We study the dynamics of a zero-temperature overdamped tracer in a bath of Brownian particles. As the bath density is increased, numerical simulations show the tracer to transition from an active dynamics, characterized by boundary accumulation and ratchet currents, to an effective equilibrium regime. To account for this analytically, we eliminate the bath degrees of freedom under the assumption of linear coupling to the tracer and show convergence, in the large density limit, to an equilibrium dynamics at the bath temperature. We then develop a perturbation theory to characterize the tracer’s departure from equilibrium at large but finite bath densities, revealing an intermediate time-reversible yet non-Boltzmann regime, followed by a fully irreversible one. Finally, we show that when the bath particles are connected as a lattice, mimicking a gel or a soft active solid, the cold tracer drives the entire bath out of equilibrium, leading to a long-ranged suppression of bath fluctuations.

We explore the joint measurability of incompatible qubit observables on ensembles of parallel and antiparallel spin-$\frac{1}{2}$ pairs. In parallel configuration, both spins are prepared in the same state, whereas in antiparallel case, each spin is paired with its flipped counterpart. We show that the antiparallel configuration uniquely enables exact simultaneous prediction of three mutually orthogonal spin components—an advantage not achievable with parallel states. Extending beyond three observables, we examine joint measurability for larger sets of spin measurements and further generalize our analysis to state configurations beyond the parallel and antiparallel cases. As we show, our results reveal a deep connection to the “mean King retrodiction task” proposed by Vaidman, Aharonov, and Albert and have implications for a cryptographic protocol introduced by Jeffrey Bub. We further demonstrate how the enhanced compatibility in the antiparallel configuration can facilitate efficient estimation of unknown measurement devices. Finally, we discuss prospects for experimentally realizing the enhanced measurement compatibility in antiparallel configuration by analyzing the effect on finite subensembles of states.

In four dimensions, it has long been established that gravity coupled to matter exhibits ultraviolet divergences at one loop, irrespective of supersymmetry. Notably, the four-matter one-loop amplitudes of half-maximal supergravity coupled to Maxwell multiplets were shown in the 1970s to be divergent. Surprisingly, we demonstrate in this Letter that half-maximal theories can nevertheless become one-loop finite when uplifted to higher dimensions, contrary to naive expectations. Specifically, we study the ultraviolet properties of the four-matter and two-matter two-graviton amplitudes in six-dimensional $\mathcal{N}=(2,0)$ and $\mathcal{N}=(1,1)$ supergravities, coupled to $n_T$ tensor and $n_V$ vector multiplets, respectively. We find that the one-loop amplitudes are finite for $n_T=21$ and $n_V=20$. This finiteness is unexpected, as symmetry-preserving counterterms do exist. Interestingly, both values exactly correspond to low-energy limits of type II string theories compactified on K3, which hints at possible origins to the surprising cancellations.

We discuss dimensional continuation of the massless scalar field theory with the $i{\varphi }^5$ interaction term. It preserves the so-called $\mathcal{P}\mathcal{T}$ symmetry, which acts by $\varphi \to -\varphi$ accompanied by $i\to -i$. Below its upper critical dimension $10/3$, this theory has interacting infrared fixed points. We argue that the fixed point in $d=2$ describes the nonunitary minimal conformal model $M(2,7)$. We identify the operators $\varphi$ and ${\varphi }^2$ with the Virasoro primaries ${\varphi }_{1,2}$ and ${\varphi }_{1,3}$, respectively, and $i{\varphi }^3$ with a quasiprimary operator, which is a Virasoro descendant of ${\varphi }_{1,3}$. Our identifications appear to be consistent with the operator product expansions and with considerations based on integrability. Using constrained Padé extrapolations, we provide estimates of the critical exponents in $d=3$. We also comment on possible lattice descriptions of $M(2,7)$ and discuss RG flows to and from this conformal field theory (CFT). Finally, we conjecture that the minimal models $M(2,2n+1)$ are described by the massless scalar field theories with the $i{\varphi }^{2n-1}$ interaction terms.

We present a new class of evolution equations that govern the high-energy behavior of power-suppressed scattering amplitudes. The equations can be viewed as a renormalization group flow with respect to the relevant effective field theory cutoff. A distinct feature of the method is in the use of a multidimensional cutoff to separate the relevant scales in problems characterized by a complex factorization structure. By adjusting the renormalization group variables to the geometry of the effective theory modes, our method naturally extends to a broad spectrum of physical problems that include massive, massless, small, and wide-angle scattering. We present applications to the benchmark processes of electron-positron forward annihilation and light-quark-mediated Higgs boson production and decays.

The differential radial flow fluctuation $v_0(p_{\mathrm{T}})$ has emerged as a new probe of the quark-gluon plasma. However, its characteristic rise-and-fall pattern with $p_{\mathrm{T}}$, resembling anisotropic flow, remains unexplained. I introduce a momentum rescaling framework that factorizes $v_0(p_{\mathrm{T}})$ into kinematic and dynamical components: $v_0(p_{\mathrm{T}})/v_0=-[d\ln ⟨n(p_{\mathrm{T}})⟩/d\ln p_{\mathrm{T}}+1]\times g(p_{\mathrm{T}})$. The first factor, determined by spectral shape, generates the rise-and-fall pattern as the spectra transition from exponential to power-law behavior. The dynamical component $g(p_{\mathrm{T}})$ isolates $p_{\mathrm{T}}$-dependent dynamics: $<1$ signals suppressed fluctuations, $>1$ indicates enhancement. Analysis of LHC data reveals $g(p_{\mathrm{T}})$ deviates from unity by 20%–40% in central collisions. Predictions for RHIC show that spectral shape alone generates the rise-and-fall baseline pattern with substantial energy dependence. This framework enables tighter medium property constraints by separating kinematic from dynamical effects, with broad applications to anisotropic flow and higher-order radial flow fluctuations.

Nuclear Schiff moments (NSMs) are sensitive probes for physics beyond the standard model of particle physics signaling violations of time-reversal and parity-inversion symmetries in atomic nuclei. In this Letter, we report the first-ever calculation of a NSM in a nuclear ab initio framework, employing the no-core shell model to study the fluorine isotope ${}^{19}\mathrm{F}$. We further perform quantum-chemistry calculations to evaluate the sensitivity of the hafnium monofluoride cation, ${\mathrm{HfF}}^{+}$, to the NSM of ${}^{19}\mathrm{F}$. Combined with recent high-precision measurements of the molecular electric dipole moment of ${\mathrm{HfF}}^{+}$ [Roussy et al., An improved bound on the electron’s electric dipole moment, Science 381, 46 (2023).], our results enable the first experimental bound on the NSM of ${}^{19}\mathrm{F}$. Although the resulting bounds on the pion-nucleon-nucleon ($\pi \mathrm{NN}$) coupling constants are not yet the most stringent, this Letter establishes the foundation for constraining $\pi \mathrm{NN}$ interactions using nuclear ab initio methods.

Axionlike particles and similar new pseudoscalar as well as vector bosons coupled to nucleons and electrons are predicted to lead to spin-dependent forces in atoms and ions. We argue that hyperfine structure measurements in hydrogenlike and lithiumlike charge states are a sensitive probe to this effect. Employing specific differences of these splittings reduces uncertainties due to nuclear effects in hyperfine structure calculations and measurements. Using this, we show that existing measurements on Be provide competitive limits in the region $m_{\varphi }\gtrsim 100\mathrm{keV}$, confirming, or improving by up to a factor of 2, existing constraints for pseudoscalar couplings, depending on the nuclear model. We also find that future measurements on Cs have a further factor of 2-2.5 improved discovery potential for pseudoscalars and an order of magnitude for new vector bosons when compared with the corresponding current constraints.

Despite growing interest, there is a scarcity of known predictions in the regime where both quantum and general relativistic effects become observable. Here, we investigate a combined atom-field system in a curved spacetime, with a specific focus on gravitational-wave backgrounds. We demonstrate that a plane gravitational wave alters spontaneous emission from a single atom, manifesting itself as a direction-dependent change in the emission spectrum. Although the total decay rate remains unchanged, implying that no information about the gravitational wave is stored in the atomic internal state alone, the wave leaves imprints on the evolution of the composite atom-field system. To quantify how well this effect can be measured, we analyze both the classical Fisher information associated with photon number measurements and the quantum Fisher information. Our analysis indicates that the effect could be measured in state-of-the-art cold-atom experiments and points to spontaneous emission as a potential probe of low-frequency gravitational waves.

Superradiant burst often arises from photon-mediated interactions within an excited ensemble of emitters, where collective emission leads to a sharp increase in photon intensity. Here, we focus on a nuclear ensemble with cooperative excitations and propose, for the first time, the generation of superradiant burst of x-ray photons. We establish the required excitation conditions to observe x-ray superradiant bursts, offering guidance for the experiments utilizing state-of-the-art x-ray sources. Building on this, we investigate the dynamics of the x-ray superradiant burst and reveal novel nonlinear relationships between the time required to reach the peak intensity and the excitation number, as well as between the peak intensity and the excitation number. Additionally, we introduce a dimensionless parameter that serves as an indicator of the system, reflecting how strongly the burst can occur in a given configuration. Our work provides fresh insights into the understanding of collective emission in many-body x-ray-nuclei systems within the nonlinear regime, opening new avenues for x-ray quantum optics.

Common wisdom says that the entanglement of fermionic systems can be low in the second quantization formalism but is extremely large in the first quantization. Hence matrix product state (MPS) methods based on moderate entanglement have been overwhelmingly formulated in the second quantization. Here we introduce a first-quantized MPS approach to simulate quantum many-body systems. We show that, by reformulating the way the fermionic antisymmetry is handled, we arrive at MPS with a level of entanglement comparable to the usual one found in the second quantization. We demonstrate our scheme on the one-dimensional $t\text{−}V$ model (spinless fermions with density-density interaction) for both ground state and time evolution. For the ground state, we find that the first quantization approach naturally extends to long-range interaction in the Wigner crystal regime. For time evolution, we find that the entanglement entropy in first quantization is significantly smaller than in its second quantization counterpart.

For spin and fermionic systems in any spatial dimension, we establish that the superpolynomial decay behavior of mutual information and conditional mutual information is a universal property of gapped pure- and mixed-state phases; i.e., all systems in such a phase possess this property if one system in this phase possesses this property. We further demonstrate that the (conditional) mutual information indeed decays superpolynomially in a large class of phases, including chiral phases. As a by-product, we sharpen the notion of mixed-state phases.

Interconversion between charge and spin currents via spin-orbit coupling underpins spin orbitronics. Magnons, which are the quanta of spin waves, can exchange angular momentum with conduction-electron spins through spin-flip scattering, suggesting a direct route for charge-to-magnon conversion. Here, we predict that in single-layer ferromagnets, an applied electric current induces a transverse magnon current, producing electrical magnon Hall and inverse magnon Hall effects that share the symmetry of the spin Hall and inverse spin Hall effects. This effect gives rise to a magnon Hall magnetoresistance in CoFeB and NiFe, with an efficiency comparable to the spin Hall effect and a characteristic decay length on the order of micrometers, far exceeding typical electron spin diffusion lengths. By enabling the direct generation and detection of long-range magnon currents, our findings open new pathways for low-loss, on-chip spin-based logic and energy-harvesting devices.

The recent discovery that electrons in nanoscale conductors can act like a highly viscous liquid has triggered a surge of activities investigating consequences of this surprising fact. Here we demonstrate that the electronic viscosity has an enormous influence on the operation of a prototypical ac-current-driven nanomotor. The design of this prototype consists of a diatomic molecule immersed in an otherwise homogeneous electron liquid which carries an ac current. The motion of the diatomic is determined by a subtle balance between the current-induced forces and electronic friction. By ab initio time-dependent density-functional simulations we demonstrate that the diatomic performs a continuous rotation provided the amplitude and frequency of the imposed ac current lie within certain zones of stability. Outside these zones the nuclear motion is either irregular or comes to a standstill. The proposed design of the nanomotor is, arguably, the conceptually simplest realization of the idea of a molecular waterwheel sandwiched between conducting leads.

We present a quantum quadratic gravity inflationary scenario that can accommodate the new cosmological constraints, which have disfavored Starobinsky inflation. The theory is asymptotically free in the ultraviolet, but 1-loop running is found to dynamically lead to slow-roll inflation toward the infrared. When a large number of matter fields contribute to the beta functions, the spectral index and the tensor-to-scalar ratio can be phenomenologically viable. We find that as inflation ends, the theory approaches its strong coupling regime and general relativity must emerge, as an effective field theory, as the universe must reheat and enter its standard radiation era. In order to avoid strong coupling, a minimum tensor-to-scalar ratio of 0.01 is predicted for this theory. Our framework offers a laboratory for connecting a concrete ultraviolet completion (quantum quadratic gravity) with inflationary dynamics, reheating, and precise cosmological observations.

We report the refined dark-photon exclusion bound from Dark SRF’s pathfinder run. Our new result is driven by improved theoretical modeling of frequency instability in high-quality resonant experiments. Our analysis leads to a constraint that is an order of magnitude stronger than previously reported (corresponding to a signal-to-noise ratio that is 4 orders of magnitude larger). This result represents the world-leading constraint on non-dark-matter dark photons over a wide range of masses below $6\mathrm{\mu }\mathrm{eV}$ and translates to the best laboratory-based limit on the photon mass $m_{\gamma }<2.9\times {10}^{-48}\mathrm{g}$.

The analysis of local minima in time series data and random landscapes is essential across numerous scientific disciplines, offering critical insights into system dynamics. Recently, Kundu et al. derived the exact distribution of the number of local minima for a broad class of Markovian symmetric walks [Phys. Rev. E 110, 024137 (2024)]; however, many real-world systems are non-Markovian, typically due to interactions with possibly hidden degrees of freedom. This Letter investigates the statistical properties of local minima in discrete-time samples of fractional Brownian motion, a non-Markovian Gaussian process with stationary increments, widely used to model complex, anomalous diffusion phenomena. We derive a complete asymptotic characterization of the fluctuations of the number of local minima $m_N$ in an $N$-step discrete-time fractional Brownian motion. We show that the fluctuations of $m_N$ exhibit a sharp transition at the Hurst exponent $H=3/4$: for $H\le 3/4$ they satisfy a central limit theorem with Gaussian limiting law, whereas for $H>3/4$ they converge to a non-Gaussian Rosenblatt process. The convergence at the process level gives us full statistical description at all times. We exemplify it on the covariance of the rescaled minima process, which displays two qualitatively distinct regimes matching Brownian and Rosenblatt covariances on either side of this threshold. Our analysis relies on a Hermite or Wick decomposition of the local-minimum indicator, which isolates a quadratic functional of an effective long-memory mode as the unique driver of the anomalous statistics. These results identify the count of local minima as a simple and robust diagnostic of long-range dependence in non-Markovian Gaussian processes, a conclusion supported by numerical simulations.

We study the typical behavior of random walkers on the microcanonical configuration space of mean-field disordered systems. Passive walks have an ergodicity-breaking transition at precisely the energy density associated with the dynamical glass transition, but persistent walks remain ergodic at lower energies. In models where the energy landscape is thoroughly understood, we show that, in the limit of infinite persistence time, the ergodicity-breaking transition coincides with a transition in the topology of microcanonical configuration space. We conjecture that this correspondence generalizes to other models, and use it to determine the topological transition energy in situations where the landscape properties are ambiguous.