Many quantum many-body wavefunctions, such as Jastrow-Slater, tensor network, and neural quantum states, are studied with the variational Monte Carlo technique, where stochastic optimization is usually performed to obtain a faithful approximation to the ground state of a given Hamiltonian. While first-order gradient descent methods are commonly used for such optimizations, quasi-second-order optimization formulations offer the potential of faster convergence under certain theoretical conditions, but with a similar cost per sample to first-order methods. However, the relative performance of first- and second-order optimizers is influenced in practice by many factors, including the sampling requirements for a faithful optimization step, the influence of wavefunction quality, as well as the wavefunction parametrization and expressivity. Here, we analyze these performance characteristics of first- and quasi-second-order optimization methods for a variety of Hamiltonians, with the additional context of understanding the scaling of these methods (for good performance) as a function of system size. Our findings help clarify the role of first- and quasi-second-order methods in variational Monte Carlo calculations and the conditions under which they should respectively be used. In particular, we find that unlike in deterministic optimization, where closeness to the variational minimum determines the suitability of second-order methods, in stochastic optimization the main factor is the overall expressivity of the wavefunction: Quasi-second-order methods lead to an overall reduction in cost relative to first-order methods when the wavefunction is sufficiently expressive to represent the ground state, even when starting far away from the ground state. This makes quasi-second-order methods an important technique when used with wavefunctions with arbitrarily improvable accuracy.

An exceptional point (EP) in PT (parity-time)-symmetric systems featuring loss and nonlinear saturable gain has been recently utilized to enhance sensing, leading to the cube-root singularity in a two-level system. However, a shift in the frequency eigenvalue comes with a relatively small scaling factor, which could potentially reduce the sensitivity augmented by the cube-root singularity, and achieving the cube-root singularity necessitates sacrificing the sensing dynamic range. Instead of detecting frequency eigenvalue, this article explores the phase shift between both resonators in parity-time symmetric film bulk acoustic resonators (FBARs) with a nonlinear saturable gain. When the FBAR is linked to an external (inductor-capacitor loop with a nonlinear negative resistance, it forms PT-symmetric resonators with loss and nonlinear gain. Our findings show that the phase difference between the loss and gain resonators displays both a cube-root dependence on perturbation and an increased scaling factor. Experimental results from our FBAR humidity sensors indicate that the sensitivity of phase shift has increased by several orders of magnitude compared to the conventional frequency shift. These discoveries have potential applications for piezoelectric resonant sensors requiring ultrahigh sensitivity across various fields.

We address the Bose polaron problem of a mobile impurity interacting strongly with a host Bose-Einstein condensate through a Feshbach resonance. On the repulsive side at strong couplings, theoretical approaches predict two distinct polaron branches corresponding to attractive and repulsive polarons, but it remains unclear how the two are related. This is partly due to the challenges resulting from a competition of strongly attractive (destabilizing) impurity-boson interactions with weakly repulsive (stabilizing) boson-boson interactions, whose interplay is difficult to describe with contemporary theoretical methods. Here, we develop a powerful variational framework that combines Gaussian correlations among impurity-boson scattering states, including up to an infinite number of bosonic excitations, with exact non-Gaussian correlations among bosons occupying an impurity-boson bound state. This variational scheme enables a full treatment of strong nonlinearities arising in the Feshbach molecule on the repulsive side of the resonance. Within this framework, we demonstrate that the interplay of impurity-induced instability and stabilization by repulsive boson-boson interactions results in a discrete set of metastable many-body bound states at intermediate energies between the attractive and repulsive polaron branches. These states exhibit strong quantum statistical characteristics in the form of non-Gaussian quantum correlations, requiring nonperturbative beyond mean-field treatments for their characterization. Furthermore, these many-body bound states have sizable molecular spectral weights, accessible via molecular spectroscopy techniques. This work provides a unified theory of attractive and repulsive Bose polarons on the repulsive side of the Feshbach resonance.

The $(\mathrm{n}, \gamma )$ cross section of silver is crucial for the nucleosynthesis process of nuclear astrophysics, the design of silver-indium-cadmium absorber rod in the nuclear reactors, and neutron resonance capture analysis for determining the elemental and isotopic composition of cultural heritage. The high-precision $\mathrm{Ag}(\mathrm{n}, \gamma )$ cross section was measured from 1 eV to 1 MeV at the Back-n facility of the Chinese Spallation Neutron Source using the time-of-flight method and pulse height weighting technique. The resonance parameters were accurately extracted in the resolved resonance region by the multilevel multichannel $R$-matrix code sammy. The talys code was used to describe the average cross sections in the unresolved resonance region, and the Maxwellian-averaged cross sections (MACSs) of ${}^{107}\mathrm{Ag}$ and ${}^{109}\mathrm{Ag}$ were further given in the 5–100 keV range. The MACS values at $kT=30 \mathrm{keV}$ for ${}^{107}\mathrm{Ag}$ $(887 \pm 89 \mathrm{mb})$ and ${}^{109}\mathrm{Ag}$ $(903 \pm 90 \mathrm{mb})$ are slightly higher than the recommended values in the Karlsruhe Astrophysical Database of Stellar Nucleosynthesis.

Linear response theory is a well-established method in physics and chemistry for exploring excitations of many-body systems. In particular, the quasiparticle random-phase approximation (QRPA) provides a powerful microscopic framework by building excitations on top of the mean-field vacuum; however, its high computational cost limits model calibration and uncertainty quantification studies. Here, we present two complementary QRPA surrogate models and apply them to study response functions of finite nuclei. One is a reduced-order model that exploits the underlying QRPA structure, while the other utilizes the recently developed parametric matrix model algorithm to construct a map between the system’s Hamiltonian and observables. Our benchmark applications, the calculation of the electric dipole polarizability of ${}^{180}\mathrm{Yb}$ and the $\beta$-decay half-life of ${}^{80}\mathrm{Ni}$, show that both emulators can achieve 0.1%–1% accuracy while offering a 6–7 orders of magnitude speedup compared to state-of-the-art QRPA solvers. These results demonstrate that the developed QRPA emulators are well positioned to enable Bayesian calibration and large-scale studies of computationally expensive physics models describing the properties of many-body systems.

When a water drop falls from a faucet, the drop is created with the formation of an axisymmetric constriction region, which thins down to breakup. Such formation of a fluid drop has been extensively studied as a representative of the singular dynamics widely observed in nature. The singular dynamics is often self-similar, i.e., shapes at different times collapsing onto a master curve after rescaling, and the self-similar dynamics has been categorized as either “universal” or “nonuniversal”: The master curve is independent of or dependent on the length scales that set the initial boundary conditions, as if memory is erased or retained. Here, we focus on the post-breakup dynamics and confine the system to break the axisymmetry, introducing three length scales, which leads to a third category: “incomplete universality” where memory is partially retained: The master curve could be dependent on the smallest scale but independent of the other two scales. The fact that only the smallest length scale can affect the shape of the master curve suggests that scale separation is deeply related to the emergence of universality. The present study suggests a promising direction for the study on the singular dynamics by exploring the symmetry.

Motivated by the recent introduction of a $U\left(1\right)$-symmetric toric code (TC) model, we investigate symmetry-based deformations of topological order by systematically deconstructing the Gauss-law-enforcing star terms of the TC Hamiltonian. This “term-dropping” protocol introduces global symmetries that go beyond the alternative framework of “ungauging” topological order in symmetry-deformed models and gives rise to models such as the $U\left(1\right)\mathrm{TC}$ or $XY\mathrm{TC}$. These models inherit (emergent) subsystem symmetries (from the original 1-form symmetry of the TC) that can give rise to (subextensive) ground-state degeneracies, which can still be organized by the eigenvalues of Wilson loop operators. However, we demonstrate that these models do not support topological or fracton order (as has been conjectured in the literature) due to the loss of (emergent) gauge symmetry. An extreme deformation of the TC is the quantum dimer model (QDM), which we discuss along the family of symmetry-deformed models from the perspective of subsystem symmetries, sublattice modulation, and quantum order-by-disorder mechanisms resulting in rich phase diagrams. For the QDM, this allows us to identify an emergent SO(2) symmetry for what appears to be a gapless ground state (by numerical standards) that is unstable to the formation of a plaquette valence bond solid upon sublattice modulation.

Nuclear spins of noble gases exhibit exceptionally long coherence times and can potentially serve as a long-lived storage medium for quantum information. We analyze and compare the performance of two mechanisms for mapping the quantum state of light onto the collective spin state of noble gases. The first mechanism utilizes collisional exchange with the electronic spin state of metastable noble-gas atoms, while the second relies on spin-exchange collisions with ground-state alkali-metal atoms. We describe the operation of an optical quantum memory relying on these two mechanisms using a compact model and study strategies that optimize the memory storage efficiency. Through numerical simulations, we identify optimal sequences for storing optical signals with different signal bandwidths and electronic spin relaxation rates. This work highlights the qualitative difference between the two approaches for using noble gases as long-lived quantum memories at noncryogenic conditions and outlines the regimes in which they are expected to be efficient.

We introduce QC-CBT-AFQMC, a hybrid algorithm that incorporates computational basis tomography (CBT) into the quantum-classical auxiliary field quantum Monte Carlo (QC-AFQMC) method proposed by Huggins et al. [Nature (London) 603, 416 (2022)], replacing the use of classical shadows. While the original QC-AFQMC showed high accuracy for quantum chemistry calculations, it required exponentially costly postprocessing. Subsequent work using Matchgate shadows [Wan et al., Commun. Math. Phys. 404, 629 (2023)] improved scalability, but still suffers from prohibitive computational requirements that limit practical applications. Our QC-CBT-AFQMC approach uses shallow Clifford circuits with a quadratic reduction of two-qubit gates over the original algorithm, significantly reducing computational requirements and enabling accurate calculations under limited measurement budgets. We demonstrate its effectiveness on the hydroxyl radical, ethylene, and nitrogen molecule, producing potential energy curves that closely match established benchmarks. We also examine the influence of CBT measurement counts on accuracy, showing that subtracting the active space AFQMC energy mitigates measurement-induced errors. Furthermore, we apply QC-CBT-AFQMC to estimate reaction barriers in $[3+2]$ cycloaddition reactions, achieving agreement with high-level references and successfully incorporating complete basis set extrapolation techniques. These results highlight QC-CBT-AFQMC as a practical quantum-classical hybrid method that bridges the capabilities of quantum devices and accurate chemical simulations.

Continuous-variable (CV) quantum computing is a promising candidate for quantum computation because it can, even with one mode, utilize infinite-dimensional Hilbert spaces and can efficiently handle continuous values. Although photonic platforms have been considered as a leading platform for CV computation, hybrid systems that use both qubits and bosonic modes, e.g., superconducting hardware, have shown significant advances because they can prepare non-Gaussian states by utilizing the nonlinear interaction between the qubits and the bosonic modes. However, the size of hybrid hardware is currently restricted. Moreover, the fidelity of the non-Gaussian state is also restricted. This work extends the projective squeezing method to establish a formalism for projecting quantum states onto the states that are unitary-transformed from the squeezed vacuum at the expense of the sampling cost. Based on this formalism, we propose methods for simulating larger quantum devices and projecting states onto the cubic phase state, a typical non-Gaussian state, with a higher squeezing level and higher nonlinearity. To make implementation practical, we can, by leveraging the interactions in hybrid systems of qubits and bosonic modes, apply the smeared projector by using either the linear-combination-of-unitaries or virtual quantum error detection algorithms. We numerically verify the performance of our methods and show that projection can suppress the effect of photon-loss errors.

Since the discovery of heavy-fermion superconductivity in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$, the material has attracted great interest, particularly with regard to the nature of the superconducting pairing and its mechanism. Consequently, it is essential to better understand the electronic Fermi surface topology and its role in strong antiferromagnetic fluctuations. The standard density functional theory method is insufficient to model the interplay of strong on-site Coulomb repulsion in localized $4f$ electrons and their hybridization with itinerant ligand-orbital electrons. We have performed electronic ground-state calculations on ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ using the Gutzwiller wave function approximation. The Gutzwiller approximation captures the quasiparticle band renormalization from the strong on-site Coulomb repulsion. We have performed an analysis of this effect on the electronic structure and the Fermi surface topology by varying the interaction strength and taking into account the crystal-field splitting. Using the de Haas-van Alphen effect, the extremal Fermi surface cross-sectional areas were calculated to quantify the effects of quasiparticle mass renormalization on the Fermi surface. Our results confirm the presence of two Fermi surface sheets corresponding to the heavy ($488{\mathit{m}}_e$) and light ($4.35{\mathit{m}}_e$) quasiparticles when the crystal-field splitting is accounted for on equal footing with the electronic correlations. This method gives the best agreement with experimental measurements as well as the renormalized band method.

Atomtronic devices are matter-wave circuits designed to emulate the functional behavior of their electronic counterparts. Motivated by superconducting quantum interference devices (SQUIDs), atomic quantum interference devices (AQUIDs) have been developed using Bose-Einstein condensates (BECs) confined in toroidal geometries. Here, we propose and numerically investigate an alternative implementation of an AQUID based on a BEC confined in a rotating-box potential. A ringlike topology is established by introducing a central depletion region via a repulsive potential barrier. We observe the hallmark AQUID feature—quantized phase winding that increases in discrete steps with angular velocity. Centrifugal effects induced by rotation degrade phase coherence and impair AQUID performance, which we mitigate by applying a counteracting harmonic confinement. Phase slips are found to be mediated by a vortex propagating from the central depletion zone to the edge of the condensate. To characterize the voltage response, we induce a bias current by translating the box along its long axis while keeping the central barrier fixed. This generates a density imbalance between the two reservoirs, exhibiting a periodic dependence on angular velocity—analogous to the voltage-flux relation in electronic SQUIDs. Our results demonstrate that rotating-box geometries provide a viable and flexible platform for realizing AQUIDs with controllable dynamics and well-defined response characteristics.

We investigate the use of driven qubits coupled to a harmonic oscillator to implement a $\sqrt{i\mathrm{SWAP}}$ gate. By dressing the qubits through an external driving field, the qubits and the harmonic oscillator can be selectively coupled, allowing for the measurement of individual qubit states, as well as leading to effective qubit-qubit interactions. We compare the qubit readout on bare and dressed qubits, and demonstrate that when coupled to low-frequency resonators, dressed qubits provide a more robust readout than bare qubits in the presence of damping and thermal effects. Furthermore, we study the impact of various system parameters on the fidelity of the two-qubit gate, identifying an optimal range for quantum computation. Our findings guide the implementation of high-fidelity quantum gates in experimental setups, for example, those employing nanoscale mechanical resonators.

We present a framework for developing quantum thermal machines—a quantum battery and a quantum Otto engine—using a single nitrogen-vacancy (NV) center coupled to a low-frequency mechanical oscillator. These machines are engineered to operate in a regime where the oscillator's frequency is much smaller than the NV center’s spin transition and relaxation rates. By employing carefully timed microwave pulses and rapid optical spin resets, we enable on-demand, coherent energy transfer between the NV center and the oscillator. In the quantum battery mode, energy is incrementally stored in or extracted from the oscillator's long-lived motional states via discrete spin flips. Furthermore, spin-driven protocols facilitate a cooling mechanism that reduces the oscillator's effective phonon population below thermal equilibrium, even for frequencies lower than the spin's decoherence scale, effectively engineering a cold bath. Integrating this spin-cooling mechanism into an Otto cycle while maintaining contact with a hot reservoir enables the realization of a single-bath quantum heat engine. This engine generates mechanical work with near-optimal efficiency in finite time without requiring environmental decoupling.

The phenomenon of quantum many-body scars (QMBS) has been studied both theoretically and experimentally, due to its unusual violation of the eigenstate thermalization hypothesis (ETH). In this paper, we extend the ETH to a description based on the grand canonical ensemble to depict the thermal properties of QMBS models. For this purpose, we embed the dynamics of kinetically constrained systems within a Lindblad-like master equation, and demonstrate that the violation of the ETH by scar eigenstates is related to their slow decay in the corresponding dissipative process. Within this open system description, we reformulate the ETH to demonstrate that both scar eigenstates and thermal ones exhibit thermalization governed by grand canonical statistics. Consequently, our revised ETH unifies scars and thermal states under a cohesive thermodynamic rule. Our work resolves the fundamental tension between constraint-induced nonergodicity and thermalization paradigms, establishing a unified route to generalized thermalization in quantum many-body systems.

We illustrate a theoretical study of a technique using high-energy muon beams (TeV class) propagating through thick (kilometer-long) crystalline rock layers subject to tectonic-induced stress, potentially capable of actively monitoring the temporal evolution of the pressure rise in seismic fault zones associated with earthquake triggering when the induced tectonic pressure reaches and overcomes the rock elastoplastic deformation limit. This technique could contribute to improving earthquake forecasting statistics in seismically active regions, offering support for seismic hazard assessment and prevention strategies. Active monitoring of the induced tectonic stress and its time evolution is achieved by remote sensing of the electric field generated in quartz crystals embedded in crystalline rocks by piezoelectric effects. In this context, tectonic pressure refers to the time-dependent stress field acting on the rock body due to tectonic forces, which adds to the time-independent lithostatic pressure resulting from the weight of overlying materials. High-energy muon beams transmitted through a rock layer subject to tectonic pressure will be affected in their transverse phase-space distributions by the piezoelectric fields, therefore transferring to a detector the information on the applied tectonic stress. Finally, we illustrate the design of a proof-of-principle experiment to be conducted in a standard accelerator laboratory, using moderate-energy muons (GeV class) propagating through granite slabs subject to a press-induced stress reaching the rupture limit. A zero-generation proof-of-principle test can also be performed using 20–150 MeV electron beams transmitted through single quartz crystals subject to variable pressure.