Optically active spin defects in solids are promising platforms for quantum technologies. Here, we present a first-principles framework to investigate intersystem crossing processes, which represent crucial steps in the optical spin-polarization cycle used to address spin defects. Considering the nitrogen-vacancy center in diamond as a case study, we demonstrate that our framework effectively captures electron correlation effects in the calculation of many-body electronic states and their spin-orbit coupling and electron-phonon interactions, while systematically addressing finite-size effects. We validate our predictions by carrying out measurements of fluorescence lifetimes, finding excellent agreement between theory and experiments. The framework presented here provides a versatile and robust tool for exploring the optical cycle of varied spin defects entirely from first principles.

The pertinent need for a microscopic understanding of magnetic exchange motivated us to go beyond the existing theories and develop a systematic method to quantify all possible mechanisms that contribute to magnetic exchange for an arbitrary pair of atoms in a given material. We apply it to the archetypal 2D magnetic monolayers ${\mathrm{CrI}}_3$ and ${\mathrm{NiI}}_2$, to reveal the previously underrated $d_{x^2-y^2}$,$d_{x^2-y^2}$ contribution as either the leading or the second largest contribution to the total magnetic exchange. We proceed to explore the microscopic mechanisms behind all the nonzero orbital contributions in both ${\mathrm{CrI}}_3$ and ${\mathrm{NiI}}_2$, and generalize the findings to other magnetic monolayers dominated by $d^8$ and $d^3$ electronic configurations of the magnetic atoms.

Observations from the Juno satellite have indicated very low electron densities, as low as ${10}^{-3}{\mathrm{cm}}^{-3}$, at high-latitudes in Jupiter’s magnetosphere. This region is strongly magnetized, with surface magnetic fields at the one-bar level up to 20 G, or 2 mT, leading to the unusual situation that the electron plasma frequency is less than the ion gyrofrequency. In this extremely low-density plasma, the Alfven wave becomes a plasma oscillation at the electron plasma frequency at shorter perpendicular wavelengths. Analysis of this mode with a kinetic low-frequency dispersion solver indicates that at large wave number, this mode has the characteristics of the Langmuir wave. Thus, this mode can be called an Alfvén-Langmuir mode. Below the plasma frequency, the high-wave number behavior of this mode exhibits a resonance cone, with frequency determined by the angle of the wave vector with the background magnetic field. These waves can be excited by the upward electron beams observed by Juno.

We report the emergence of two-dimensional itinerant half-metallicity in a two-atomic-layer thick iron-palladium alloy. Against the common belief that spin-orbit coupling is adverse to half-metallicity, the complex interplay between exchange and spin-orbit coupling counterintuitively enables it via electronic band hybridization. The 2D ferromagnet is engineered directly via controlled alloying and is verified experimentally via spin-resolved band structure analyses. First-principles calculations corroborate its tunability by providing a systematic bottom-up approach through stepwise ground-state construction. The appearance of hybridization points at specific $k$ points, via interplay between exchange and spin-orbit interactions, is responsible for the emergence of half-metallicity.

We present a search for the diffuse extremely-high-energy neutrino flux using 12.6 years of IceCube data. The nonobservation of neutrinos with energies well above 10 PeV constrains the all-flavor neutrino flux at ${10}^{18}\mathrm{eV}$ to a level of $E^2{\mathrm{\Phi }}_{{\nu }_e+{\nu }_{\mu }+{\nu }_{\tau }}\simeq {10}^{-8}\mathrm{GeV}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$, the most stringent limit to date. Using these data, we constrain the proton fraction of ultrahigh-energy cosmic rays (UHECRs) above $\simeq 30\mathrm{EeV}$ to be $\lesssim 70\%$ (at 90% CL) if the cosmological evolution of the sources is comparable to or stronger than the star formation rate. This is the first result to disfavor the “proton-only” hypothesis for UHECR in this evolution regime using neutrino data. This result complements direct air-shower measurements by being insensitive to uncertainties associated with hadronic interaction models. We also evaluate the tension between IceCube’s nonobservation and the $\sim 200\mathrm{PeV}$ KM3NeT neutrino candidate (KM3-230213A), finding it to be $\sim 2.9\sigma$ based on a joint-livetime fit between neutrino datasets.

We report a single-ion optical atomic clock with a fractional frequency uncertainty of $5.5\times {10}^{-19}$ and fractional frequency stability of $3.5\times {10}^{-16}/\sqrt{\tau /\mathrm{s}}$, based on quantum logic spectroscopy of a single ${}^{27}{\mathrm{Al}}^{+}$ ion. A cotrapped ${}^{25}{\mathrm{Mg}}^{+}$ ion provides sympathetic cooling and quantum logic readout of the ${}^{27}{\mathrm{Al}}^{+}{{}^{1}S}_0\leftrightarrow {{}^{3}P}_0$ clock transition. A Rabi probe duration of 1 s, enabled by laser stability transfer from a remote cryogenic silicon cavity across a 3.6 km fiber link, results in a threefold reduction in instability compared to previous ${}^{27}{\mathrm{Al}}^{+}$ clocks. Systematic uncertainties are lower due to an improved ion trap electrical design, which reduces excess micromotion, and a new vacuum system, which reduces collisional shifts. We also perform a direction-sensitive measurement of the ac magnetic field due to the rf ion trap, eliminating systematic uncertainty due to field orientation.

The recent experimental observation of quantum anomalous Hall (QAH) effects in the rhombohedrally stacked pentalayer graphene has motivated theoretical discussions on the possibility of quantum anomalous Hall crystal (QAHC), a topological version of Wigner crystal. Conventional topological Wigner crystals typically have one electron per unit cell. In this Letter we propose new types of topological Wigner crystals labeled as QAHC-$z$, with $z$ electrons per unit cell. In the pentalayer graphene system, we find parameter regimes where QAHC-2 and QAHC-3 have lower energy than the conventional QAHC-1 at total filling $\nu =1$ per moiré unit cell. These states all have total Chern number $C_{\mathrm{tot}}=1$ and are consistent with the QAH effect observed in the experiments. The larger period QAHC states have lower kinetic energy due to the unique Mexican-hat dispersion of the pentalayer graphene, which can compensate for the loss in the interaction energy. Unlike QAHC-1, QAHC-2, and QAHC-3 break the moiré translation symmetry and are sharply distinct from a moiré band insulator. We also briefly discuss the competition between integer QAH and fractional QAH states at filling $\nu =2/3$. Moreover, we find that a stronger moiré potential can significantly change the phase diagram and even favors a QAHC-1 ansatz with $C=2$ Chern band.

The $\sim 60000$ solar-mass ($M_{\odot }$) star cluster R136 (NGC 2070) in the Tarantula Nebula in the Large Magellanic Cloud is the host of at least 55 massive stars ($M\gtrsim 10M_{\odot }$) which move away from the cluster at projected velocities $\ge 27.5\mathrm{km}/\mathrm{s}$. The origin of the high velocities of such runaway stars have been debated since the 1960s, resulting either from dynamical ejections or from supernova explosions. Because of the Gaia satellite’s outstanding precision, we can now retrace the most recently ejected binary star, Mel 34, back to the center of R136 and reconstruct the events that 52 000 years ago led to its removal from R136; i.e., we establish its dynamical interaction and ejection history. We find that this ejection requires the participation of five stars in a strong interaction between a triple composed of the tight massive binary Mel 39, orbited by the star VFTS 590, and the binary star Mel 34. The participation of five stars is unexpected because runaway stars were not expected to result from triple interactions. The deterministic nature of the Newtonian dynamics in the scattering enables us to reconstruct the encounter that ejected Mel 34. We then predict that Mel 39 is a binary star with an $80M_{\odot }$ companion star that orbits within $\sim 1°$ in the same plane as Mel 34 and escapes the cluster with a velocity of $\sim 64\mathrm{km}/\mathrm{s}$. The five stars will undergo supernova explosions in the coming 5 Myr at a distance of $\sim 180–332\mathrm{pc}$ from their birth location (R136). The resulting black hole binaries, however, are not expected to merge within a Hubble time.

Nonadiabatic effects profoundly influence lattice dynamics, resulting in phonon renormalizations not only at the center of Brillouin zone (BZ), but also across the entire dispersion at finite momenta. These nonadiabatic phenomena exhibit clear dimensional dependencies and remain largely unexplored experimentally in low-dimensional systems. Here, we utilize high-resolution electron energy loss spectroscopy to investigate nonadiabatic phonon dispersion renormalization in monolayer graphene (MLG) and Bernal bilayer graphene (BLG). We present comprehensive phonon spectra measurements for both MLG and BLG across the full BZ. The high-resolution data reveal an intriguing “W” -shaped dispersion for the longitudinal optical phonon in MLG and a “V” -shaped dispersion in BLG near the BZ center, in contrast to the conventional “U” -shaped parabolic dispersions. Combining theoretical analysis, these anomalous phonon renormalizations are demonstrated to originate from nonadiabatic electron-phonon couplings. The comparative study of MLG and BLG gives a generic understanding of the impact of nonadiabatic effects on phonon dispersions in doped two-dimensional systems.

We combine atomistic simulations and theories to predict the free energy of polymer crystallization, which highlights the roles of backbone stiffness and orientational coupling between monomers in the formation of metastable mesophases. We show that polyethylene (PE) with sufficient backbone stiffness in the conformationally disordered state can develop uniaxial order through metastable nematic and rotator phases before conformationally ordering into the crystalline state. Isotactic polypropylene (iPP), with a lower backbone stiffness, undergoes one-step crystallization directly from the isotropic phase to the crystalline phase. The predicted bulk phase transition temperatures for iPP and PE of different molecular weights agree well with experiments.

Topological spin textures, such as merons and skyrmions, have shown significance in both fundamental science and practical applications across diverse physical systems. The optical skyrmionic textures in real space have been extensively explored, but those in momentum space are still rarely studied. Here, we report the experimental generation of momentum-space meron spin textures via bound states in the continuum (BICs) in photonic crystal slabs. We show that under circularly polarized illumination, the momentum-space vortex topology of BICs can transform light with meron spin textures in momentum space. These merons exhibit polarity-switchable configurations controlled by incident light polarization. We theoretically and experimentally verify the generation of momentum-space merons and demonstrate their operational flexibility across a broad spectral range. Our results establish a connection between different momentum-space topologies and provide a robust and compact platform for generating topological spin textures.

Cosmic rays are deemed to be generated by a process known as “Fermi acceleration” in which charged particles scatter against magnetic fluctuations in astrophysical plasmas. The process itself is, however, universal, has both classical and quantum formulations, and is at the basis of dynamical systems with interesting mathematical properties, such as the celebrated Fermi-Ulam model. Despite its effectiveness in accelerating particles, Fermi acceleration has so far eluded unambiguous verifications in laboratory settings. Here, we realize a fully controllable Fermi accelerator by colliding ultracold atoms against engineered movable potential barriers. We demonstrate that our Fermi accelerator, which is only $100\mathrm{\mu }\mathrm{m}$ in size, can produce ultracold atomic jets with velocities above $0.5\mathrm{m}/\mathrm{s}$. Adding dissipation, we also experimentally test Bell’s general argument for the ensuing energy spectra, which is at the basis of any model of cosmic ray acceleration. On the one hand, our Letter effectively opens the window to the use of cold atoms to study phenomena relevant for high energy astrophysics. On the other, the performance of our Fermi accelerator is competitive with those of best-in-class accelerating methods used in quantum technology and quantum colliders, but with substantially simpler implementation and no fundamental physics limit.

Two-dimensional topological insulators are characterized by an insulating bulk and conductive edge states protected by the nontrivial topology of the bulk electronic structure. They remain robust against moderate disorder until Anderson localization occurs and destroys the topological phase. Interestingly, disorder can also induce a topological phase—known as a topological Anderson insulator—starting from an otherwise pristine trivial phase. While topological invariants are generally regarded as global quantities, we argue that space-resolved topological markers can act as local order parameters, revealing the role of fluctuations and correlations in the local topology under Anderson disorder and vacancies. With this perspective, we perform numerical simulations of disorder-driven topological phase transitions in the Haldane and Kane-Mele models, using supercells with both open and periodic boundary conditions. We find that short-scale fluctuations of topological markers vanish upon coarse graining, except at the topological phase transition, where their correlation length peaks and large-scale fluctuations remain. Notably, such a topological correlation function is characterized by critical exponents that appear universal across disorder types, yet they can resolve different topological phase transitions.

Quantum field theory (QFT) in curved spacetimes predicts the amplification of field excitations and the occurrence of classical and quantum correlations, as in the Hawking effect for example. This raises interest in experiments in which the curvature of spacetime can be controlled and amplification measured, as in fluids going from subsonic to supersonic speeds where acoustic excitations are effectively trapped inside an acoustic horizon. Quantum fluctuations of the acoustic field are predicted to yield entangled emission across the horizon, as in black holes. Here, we introduce such a QFT simulator in a one-dimensional polaritonic fluid of light. We demonstrate the unique tunability of our system by engineering smooth and steep horizons, which respectively have quasithermal, but weak, and strong Hawking radiation. We measure the spectrum on either side of the horizon and evidence the excitation of negative energy waves in fluids of light for the first time. Notably, we explicitly show that, beyond phononic excitations as in other systems, our simulator also supports excitations with a massive, relativistic dispersion. In the future, quantum optics techniques offer the possibility to measure entanglement in unexplored regimes, giving insight in this outstanding prediction of relativistic QFT.

Understanding how statistical equilibrium can occur in out-of-equilibrium systems is of paramount interest, as it would enable the use of statistical mechanics tools to these systems. Here, we report the first experimental evidence of statistical equilibrium of the large scales of hydroelastic turbulent waves driven by small-scale random forcing. The wave field statistics at scales larger than the forcing scale, resolved in space and time, align well with the predictions of Rayleigh-Jeans equilibrium spectra over more than a decade. We measure zero net energy flux in this regime, as expected. We also determine the effective temperature, entropy, and heat capacity of this nonequilibrium system, demonstrating that classical thermodynamic concepts apply to describe large scales in statistical equilibrium of turbulent systems.

We demonstrate that the effective internal entropy of quasiparticles within the non-Abelian fractional quantum Hall effect manifests in the heat current through a tunneling barrier. We derive the electric current and heat current resulting from voltage and heat biases of the junction, taking into account the quasiparticles’ internal entropy. We find that when the tunneling processes are dominated by quasiparticle tunneling of one type of charge, the effective internal entropy can be inferred from the measurement of the heat current and the charge current. Our methods may be used to conclusively identify non-Abelian quasiparticles, such as the anyons that emerge in the $\nu =5/2$ fractional quantum Hall state.

Knowledge of the pressure of cold and dense quark matter is known to significantly constrain the equation of state of neutron-star matter, a quantity playing a key role in deciphering the stars’ internal structure. In this Letter, we make important progress toward determining the last unknown contribution to the pressure at next-to-next-to-next-to-leading order in the strong coupling constant ${\alpha }_s$, available through the sum of all four-loop vacuum diagrams of dense quantum chromodynamics. We demonstrate the cancellation of both the covariant gauge parameter and all infrared divergences in the sum, showcasing the effective-field-theory paradigm in action. For the remaining infrared-finite four-loop integrals, we demonstrate the efficacy of the dense loop tree duality method—a novel numerical framework for multiloop calculations in thermal field theory. Together, our results show that completing the next-to-next-to-next-to-leading order pressure of cold quark matter is no longer merely possible but for the first time within reach.

Quantum stochastic rectification arises from the dc response of a system subjected to both quantum noise and a small, periodic sinusoidal voltage modulation. In this Letter, we demonstrate quantum stochastic rectification by using inelastic electron tunneling spectroscopy with a scanning tunneling microscope to monitor the conformational switching of a single pyrrolidine molecule adsorbed on a Cu(001) surface. By applying quasistatic and nonequilibrium approximations, we elucidate the underlying dynamic response mechanisms. Additionally, we establish a quantitative relationship between the critical driving frequency in rectification measurements and the system’s relaxation time, facilitating the measurement of rapid relaxation rates in single molecules.

Historically, amorphous solid water (ASW) films have been created by physical vapor deposition both naturally on interstellar particles and in the laboratory. The present study focuses on the role of the mobile surface layer, which can be observed during vapor deposition and characterized by dielectric relaxation techniques. It is shown that the mobility of the surface layer can determine whether an as-deposited film is porous (p-ASW), collapsed (c-ASW), or crystallized to cubic ice ($I_c$). The latter case occurs when the fictive temperature of the surface reaches deep into the so-called “no man’s land”, whereas the actual temperature remains below that of the glass transition at $T_{\mathrm{g}}$. The dynamics of this surface layer is independent of deposition temperature in the 15 to 120 K range.

We identify hydrogen ordering in ${\mathrm{H}}_2\mathrm{O}$ ices spectroscopically in the near-infrared (NIR) range ($10000–4000{\mathrm{cm}}^{-1}/1–2.5\mathrm{\mu }\mathrm{m}$) based on the example of ices V/XIII. Previously it was thought that hydrogen ordering can only be revealed on the basis of lattice phonons, i.e., intermolecular vibrations. Here we show differences in the overtone spectrum of the intramolecular OH-stretching vibration. This makes NIR spectroscopy the first remote sensing method that is sensitive to different orientations of the water dipoles within ice. As such it will allow for future observations of the hydrogen order of ices in space by the James-Webb Space Telescope or the JUICE mission.