In reference Bravyi et al., [Nature (London) 627, 778 (2024)], Bravyi et al. found examples of bivariate bicycle (BB) codes with similar logical performance to the surface code but with an improved encoding rate. In this work, we generalize a novel parity-check circuit design principle called morphing circuits and apply it to BB codes. We define a new family of BB codes whose parity check circuits require a qubit connectivity of degree five instead of six while maintaining their numerical performance. Logical input or output to an ancillary surface code is also possible in a biplanar layout. Finally, we develop a general framework for designing morphing circuits and present a sufficient condition for its applicability to two-block group algebra codes.

Expectation values of observables are routinely estimated using so-called classical shadows—the outcomes of randomized bases measurements on a repeatedly prepared quantum state. In order to trust the accuracy of shadow estimation in practice, it is crucial to understand the behavior of the estimators under realistic noise. In this Letter, we prove that any shadow estimation protocol involving Clifford unitaries is stable under gate-dependent noise for observables with bounded stabilizer norm—originally introduced in the context of simulating Clifford circuits. In contrast, we demonstrate with concrete examples that estimation of “magic” observables can lead to highly misleading results in the presence of miscalibration errors and a worst case bias scaling exponentially in the system size. We further find that so-called robust shadows, aiming at mitigating noise, can introduce a large bias in the presence of gate-dependent noise compared to unmitigated classical shadows. Nevertheless, we guarantee the functioning of robust shadows for a more general noise setting than in previous works. On a technical level, we identify average noise channels that affect shadow estimators and allow for a more fine-grained control of noise-induced biases.

An enhanced electric octupole (E3) strength of 45(5) W.u. has been measured for the $3^{-}\to 0^{+}$ transition in ${}^{150}\mathrm{Gd}$, providing direct evidence for large octupole collectivity in this nucleus. The value was determined following two sensitive experiments, to measure the mean lifetime of the $J^{\pi }=3^{-}$ level and the weak $E3$ decay branch direct to the ground state. The systematics of the $E3$ strength across the Gd isotopic chain reveals that enhanced octupole collectivity is obtained for ${}^{150}\mathrm{Gd}$. The increasing low-lying octupole strength in the Gd isotopes is analyzed with five state-of-the-art theoretical models. It is found to be best reproduced by quasiparticle random phase approximation calculations, providing a good understanding of octupole collectivity in the rare-earth region.

The chromophore anion derived from the green fluorescent protein is one of the best-studied chromophores in the gas phase, but attempts to measure fluorescence have failed at room temperature. Here we unequivocally show that the chromophore exhibits fluorescence in the gas phase when cooled to low temperatures ($<150\mathrm{K}$), thereby validating previous hypotheses. The experimental confirmation is enabled by a unique mass-spectroscopy setup, allowing for fluorescence observation near or at the 0-0 transition without inducing heat in the ions upon photon absorption. The low-temperature conditions effectively simulate the restricted motion experienced within the protein, inhibiting internal conversion via a conical intersection along a twist motion coordinate. Fluorescence-excitation experiments at 100 K reveal an absorption-band maximum at $481.6\pm 0.2\mathrm{nm}$, while the dispersed fluorescence spectrum shows maximum emission at $483.6\pm 0.5\mathrm{nm}$. Remarkably, both values closely resemble those for proteins cooled to 77 K. We estimate that after excitation at the band maximum, radiation is the only pathway back to the ground state. Franck-Condon simulations at the $\omega \mathrm{B}97\mathrm{XD}/\mathrm{aug}\text{−}\mathrm{cc}\text{−}\mathrm{pVDZ}$ level of theory nicely reproduce the experimental spectra and identify the fluorescent form to be planar, and that an in-plane scissoring mode ($80{\mathrm{cm}}^{-1}$) is active for both absorption and emission.

Thouless pumping is an emblematic manifestation of topology in physics, referring to the ability to induce a quantized transport of charge across a system by simply varying one of its parameters periodically in time. The original concept of Thouless pumping involves a noninteracting system, and has been implemented in several platforms. One current challenge in the field is to extend this concept to interacting systems. In this Letter, we propose a Thouless pump that solely relies on nonlinear physics, within a chain of coupled Kerr resonators. Leveraging the driven-dissipative nature of the system, we modulate in space and time the on-site Kerr interaction energies, and generate $1+1$-dimensional topological bands in the Bogoliubov spectrum of excitations. These bands present the same topology as the ones obtained within the Harper-Hofstadter framework, and the Wannier states associated to each band experience a net displacement and show quantized transport according to the band Chern numbers. Remarkably, we find driving configurations leading to band inversion, revealing an interaction-induced topological transition. Our numerical simulations are performed using realistic parameters inspired from exciton polaritons, which form a platform of choice for investigating driven topological phases of matter.

Dissolution of fractured and porous media introduces a positive feedback between fluid flow and reactant transport, leading to the emergence of pronounced, fingerlike channels. We investigate the formation of these structures using a microfluidic Hele-Shaw cell with a soluble bottom. Our experiments show that the shape of dissolution fingers is invariant and reveals itself over time as the fingers extend into the system. By combining reactive-transport theory and conformal mapping techniques we derive these invariant forms. We relate these results to natural dissolution fingers in karst landscapes, and illustrate how to determine the groundwater flow rate responsible for their formation based on the finger shape.

We present an efficient automatic process explorer (APE) framework to overcome the reliance on human intuition to empirically establish relevant elementary processes of a given system, e.g., in prevalent kinetic Monte Carlo (kMC) simulations based on fixed process lists. Use of a fuzzy machine learning classification algorithm minimizes redundancy in the transition-state searches by driving them toward hitherto unexplored local atomic environments. APE application to island diffusion at a Pd(100) surface immediately reveals a large number of, up to now, disregarded low-barrier collective processes that lead to a significant increase in the kMC-determined island diffusivity as compared to classic surface hopping and exchange diffusion mechanisms.

The existence of multiple superconducting phases induced by either pressure or magnetic field is one of the most striking features of superconductivity of ${\mathrm{UTe}}_2$, among the many unusual superconducting properties of this system. Here we report thermodynamic measurements of the superconducting phase diagram combining pressure and magnetic fields up to 30 T. We show that the high-field superconducting phase at ambient pressure continuously evolves under pressure into the zero-field high-temperature superconducting phase, which occurs above 0.2 GPa.

We use density matrix renormalization group (DMRG) and variational exact diagonalization (VED) to calculate the single-electron removal spectral weight for the Hubbard-Holstein model at low electron densities. Tuning the strength of the electron-phonon coupling and of the Hubbard repulsion allows us to contrast the results for a liquid of polarons versus a liquid of bipolarons. The former shows spectral weight up to the Fermi energy, as expected for a metal. The latter has a gap in its spectral weight, set by the bipolaron binding energy, although this is also a (strongly correlated) metal. This difference suggests that angle-resolved photoemission spectroscopy could be used to identify liquids of pre-formed pairs. Furthermore, we show that the one-dimensional liquid of incoherent bipolarons is well approximated by a “Bose sea” of bosons that are hard-core in momentum space, occupying the momenta inside the Fermi sea but otherwise noninteracting. This new proposal for a strongly correlated many-body wave function opens the way for studying various other properties of incoherent (nonsuperconducting) liquids of preformed pairs in any dimension.

The electronic spectra of altermagnets are a fertile ground for nontrivial topology due to the unique interplay between time-reversal and crystalline symmetries. This is reflected in the unconventional Zeeman splitting between bands of opposite spins, which emerges in the absence of spin-orbit coupling (SOC) and displays nodes along high-symmetry directions. Here, we argue that even for a small SOC, the direction of the magnetic moments in the altermagnetic state has a profound impact on the electronic spectrum, enabling novel topological phenomena to appear. By investigating microscopic models for two-dimensional (2D) and three-dimensional (3D) altermagnets, motivated in part by rutile materials, we demonstrate the emergence of hitherto unexplored Dirac crossings between bands of the same spin but opposite sublattices. The direction of the moments determines the fate of these crossings when the SOC is turned on. We focus on the case of out-of-plane moments, which forbid an anomalous Hall effect and thus ensure that no weak magnetization is triggered in the altermagnetic state. In the 2D model, the SOC gaps out the Dirac crossings, resulting in mirror Chern bands that enable the Quantum Spin Hall Effect and undergo a topological transition to trivial bands upon increasing the magnitude of the magnetic moment. On the other hand, in the 3D model the crossings persist even in the presence of SOC, forming Weyl nodal loops protected by mirror symmetry. Finally, we discuss possible ways to control these effects in altermagnetic material candidates.

Recent advancements in time-resolved electron and photon detection enable novel correlative measurements of electrons and their associated cathodoluminescence (CL) photons within a transmission electron microscope. These studies are pivotal for understanding the underlying physics in coherent CL processes. We present experimental investigations of energy-momentum conservation on the single particle level, achieved through coincidence detection of electron-photon pairs. This not only enables unprecedented clarity in detecting weak signals otherwise obscured by nonradiative processes but also provides a new experimental pathway to investigate momentum-position correlations to explore entanglement within electron-photon pairs.

The observation of negative-temperature states in the localized phase of the discrete nonlinear Schrödinger equation has challenged statistical mechanics for a long time. For isolated systems, they can emerge as stationary extended states through a large-deviation mechanism occurring for finite sizes, while they are formally unstable in grand canonical setups, being associated to an unlimited growth of the condensed fraction. Here, we show that negative-temperature states in open setups are metastable and their lifetime $\tau$ is exponentially long with the temperature, $\tau \approx \exp (\lambda |T|)$ (for $T<0$). A general expression for $\lambda$ is obtained in the case of a simplified stochastic model of noninteracting particles. In the discrete nonlinear Schrödinger model, the presence of an adiabatic invariant makes $\lambda$ even larger because of the resulting freezing of the breather dynamics. This mechanism, based on the existence of two conservation laws, provides a new perspective over the statistical description of condensation processes.

Neutron spin echo experiments on deuterated and protonated molecular liquids with different interactions have revealed their coherent and incoherent scattering functions at the mesoscale. The coherent one decays by a nondiffusive exponential relaxation process that can be related to stress fluctuations. For self-dynamics, we show that even though diffusion is, as expected, the predominant relaxation channel at the mesoscale, the process observed for collective dynamics also appears at short times, evidencing the influence of stress fluctuations on translational motions even at high temperatures.

Solute gradients next to an interface drive a diffusioosmotic flow, the origin of which lies in the intermolecular interactions between the solute and the interface. These flows on the surface of colloids introduce an effective slip velocity, driving their diffusiophoretic migration. In confined environments, the interplay between diffusiophoresis and diffusioosmosis governs the motion of colloids. Previous studies have indeed demonstrated the quantitative modulation of phoretic migration by the osmotic flows. Here, we show that diffusioosmotic flows can lead to qualitatively distinct outcomes, reversing the direction of colloidal focusing expected from diffusiophoresis alone. Using microfluidic experiments in a T-junction, numerical simulations, and theoretical modeling, we explain our observations to be due to an interplay between diffusiophoretic migration of colloids toward the walls and their entrainment in a diffusioosmotic vortex. We show this focusing to be persistent for a range of salt types, salt gradients, and flow rates, and establish a criterion for its emergence. Our work sheds light on how boundaries modulate the solute-mediated transport of colloids in confined environments and how the colloidal trajectories can be utilized to infer the surface properties.

Device-independent randomness certification based on Bell nonlocality does not require any assumptions about the devices, thus provides adequate security. Great effort has been made to demonstrate that nonlocality is necessary for generating quantum randomness, but the minimal resource required has not been clarified. Here we prove and experimentally demonstrate that violating any two-input Bell inequality is both necessary and sufficient for certifying randomness, however, for the multiple-input cases, this sufficiency ceases to apply, leading to certain states exhibiting Bell nonlocality without the capability to certify randomness. We examine two typical classes of Bell inequalities with multiple input and multiple output, the facet inequalities and Salavrakos-Augusiak-Tura-Wittek-Acín-Pironio inequalities, in the high-dimensional photonic system, and observe that the violation of the latter one can always certify randomness which is not true for the former. The private randomness with a generation rate of $1.867\pm 0.018\mathrm{bits}$ per photon pair is obtained in the scenario of Salavrakos-Augusiak-Tura-Wittek-Acín-Pironio inequalities with 3-input and 4-output. Our Letter unravels the internal connection between randomness and nonlocality, effectively enhancing the performance of tasks such as device-independent random number generation.