Atom interferometers represent a promising approach for gravitational wave detection in the decihertz frequency band, complementary to existing light-based detectors. The South Pole offers unique advantages for such experiments: exceptionally low seismic noise, established infrastructure for large scientific projects, and a location that strengthens gravitational wave source localization through global triangulation. Here, we discuss the scientific case and practical considerations for deploying a long-baseline atom interferometer at the South Pole, which has the potential to expand the global network of gravitational wave detectors while enabling precision tests of fundamental physics.

Magnetic-rare-earth pyrochlore iridates exhibit a rich variety of unconventional phases, driven by the complex interactions within and between the rare-earth and the iridium sublattices. In this study, we investigate the peculiar magnetic state of ${\mathrm{Tb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, where a component of the ${\mathrm{Tb}}^{3+}$ moment orders perpendicular to its local Ising anisotropy axis. By means of neutron diffraction and inelastic neutron scattering down to dilution temperatures, complemented by specific heat measurements, we show that this intriguing magnetic state is fully established at $1.5\mathrm{K}$ and we characterize its excitation spectrum across a broad range of energies. Our calculations reveal that bilinear interactions between ${\mathrm{Tb}}^{3+}$ ions subjected to the Ir molecular field capture several key features of the experiments, but need to be supplemented to fully reproduce the observed behavior.

While three-dimensional Dirac semimetals have been extensively studied in centrosymmetric materials such as ${\mathrm{Cd}}_3{\mathrm{As}}_2$ and ${\mathrm{Na}}_3\mathrm{Bi}$, the recent colloidal synthesis of wurtzite HgTe (wz-HgTe) opens new opportunities for exploring topological quantum phases in noncentrosymmetric systems. Building on recent evidence that bulk wz-HgTe is a noncentrosymmetric Dirac semimetal, we investigate how strain and quantum confinement reshape its topology. To this end, we combine ab initio density-functional theory, semiempirical tight-binding, and $\mathbf{k}·\mathbf{p}$ approaches. Bulk wz-HgTe hosts two Dirac points along the $\mathrm{\Gamma }–\mathrm{A}$ line, i.e., along the $c$ axis, located only 37 meV above the Fermi level, indicating that the Dirac regime should be experimentally accessible by doping or electrostatic gating. We show that biaxial strain drives a sequence of topological phase transitions from a Dirac semimetal to a three-dimensional topological insulator and eventually to a multiband metal. In (0001) thin films, quantum confinement induces oscillatory transitions between trivial and topological insulating phases. We demonstrate that this behavior is governed not only by the underlying $s–p$ band inversion, but also by the crystal-field splitting specific to the wurtzite structure and by the presence of bulk Dirac points. In contrast, $\left(1\overline{1}00\right)$ films exhibit a single topological transition and, above a critical thickness, develop surface states forming Fermi-arc-like contours that connect the projections of the Dirac points in the two-dimensional Brillouin zone. In the same weak-confinement regime, semimetallic nanowires oriented along the $c$ axis are predicted to host localized edge states that should be accessible to transport measurements or scanning tunneling spectroscopy. These results establish the topological phase diagram of wz-HgTe beyond the strong-confinement regime explored so far and reveal how crystal symmetry, inversion breaking, strain, and confinement combine to reshape the topology of a Dirac material. More broadly, they identify wurtzite HgTe as a promising platform for engineering topological phases in experimentally relevant nanostructures.

When a superconductor is placed in contact with a normal material, Cooper pairs penetrate the latter and induce superconductivity via the proximity effect. Despite its central role in quantum materials, superconducting devices, and topological platforms, a predictive first-principles description of the proximity effect at realistic interfaces has remained computationally prohibitive so far. Here, we fill this gap by developing a Green's-function framework based on real-space dynamical embedding that enables first-principles simulations of superconducting proximity in mesoscopic systems. We show that the proximity effect admits a transparent diagrammatic formulation in terms of normal and anomalous embedding self-energies, which disentangle and quantify the distinct renormalization mechanisms generated by coupling to a superconducting bath. By combining this formalism with recursive schemes, we compute local spectral functions and proximity lengths extending over hundreds of nanometers into the bulk without resorting to thick interface slabs. We deploy the approach on tight-binding models (Qi-Hughes-Zhang and Fu-Kane-Mele), where we analyze mixed-parity superconductivity in topological insulators proximitized by $s$-wave superconductors, and on first-principles simulations of ${\mathrm{NbSe}}_2/{\mathrm{CrBr}}_3$ heterostructures based on density-functional theory and maximally localized Wannier functions, the latter enabling direct comparison with scanning tunneling spectroscopy experiments. Our work provides a scalable and conceptually unified framework that bridges microscopic electronic structure and mesoscale proximity physics, enabling predictive atomistic simulations of superconducting interfaces.

Throughout its history, the theory of quantum error correction has heavily benefited from translating classical concepts into the quantum setting. In particular, classical notions of weight enumerators, which relate to the performance of an error-correcting code, and MacWilliams’ identity, which links enumerators of a code to the ones of its dual, have been generalized to the quantum case. In this work, we establish a relationship between the theoretical machinery of quantum weight enumerators and a seemingly unrelated physics experiment: we prove that Rains’ quantum shadow enumerators—a powerful mathematical tool—arise as probabilities of observing fixed numbers of triplets in a two-copy Bell sampling experiment. This insight allows us to develop here a rigorous framework for the direct measurement of quantum weight enumerators, thus enabling experimental and theoretical studies of the entanglement structure of any quantum error-correcting code or quantum state under investigation. On top of that, we derive concrete sample complexity bounds and physically motivated robustness guarantees against unavoidable experimental imperfections. Finally, we demonstrate the feasibility of experimentally learning weight enumerators on a trapped-ion quantum computer. Our experimental findings are in good agreement with theoretical predictions and illuminate how entanglement theory and quantum error correction cross-fertilize each other once two-copy Bell sampling experiments are combined with the theoretical machinery of quantum weight enumerators.

The response of nonlinear resonators to multifrequency driving reveals rich dynamics beyond conventional single-tone theory. We study a Duffing resonator under bichromatic excitation and identify a competition between the two drives, governed by their detuning and relative amplitudes. In the slow-beating regime, where the tones are closely spaced, the secondary tone acts as a modulation that induces dynamical phase transitions between coexisting stationary states. We introduce the cycle-averaged amplitude as an order parameter and map the resulting phase diagram as a function of the drive detuning and amplitude ratio, capturing the pronounced asymmetry observed for blue versus red detuning between the drives in experiment. We link the onset of these transitions to the resonance properties around the nonlinear stationary mode of the system. We apply a two-tone ansatz to this system, for the first time, to provide a comprehensive map of stationary states. Our results provide a framework for controlling driven nonlinear systems, enabling state manipulation, and sensing in nanomechanical, optical, and superconducting circuit platforms.

In red-detuned magneto-optical traps (MOTs) of molecules, sub-Doppler heating competes with Doppler cooling, resulting in high temperature and low density. A solution is offered by the blue-detuned MOT where sub- Doppler cooling dominates and the cloud is compressed. Several blue-detuned molecular MOTs have been implemented. A recent implementation relies on a pair of orthogonally polarized components whose frequency separation is smaller than the transition linewidth. We identify the trapping force in these MOTs. At a certain magnetic field, there is a state that is dark to the laser propagating in one direction, but not to the counterpropagating one. This Zeeman-induced dark state (ZIDS) sets up an imbalance in the photon scattering rate, leading to a restoring force. We also study the role of the moving lattices generated by the closely spaced frequency components of the light. We show that there is a velocity-dependent force that drives the molecules toward the speeds of these moving lattices, and that over a relevant range of magnetic fields this combines with the ZIDS force to transport molecules toward the center of the MOT. Here, gray molasses cooling, assisted by nonadiabatic transitions driven by the time-varying polarization of the light field, cools the molecules toward zero velocity. We study these mechanisms for model systems with simple level structures and then extend them to molecules with ground-state hyperfine structure.

Current quantum computers are limited by errors, but have not yet achieved the scale required to benefit from active error correction in large computations. We show how simulations of dissipative quantum circuits can benefit from error detection. In particular, we use Quantinuum’s H2 quantum computer to perform 24-qubit logical simulations of a nonequilibrium phase transition using many [[4,2,2]] code blocks. Importantly, by converting detected errors into random resets, which are an intended part of the dissipative quantum dynamics being studied, we avoid any postselection in our simulations, thereby eliminating the exponential cost typically associated with error detection. The encoded simulations perform near breakeven with unencoded simulations at short times.

Active fluids display spontaneous turbulentlike flows known as active turbulence. Recent work revealed that these flows have universal features, independent of the material properties and of the presence of topological defects. However, the differences between defect-laden and defect-free active turbulence remain largely unexplored. Here, by means of large-scale numerical simulations, we show that defect-free active nematic turbulence can undergo dynamical arrest. This state is characterized by an emergent network of nematic domain walls that channels coherent streams and suppresses chaotic flows. As the system evolves, the branched wall network produces a large-scale pattern with treelike topological properties. We find that flow alignment—the tendency of nematics to reorient under shear—enhances large-scale chaotic jets in contractile rodlike systems while promoting dynamical arrest in extensile systems. We further show that dynamical arrest arises regardless of whether defects are prohibited by construction or simply fail to form due to a high energy cost of defect cores. Taken together, our findings reveal a striking pattern-formation mechanism, with labyrinths emerging from active turbulence, and illuminate the rich transitional regime between defect-free and defect-laden dynamics. These behaviors call for the experimental realization of active nematics at vanishing or low defect densities, and underscore that, in extensile rodlike nematics, topological defects enable turbulence by preventing dynamical arrest.

We analyze powder-averaged inelastic neutron scattering and magnetization data for the distorted honeycomb compound ${\mathrm{Cu}}_5{\mathrm{SbO}}_6$ using a first-order dimer expansion calculation and quantum Monte Carlo simulations. We show that, in contrast to the previously proposed honeycomb-lattice model, ${\mathrm{Cu}}_5{\mathrm{SbO}}_6$ accommodates interacting dimerized spin chains with alternating ferromagnetic-antiferromagnetic couplings along the chain. Moreover, unlike the typical couplings observed in other ${\mathrm{Cu}}^{2+}$-based distorted honeycomb magnets, the spin chains in ${\mathrm{Cu}}_5{\mathrm{SbO}}_6$ primarily couple through an antiferromagnetic coupling that arises between the honeycomb layers, rather than the expected interchain coupling in the layers. This finding reveals a different magnetic coupling scheme for ${\mathrm{Cu}}_5{\mathrm{SbO}}_6$. In addition, utilizing x-ray spectroscopy and transmission electron microscopy, we also refine the crystal structure and stacking-fault model of the compound.

To investigate the interplay between charge density wave (CDW) and superconductivity, we performed ultralow-temperature spectroscopic-imaging scanning tunneling microscopy on the cleaved surface of the layered superconductor $2H\text{−}{\mathrm{NbSe}}_2$. We found that the superconducting-gap spectrum exhibits intricate structures reflecting the anisotropic gaps opening on multiple Fermi surfaces. Notably, none of the characteristic energy scales apparent in the spectral gap show appreciable spatial variations, suggesting that the finite-momentum pairing is negligible. Instead, the spectral weight near the coherence peak is modulated with the same periodicity as the CDW. The maximum position of the coherence-peak-weight modulation coincides with neither the peak nor the bottom of the CDW modulation; rather, it aligns with the center of one of the two inequivalent triangular plaquettes that comprise the CDW unit cell. This distribution pattern of Bogoliubov quasiparticles directly results from the broken in-plane inversion symmetry at the surface of $2H\text{−}{\mathrm{NbSe}}_2$, which may activate Ising spin-orbit coupling.

We show that molecular nanomagnets provide an ideal platform to investigate entanglement in qubit-qudit and qudit-qudit pairs. A prototypical example is ${}^{173}\mathrm{Yb}$ (trensal), embedding an electronic spin qubit coupled to a nuclear spin qudit. We demonstrate violation of generalized Bell inequalities on this molecule by developing an optimized pulse sequence and by performing realistic numerical simulations using experimentally measured Hamiltonian parameters and coherence times. We find that the inequalities are safely violated in a wide range of parameters, proving the robustness of entanglement in the investigated system. Furthermore, we propose a scheme to study qudit-qudit entanglement on a molecular spin trimer, in which two spins 3/2 are linked via an interposed switch to turn on and off their mutual interaction.

In morphogenesis and disease, biological tissues may exhibit diverse mechanical properties due to their capacity to switch between fluidlike and solidlike states. Here, we introduce a foam model to investigate how active mechanical properties and cellular interactions govern this fluid-to-solid transition in active cell monolayers. This model explicitly represents three-dimensional cell shapes and describes cell-cell interactions via discrete interacting surfaces. Simulations reveal that cell-cell adhesive tension promotes tissue fluidization in high-adhesive tissues, where it mainly promotes cell deformability, while it induces solidification in the low-adhesive regime, where it prevents cell-cell debonding. Moreover, we study the dynamic role of adhesive ligand turnover through an effective intercellular friction. Through simulated shear experiments, we find that intercellular friction strongly suppresses neighbor exchanges, but does not lead to solidlike tissue properties. We discuss the implications of our results for understanding the relationship between fluidization and the partial epithelial-mesenchymal transition, highlighting how differences in adhesion dynamics and intercellular friction may reconcile conflicting observations in tissue mechanics and cancer metastasis.

We investigate the nature of the $5f$ electrons in the unconventional odd-parity superconductor ${\mathrm{UTe}}_2$, focusing on the degree of covalency, localization versus itinerancy, and dominant electronic configuration. This is achieved using density functional theory (DFT) in combination with dynamical mean-field theory (DMFT) calculations. A key aspect of our approach is the material-specific tuning of the double-counting correction parameter, ${\mu }_{\mathrm{dc}}$, within the $\mathrm{DFT}+\mathrm{DMFT}$ part. This tuning is guided by the energy dependence of photoionization cross sections in valence band photoelectron spectroscopy. The reliability of the parameters is confirmed by the accurate reproduction of the angle-resolved valence-band photoemission spectra and the $\mathrm{U}4f$ core-level data. The $\mathrm{DFT}+\mathrm{DMFT}$ model reveals that in ${\mathrm{UTe}}_2$ $\mathrm{U}5f^n$ configurations with $n=1$–4 contribute to the ground state, with the $5f^2$ configuration being most prevalent and an average $5f$ shell filling close to 2.5. The model further suggests that the $5f$ electrons form narrow bands and that charge fluctuations due to degeneracy play a role in addition to coherent valence dynamics arising from hybridization with the conduction bath. Additionally, the significance of the $\mathrm{U}6d$ states in ${\mathrm{UTe}}_2$ is discussed.

Vortex singularities in speckle patterns formed from random superpositions of waves are an inevitable consequence of destructive interference and are consequently generic and ubiquitous. Singularities are topologically stable, meaning that they persist under small perturbations and can only be removed via pairwise annihilation. They have applications including sensing, imaging, and energy transfer in multiple fields such as optics, acoustics, and elastic or fluid waves. We generalize the concept of singularity speckle patterns to arbitrary two-dimensional parameter spaces and any complex scalar function that describes wave phenomena involving complicated scattering. In scattering systems specifically, we are often concerned with singularities associated with complex zeros of various functions of the scattering matrix $S$. Some examples are coherent perfect absorption (CPA), reflectionless scattering modes, transmissionless scattering modes, and $S$-matrix exceptional points. Experimentally, we find that all scattering singularities share a universal statistical property: Any quantity that diverges as a simple pole at a singularity, such as $\text{det}S$ at CPA, has a probability distribution function with a $-3$ power-law tail. The heavy tail of the distribution provides an estimate for the likelihood of finding a given singularity in a generic scattering system. We use these universal statistical results to determine that homogeneous system loss is the most important parameter determining singularity density in a given parameter space of an absorptive scattering system. Finally, we discuss events where distinct singularities coincide in parameter space, which result in higher-order singularities that have applications beyond the capabilities of isolated singularities. These higher-order singularities are not topologically protected, and we do not find universal statistical properties for them. We support our empirical results from microwave experiments with random matrix theory simulations and conclude that the statistical results presented hold for all generic non-Hermitian scattering systems in which singularities can occur.

Laser cooling of large, complex molecules is a long-standing goal, instrumental for enabling new quantum technology and precision measurements. A primary consideration for the feasibility of laser cooling, which determines the efficiency and technical requirements of the process, is the number of excited-state decay pathways leading to vibrational excitations. Therefore, the assessment of the laser-cooling potential of a molecule begins with estimate of the vibrational branching ratios of the first few electronic excited states theoretically to find the optimum cooling scheme. Such calculations, typically done within the Born-Oppenheimer and harmonic approximations, have suggested that one leading candidate for large, polyatomic molecule laser cooling, alkaline earth phenoxides, can most efficiently be laser cooled via the third electronically excited ($\tilde{C}$) state. Here, we report the first detailed spectroscopic characterization of the $\tilde{C}$ state in CaOPh and SrOPh. We find that nonadiabatic couplings between the $\tilde{A}, \tilde{B}$, and $\tilde{C}$ states lead to substantial mixing, giving rise to vibronic states that enable additional decay pathways. Based on the intensity ratio of these extra decay channels, we estimate a nonadiabatic coupling strength of $\sim 0.1{\mathrm{cm}}^{-1}$. While this coupling strength is small, the large density of vibrational states available at photonic energy scales in a polyatomic molecule leads to significant mixing. Only the lowest excited state $\tilde{A}$ is exempt from this coupling because it is highly separated from the ground state. Thus, this result is expected to be general for large molecules and implies that only the lowest electronic excited state should be considered when judging the suitability of a molecule for laser cooling.

Molecular nanomagnets are quantum spin systems potentially serving as qudits for future quantum technologies thanks to their many accessible low-energy states. At low temperatures, the primary source of error in these systems is pure dephasing, caused by their interactions with the bath of surrounding nuclear spins degrees of freedom. Most importantly, as the system’s dimensionality grows going from qubits to qudits, the control and mitigation of decoherence become more challenging. Here, we analyze the characteristics of pure dephasing in molecular qudits under spin-echo sequences. We use a realistic description of their interaction with the bath, whose non-Markovian dynamics is accurately computed by the cluster correlation expansion technique. First, we show that the differences in the expectation values of the local spin operators on the eigenstates of the qudit are the source of decoherence in these systems. Indeed, we demonstrate that this is a necessary and sufficient condition to prevent the decay of coherence with time, also introducing a parameter to quantify the deviation from such ideal condition. We illustrate this with two paradigmatic systems: a single giant spin and a composite antiferromagnetic spin system. We then advance a proposal for optimized nanomagnets, identifying key ingredients for engineering robust qudits for quantum technologies.

Topologically ordered quantum liquids are highly sought-after quantum phases of matter, and recently, fractional Chern insulators (FCIs) joined the few experimental realizations of such phases. Here, we ask whether a gapped classical, highly degenerate liquid can be the birthplace of FCIs upon the addition of suitable quantum fluctuations. Two competing tendencies can be anticipated: (1) Following the quantum order-by-disorder paradigm, quantum fluctuations could induce symmetry-breaking (charge) order, or (2) the classical liquid builds up long-range entanglement and turns into a quantum liquid. We study spinless fermions on a honeycomb lattice subject to cluster-charging interactions and introduce quantumness through a Haldane kinetic term, featuring complex second-nearest-neighbor hopping. Based on extensive exact diagonalization calculations and high-order perturbation theory, we find that neither scenario (1) nor (2) prevails, but (1) and (2) manifest sequentially as the kinetic energy is increased. We demonstrate how the gradual lifting of kinematic constraints gives rise to this sequence of phases. Our results relate to the regime of intermediate-scale interactions present in moiré systems, where band projections are not suitable to model FCIs and competing charge-ordered phases have been identified.

We explore a deep connection between fracton order and product codes. In particular, we propose and analyze conditions on classical seed codes which lead to fracton order in the resulting quantum product codes. Depending on the properties of the input codes, product codes can realize either Type-I or Type-II fracton models, in both nonlocal and local constructions. For the nonlocal case, we show that a recently proposed model of lineons on nonlocal graphs can be obtained as a hypergraph product code. Interestingly, constrained mobility in this model arises only from energy barriers associated with the graph. For the local case, we introduce a novel type of classical LDPC code defined on a planar aperiodic tiling. By considering the specific example of the pinwheel tiling, we demonstrate the systematic construction of local Type-I and Type-II fracton models as product codes. Our work establishes product codes as a natural setting for exploring fracton order.

We present a unique method for measuring the half-life of highly charged radioisotopes by nondestructive nuclear recoil detection in a Penning ion trap. A specific emphasis is placed on ${}^7\mathrm{Be}^{3+}$, which plays a crucial role in stellar evolution and the production of solar neutrinos. The determination of the half-life is necessary to constrain the free electron capture rate in the solar environment, but is difficult to measure by existing techniques. Simulations of the sympathetic cooling of the recoiled daughter nuclei (${}^7\mathrm{Li}^{3+}$) with the trapped cloud of ${}^7\mathrm{Be}^{3+}$ demonstrate a decay detection efficiency of $99.5\%$. A statistical analysis of half-life measurements on ensembles containing hundreds of ions shows that a final statistical uncertainty of less than $5\%$ is achieved with only 500 measured decays. By coherent control of hyperfine populations in trapped ions, the fidelity of the technique we describe enables the direct measurement and manipulation of state-dependent decay branching ratios for the first time.