnetic space group $P{2}_1{2}_1^{\prime }{2}^{\prime }$, exhibits a tilted ferrimagnetic configuration driven by the Dzyaloshinskii-Moriya interaction, reflecting the noncentrosymmetric nature of the lattice. These results extend the applicability of mean-field theory to complex insulating magnets and establish ${\mathrm{Eu}}_2{\text{MnSi}}_2{\mathrm{O}}_7$ as a platform for exploring ferrimagnetism and long-range interactions. To our knowledge, this is the first insulating ferrimagnet in which dipolar interactions drive mean-field criticality.

Two ultranarrow absorption peaks around 1.5 eV, which are widely believed to originate from a transition from a spin-orbital entangled triplet to a singlet state, in the two-dimensional van der Waals crystal ${\mathrm{NiPS}}_3$, have attracted considerable attention because of their pronounced spin-dependent character. An interesting question is whether ultrahigh magnetic fields modify an interaction-driven hybridization between those two peaks. In this Letter we perform systematic magneto-optical measurements of ${\mathrm{NiPS}}_3$ in pulsed magnetic fields of up to 178 T and observe a pronounced mutual repulsion between the two sharp exciton peaks accompanied by a redistribution of oscillator strength, while the band edge shows no detectable field-induced shift within our experimental resolution. We construct a minimal two-level interaction model and compare it semiquantitatively with the experimental data. Our results reveal a magnetic-field-tunable exciton-exciton coupling as the dominant high-field response of ${\mathrm{NiPS}}_3$, and clarify this material as a new experimental platform for exploring strongly correlated exciton physics in magnetic van der Waals insulators.

In a finite-time continuous phase transition, topological defects emerge as the system undergoes spontaneous symmetry breaking. The Kibble-Zurek mechanism predicts how the defect density scales with the quench rate. During such processes, dissipation also arises as the system fails to adiabatically follow the control protocol near the critical point. Quantifying and minimizing this dissipation is fundamentally relevant to nonequilibrium thermodynamics and practically important for energy-efficient computing and devices. However, experimentally measuring dissipation, and optimizing control protocols to reduce it, remains almost completely unexplored. In addition, it is an open question to what extent dissipation is correlated with the formation of defects. Here, we directly measure the dissipation generated during the voltage-driven Fréedericksz transition of a liquid crystal with a sensitivity equivalent to a $\sim 10$ nanokelvin temperature rise. We observe Kibble-Zurek scaling of dissipation and its breakdown, both in quantitative agreement with existing theoretical works. We further implement a fully automated in situ optimization approach that discovers more optimal driving protocols, reducing dissipation by a factor of 3 relative to a simple linear protocol.

Time-translation symmetry strongly constrains physical dynamics, yet systematic characterization for continuous-variable systems lags behind its discrete-variable counterpart. We close this gap by providing a rigorous classification of Gaussian quantum operations that are covariant under time translations, termed Gaussian covariant operations. We show that several key results known for discrete-variable covariant operations break down in the Gaussian optical setting: discrepancies arise in physical and thermodynamic implementation, in the extensivity of asymmetry, and in catalytic advantages. Our results provide comprehensive mathematical and operational toolkits for Gaussian covariant operations, including a peculiar pair of asymmetry measures that are completely nonextensive. Our findings also reveal surprising consequences of the interplay among symmetry, Gaussianity, and thermodynamic constraints, suggesting that real-world scenarios with multiple constraints have a rich structure not accessible from examining individual constraints separately.

In cavity quantum electrodynamics (QED), photons leaving the cavity can be irreversibly lost or reused as a power source. This dichotomy is reflected in two different thermodynamic bookkeeping methods of the light field, both corresponding to valid thermodynamic frameworks. In this Letter, we formulate a rigorous semiclassical limit of cavity QED and show that the resulting thermodynamic description may qualitatively differ from that of the fully quantized model. We find that violations of the thermodynamic uncertainty relations are recovered in the semiclassical limit only by one of the two thermodynamic frameworks: the one which treats part of the photon flux as a power source. We illustrate our findings in a three-level system coupled to a driven cavity.

We explore the lowest mass limit that can be placed on the halo mass function in cold dark matter (CDM) using 28 strong gravitational lenses. For this purpose, we study an extreme model in which the halo mass function and mass-concentration relation follow CDM, with a sharp cutoff at some mass scale, $m_{\mathrm{low}}$. Lensing provides a unique window into this quantity as it does not depend on the presence of baryons in dark matter halos and allows for the detection of low mass halos at cosmological distances, both in the lens galaxies and along the line of sight. Our model incorporates the effects of tidal stripping of subhalos, leading to the presence of many subhalos below a given model cutoff scale. We place an upper limit on the low-mass cutoff of the halo mass function of $m_{\mathrm{low}}<{10}^{8.3}{\mathit{M}}_{\odot }$ at 10∶1 odds using a prior for the normalization of the subhalo mass function from the semianalytic model galacticus and $m_{\mathrm{low}}<{10}^{8.2}{\mathit{M}}_{\odot }$ at 10∶1 odds using a prior from $N$-body simulations. These limits are comparable to, or stronger than, existing constraints based on Milky Way satellite galaxies. Based on these results, we forecast more than an order of magnitude improvement with a sample of 200 quadruply imaged quasar lenses. This number represents a small subset of the thousands that are anticipated to be discovered by Rubin, Euclid, and Roman. Furthermore, with this larger sample of lenses we expect to directly constrain the normalization of the subhalo mass function, thereby eliminating a major source of uncertainty in our current measurements.

I present a new indirect search for dark matter (DM) using hydrogen-$\alpha$ ($\mathrm{H}\alpha$) recombination emission. DM annihilation or decay products can ionize neutral gas; subsequent recombination cascades generate $\mathrm{H}\alpha$ photons through the $3\to 2$ transition. In quiet gas-rich dwarf galaxies, the $n=2$ population is negligible, so $\mathrm{H}\alpha$ is effectively unabsorbed and traces the DM-energy injection site. Using the nondetection of extended $\mathrm{H}\alpha$ emission in the Leo T dwarf galaxy with Multi Unit Spectroscopic Explorer observations, I derive the first $\mathrm{H}\alpha$-based limits on DM annihilation and decay, reaching leading sensitivity for parts of the eV–GeV mass range. Existing and upcoming telescopes can further extend this reach, establishing $\mathrm{H}\alpha$ imaging as a powerful DM search strategy.

This Letter reports on the first evidence of inclusive Higgs-boson production at high transverse momentum in the $b\overline{b}$ final state, reconstructed in a single large-radius jet. The results are based on proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider at a center-of-mass energies of 13 and 13.6 TeV, corresponding to a total integrated luminosity of $301{\mathrm{fb}}^{-1}$. The study profits from the large background suppression provided by the use of a new transformer-based algorithm for jet identification and the sharper mass and transverse momentum, $p_{\mathrm{T}}$, resolution from a dedicated regression model. The yield relative to the standard model prediction for Higgs bosons produced at $p_{\mathrm{T}}$ larger than 450 GeV is measured to be $1.53\pm 0.2{7)}_{-0.27}^{+0.33}\pm 0.17$, where the first uncertainty is statistical, the second is experimental systematic, and the third accounts for theory systematic, corresponding to an observed (expected) significance of $3.8\sigma (2.5\sigma )$ relative to the background-only hypothesis. Results are also obtained in three Higgs boson $p_{\mathrm{T}}$ intervals and found to be compatible with standard model predictions, where the first uncertainty is statistical, the second is experimental systematic, and the third accounts for theory systematic.

The onset of nuclear effects in light-ion collisions is studied by measuring the ratio of $D^0$ meson production between NeNe and OO collisions at a center-of-mass energy per nucleon pair of 5.36 TeV recorded by the LHCb detector. The $D^0$ meson yields are measured differentially in transverse momentum ($p_{\mathrm{T}}$) for $0.5<p_{\mathrm{T}}<20\mathrm{GeV}$ in the rapidity ($y$) region $2.0<y<4.5$, and are normalized to the total number of recorded inelastic nucleus-nucleus collisions in each data sample. The resulting production ratio shows evidence of variation as a function of $p_{\mathrm{T}}$, which is inconsistent with predictions based on nuclear modification of nucleon structure alone. This measurement is consistent with the onset of quark-gluon plasma production with increasing nucleus size in high-energy nuclear collisions.