A parametrically driven oscillator has two stable vibrational states at half the modulation frequency. The states have opposite phases and equal amplitudes. An extra drive at half the modulation frequency provides an effective bias that lifts the state symmetry. Quantum fluctuations lead to switching between the states, i.e., to phase-flip transitions. We develop a semiclassical approach that allows us to find the dependence of the switching rates on the amplitude of the bias and the parameters of the modulating field. We find that the rate of switching from a “shallow” state can become anomalously small at certain parameter values, leading to an efficient localization in this state. This is a consequence of the change of the topology of the oscillator phase trajectories. The results pave the way for implementing nonreciprocal quantum Ising systems based on parametric oscillators.

Semiconductor-superconductor hybrid nanocircuits are of high interest because of their potential applications in quantum computing. Semiconductors with strong spin-orbit coupling and large $g$ factor are particularly attractive since they are the basic building blocks of novel qubit architectures. However, for the engineering of these complex circuits, the building blocks must be characterized in detail. We have investigated a Josephson junction where the weak link is a two-dimensional electron gas (2DEG) hosted in an InAs/InGaAs heterostructure grown on a GaAs substrate. We employed the in situ epitaxially grown Al layer as a superconducting contact to form an rf SQUID, and also to create a microwave resonator for sensing the Josephson inductance. We determined the gate-dependent current-phase relation and observed supercurrent interference in out-of-plane magnetic fields. With the application of an in-plane magnetic field, we induced asymmetry in the interference pattern, which was found to be anisotropic in the device plane.

We introduce and analyze an active steering protocol designed to target multipartite entangled states. The protocol involves multiple qubits subjected to weak Bell-pair measurements with active feedback, where the feedback operations are optimized to maximize the quantum Fisher information. Our scheme efficiently reaches a genuinely entangled one-parameter state manifold. Numerical simulations for systems with up to 22 qubits suggest that the protocol is scalable and allows for high multipartite entanglement across the system.

The investigation of methods to control and optimize the physical properties of the intrinsic magnetic topological insulator (TI) ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ is a critical challenge for the development of functional materials for quantum technologies and spintronics. The promising approach is to substitute Mn atoms with Ge, Sn, or Pb atoms in the $\left({\mathrm{Mn}}_{1-x}{A}_x^{\mathrm{IV}}\right){\mathrm{Bi}}_2{\mathrm{Te}}_4$ ($A^{\mathrm{IV}}=\mathrm{Ge},\mathrm{Sn},\mathrm{Pb}$) solid solutions. This substitution enables manageable tuning of the system's magnetic and electronic properties. In this study, we present a detailed investigation of the electronic structure evolution in $\left({\mathrm{Mn}}_{1-x}{\mathrm{Pb}}_x\right){\mathrm{Bi}}_2{\mathrm{Te}}_4$ as a function of Pb concentration, using a variety of angle-resolved photoemission spectroscopy based techniques, including photon-energy-dependent studies, spin-resolved, and circular dichroism (CD) measurements. Special emphasis was placed on identifying experimental evidence of the theoretically predicted topological phase transitions (TPTs) in $\left({\mathrm{Mn}}_{1-x}{\mathrm{Pb}}_x\right){\mathrm{Bi}}_2{\mathrm{Te}}_4$ near $x\approx 50$%. The criteria for detecting TPT include the presence or absence of topological surface states (TSS) in the electronic structure, which can be identified by their characteristic helical spin structure in spin-resolved or CD spectra, as well as the closure of the bulk band gap. Our results show that the bulk gap in the ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ -like electronic structure decreases gradually with increasing Pb concentration up to 40%, where it almost closes. From 40% to 60%, the band gap remains unchanged, and above 80%, the ${\mathrm{PbBi}}_2{\mathrm{Te}}_4$ -like electronic structure emerges, with the bulk gap reopening. Additionally, the TSS were detected in $\left({\mathrm{Mn}}_{1-x}{\mathrm{Pb}}_x\right){\mathrm{Bi}}_2{\mathrm{Te}}_4$ samples with Pb concentrations up to at least 30% and beyond 80%, correlating with the regions where the bulk gap is open. However, no TSS were observed at $x\approx 55$%, indicating that the system is in topologically distinct phase compared to ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ or ${\mathrm{PbBi}}_2{\mathrm{Te}}_4$. At this concentration, the system may be in a semimetallic state or a trivial insulator phase with a very narrow bulk gap. The demonstrated tunability of the electronic structure in $\left({\mathrm{Mn}}_{1-x}{\mathrm{Pb}}_x\right){\mathrm{Bi}}_2{\mathrm{Te}}_4$ highlights its potential for further exploration in topological and spintronic applications.

The codespace of a quantum error-correcting code can often be identified with the degenerate ground space within a gapped phase of quantum matter. We argue that the stability of such a phase is directly related to a set of coherent error processes against which this quantum error-correcting code (QECC) is robust: such a quantum code can recover from adiabatic noise channels, corresponding to random adiabatic drift of code states through the phase, with asymptotically perfect fidelity in the thermodynamic limit, as long as this adiabatic evolution keeps states sufficiently “close” to the initial ground space. We further argue that when specific decoders—such as minimum-weight perfect matching—are applied to recover this information, an error-correcting threshold is generically encountered within the gapped phase. In cases where the adiabatic evolution is known, we explicitly show examples in which quantum information can be recovered by using stabilizer measurements and Pauli feedback, even up to a phase boundary, though the resulting decoding transitions are in different universality classes from the optimal decoding transitions in the presence of incoherent Pauli noise. This provides examples where nonlocal, coherent noise effectively decoheres in the presence of syndrome measurements in a stabilizer QECC.

Microwave shielding is an important technique that can suppress the losses that arise from collisions of ultracold polar molecules. It has been instrumental in achieving molecular Bose-Einstein condensation (BEC) for NaCs [Nature (London) 631, 289 (2024)]. We demonstrate that microwave shielding is universal, in the sense that the two-body collision properties of different molecules are very similar when expressed in suitable reduced units of length and energy. This applies to rate coefficients for inelastic scattering and loss, to scattering lengths, and to the properties of two-molecule bound states. We also explore the small deviations from universality that arise at very large Rabi frequencies. In general, the collision properties are near-universal except when the Rabi frequency exceeds a few percent of the molecular rotational constant. The universality extends to elliptically polarized microwaves and to combinations of multiple fields. Our results indicate that the methods that have been used to achieve BEC for NaCs can be transferred directly to most other polar molecules.

We introduce the concept of quantum weight as a ground-state property of quantum many-body systems that is encoded in the static structure factor and characterizes density fluctuation at long wavelengths. The quantum weight carries a wealth of information about dielectric responses and optical properties of the system and is closely related to its quantum geometry. For systems with short-range interactions or low-dimensional Coulomb systems, we show that the many-body quantum metric (which measures the change of the ground state under twisted boundary conditions) can be determined directly from the quantum weight. Notably, the quantum weight is a property of a single ground state and independent of boundary conditions in the thermodynamic limit. Our finding thus enables direct experimental measurement and numerical calculation of the many-body quantum metric. On the other hand, for three-dimensional Coulomb systems, we show that the quantum weight is distinct from the many-body quantum metric due to dielectric screening in three dimensions. We further use dielectric sum rules to derive upper and lower bounds on the quantum weight of real materials in terms of their electron density, static dielectric constant, and plasmon energy. Our work highlights quantum weight as a fundamental material parameter, which can be experimentally determined by x-ray scattering or electron-loss spectroscopy.

Arrays of superconducting qubits and cavities offer a promising route for realizing artificial materials. However, many analog simulations on superconducting circuit hardware have focused on bosonic systems. Fermionic simulations, in contrast, have largely relied on digital approaches. Here, we propose and study an alternative approach for analog fermionic quantum simulation based on arrays of coherently coupled mesoscopic Josephson junctions. These Josephson junction arrays implement an effective superlattice of Andreev bound state atoms that can trap individual fermionic quasiparticles and, due to their wave-function overlap, mediate quasiparticles hoppings. By developing a Wannier function approach, we show that these Andreev bound state arrays form an all-superconducting platform for emulating lattice models of fermionic quasiparticles that are phase and gate programmable. Interestingly, the junction lattices can also undergo a topological transition and host fermionic boundary modes that can be probed by conductance measurements. We hope our results will inspire the realization of artificial and topological materials on Andreev bound state quantum simulators.

We study quantum-fluctuation-driven fractionalized phases in the vicinity of altermagnetic order. First, the long-range magnetic orders in the vicinity of collinear altermagnetism are identified; these feature a noncoplanar orbital altermagnet, which has altermagnetic symmetries in spin-rotation-invariant observables. We then describe neighboring fractionalized phases with topological order reached when quantum fluctuations destroy long-range spin order, within Schwinger boson theory and an SU(2) gauge theory of fluctuating magnetism. Discrete symmetries remain broken in some of the fractionalized phases, with the orbital altermagnet becoming an altermagnetic spin liquid. We compute the electronic spectral function in the doped system, which is characterized by split Fermi surfaces with preserved spin-rotation symmetry.

We report on the discovery of superconductivity in the previously unknown compound ${\mathrm{Ta}}_4{\mathrm{Rh}}_2{\mathrm{C}}_{1-\delta }$. ${\mathrm{Ta}}_4{\mathrm{Rh}}_2{\mathrm{C}}_{1-\delta }$ crystallizes in the $\eta$-carbide structure type, in the cubic space group $Fd\overline{3}m$ (No. 227) with a unit cell parameter of $a=11.7947$ Å. Temperature-dependent magnetic susceptibility, resistivity, and specific-heat capacity measurements reveal that ${\mathrm{Ta}}_4{\mathrm{Rh}}_2{\mathrm{C}}_{1-\delta }$ is a type-II bulk superconductor with a critical temperature of $T_{\mathrm{c}}=6.4\mathrm{K}$ and a normalized specific-heat jump $\mathrm{\Delta }C/\gamma T_{\mathrm{c}}=1.56$. Notably, we find that ${\mathrm{Ta}}_4{\mathrm{Rh}}_2{\mathrm{C}}_{1-\delta }$ has a high upper critical field of ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)=17.4\mathrm{T}$, which exceeds the BCS weak-coupling Pauli limit of ${\mu }_0{H}_{\mathrm{Pauli}}=11.9\mathrm{T}$.

Prethermalization occurs as an important phase in the dynamics of isolated many-body systems when coupled degrees of freedom relax in a subspace separated from a state of complete thermodynamic equilibrium by a gap in energy or other conserved quantity. Slow equilibration from a prethermal phase can localize the dynamics of natural and model systems despite high dimensionality and limited disorder. Here, we report the signature of an enduring prethermal regime of arrested relaxation in the molecular ultracold plasma that forms following the avalanche of a state-selected Rydberg gas of nitric oxide. For a wide range of initial conditions, this system enters a critical phase in which a density of ${\mathrm{NO}}^{+}$ and electrons balances a population of Rydberg molecules. Electron collisions mix orbital angular momentum, scattering Rydberg electrons to states of very high $\ell$. The rapid predissociation of molecules in states of low-$\ell$ purifies this high-$\ell$ ensemble. The angular momentum barrier that separates ${\mathrm{NO}}^{+}$ ions and electrons creates an extraordinary gap between the plasma states of $n\approx \ell$, with measured $n>200$ and penetrating states of $\ell =0,1$, and 2. Evolution to a statistically equilibrated state of N and O atoms cannot occur without Rydberg electron penetration, and this gap blocks relaxation for a millisecond or more. A weak radiofrequency (RF) field drives $\ell$-mixing electron collisions that erase this gap, causing wholesale dissipation. Remarkably, a local quantum-state transition promoting an exceedingly small fraction of the molecules in the prethermalized ensemble to a predissociative state also acts with a global effect, driving the entire system to a dissipative equilibrium. Using the Lindblad master equation, we illustrate qualitatively similar dynamics for a toy model of an open quantum system that consists of a localized set of spins on which dissipation acts at a single site as a gateway to equilibrium.

We demonstrate that the widely used plasma analogy is unreliable at predicting edge properties of quantum Hall states. This discrepancy arises from a fundamental difference between quantum Hall droplets and plasmas (Coulomb gases): The former are incompressible liquids subject to area-preserving deformations, while the latter are governed by electrostatics and thus involve conformal transformations. Consequently, the plasma analogy fails at the edge, except in fine-tuned geometries, as it does not account for the emergent local edge velocity. We quantitatively show how the analogy's failure affects physical quantities, such as fluctuations of local observables and absorption rates in microwave spectroscopy, measurable in both solid-state experiments and quantum simulators.

Exceptional points are special degeneracy points in parameter space that can arise in (effective) non-Hermitian Hamiltonians describing open quantum and wave systems. At an $n\mathrm{th}$-order exceptional point, $n$ eigenvalues and the corresponding eigenvectors simultaneously coalesce. These coalescing eigenvalues typically exhibit a strong response to small perturbations that can be useful for sensor applications. A so-called generic perturbation with strength $\epsilon$ changes the eigenvalues proportional to the $n\mathrm{th}$ root of $\epsilon$. A different eigenvalue behavior under perturbation is called nongeneric. An understanding of the behavior of the eigenvalues for various types of perturbations is desirable and also crucial for applications. We advocate a graph-theoretical perspective that contributes to the understanding of perturbative effects on the eigenvalue spectrum of higher-order exceptional points, i.e., $n>2$. Our approach is mathematically derived and the intuitive consequences elucidated. To highlight the relevance of nongeneric perturbations and to give a physical interpretation for their occurrence, we consider an illustrative example, a system of microrings coupled by a semi-infinite waveguide with an end mirror. Furthermore, the saturation effect occurring for cavity-selective sensing in such a system is naturally explained within the graph-theoretical picture.

Quantum matter phases may coexist microscopically even when they display competing tendencies. A fundamental question is whether such a competition can be avoided through the elimination of one phase while the other one condenses into the ground state. Here, we present a high-resolution neutron spectroscopy study of the low-energy spin excitations in the high-temperature superconductor ${\mathrm{La}}_{1.855}{\mathrm{Sr}}_{0.145}{\mathrm{CuO}}_4$. In the normal state, we find low-energy magnetic fluctuations at incommensurate reciprocal lattice positions where spin-density-wave order emerges at lower Sr concentration or at high magnetic fields. While these spin excitations are largely suppressed by the emergence of the superconducting spin gap, some low-energy magnetic fluctuations persist deep inside the superconducting state. We interpret this result in terms of a dynamic competition between superconductivity and magnetism, where superconductivity impedes the condensation of low-energy magnetic fluctuations through the formation of magnetically mediated Cooper pairs.

In the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A =$ K, Rb, Cs) kagome materials, superconductivity coexists with a charge density wave (CDW), constituting a new platform to study the interplay of these two orders. Despite extensive research, the symmetry of the superconducting order parameter remains disputed, with experiments seemingly supporting different conclusions. As key aspects of the physics might lie in the intertwining of electronic orders, a better understanding of the impact of the CDW on superconductivity is crucial. In this paper, we develop a phenomenological framework to study the interplay of superconductivity and CDW order. In particular, we derive a Ginzburg-Landau free energy for both superconducting and CDW order parameters. Given the unclear nature of the superconducting state, we discuss general pairing symmetries with a focus on $s$-wave, $d$-wave, and pair-density-wave order parameters. Motivated by experiments, we consider the additional breaking of time-reversal or point-group symmetries of the CDW and determine in detail the consequences for the superconducting state. Our results show how the superconducting state mimics the broken symmetries of the CDW and can guide future microscopic calculations, as well as the experimental identification of the superconducting state in the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ compounds.

Electron-hole pairs in semiconductors are essential for solar cells and fast electronic circuitry, but the competition between carrier transport and relaxation into heat limits the efficiency and speed. Here we use ultrafast electron diffraction with terahertz pulse compression to measure the electron-phonon decay rate in single-crystal silicon as a function of laser excitation strength. We find that the excited electrons relax slower into phonons for higher carrier densities. The electron-phonon scattering rate changes in a nonlinear way from 400 fs at $\sim 2\times {10}^{20}/\mathrm{c}{\mathrm{m}}^3$ to 1.2 ps at $\sim 4\times {10}^{20}/\mathrm{c}{\mathrm{m}}^3$. These results indicate that a hot electron gas quenches the scattering into phonons in a temperature-dependent way. Ultrafast electronic circuitry of silicon should therefore work faster and provide higher bandwidths at lower carrier densities.

Density functional theory (DFT) offers a desirable balance between quantitative accuracy and computational efficiency in practical many-electron calculations. Its central component, the exchange-correlation energy functional, has been approximated with increasing levels of complexity ranging from strictly local approximations to nonlocal and orbital dependent expressions with many tuned parameters. In this paper, we formulate a general way of rewriting complex density functionals using deep neural networks in a way that allows for simplified computation of Kohn-Sham potentials as well as higher functional derivatives through automatic differentiation, enabling access to highly nonlinear response functions and forces. These goals are achieved by using a recently developed class of robust neural network models capable of modeling functionals, as opposed to functions, with explicitly enforced spatial symmetries. Functionals treated in this way are then called global density approximations and can be seamlessly integrated with existing DFT workflows. Tests are performed for a dataset featuring a large variety of molecular structures and popular meta–generalized gradient approximation density functionals, where we successfully eliminate orbital dependencies coming from the kinetic energy density, and discover a high degree of transferability to a variety of physical systems. The presented framework is general and could be extended to more complex orbital and energy dependent functionals as well as refined with specialized datasets.

We explore the behavior of the Hall response of a Bose-Hubbard triangular ladder in a magnetic field as a function of the repulsive on-site atomic interactions. We consider a wide range of interaction strengths, from the weakly interacting limit to the hard-core regime. This is realized by computing the Hall polarization following the quench of a weak linear potential, which induces the flow of a current through the system, using time-dependent matrix product state numerical simulations. We complement our understanding in the regime of small magnetic fields by analytical calculations of the equilibrium value of the Hall polarization for noninteracting bosonic atoms and under a mean-field assumption. The Bose-Hubbard triangular flux ladder exhibits a rich phase diagram, containing Meissner, vortex, and biased-chiral, superfluid phases. We show that the Hall response can be employed to fingerprint the various chiral states, the frustration effects occurring in the limit of strong interactions, and the phase boundaries of the equilibrium phase diagram.

Magnon condensation, which emerges in pumped bosonic systems at room temperature, continues to garner great interest for its long-lived coherence. While traditionally formulated in terms of Bose-Einstein condensation, which typically occurs at ultralow temperatures, it could potentially also be explained by Fröhlich condensation, a hypothesis of Bose-Einstein-like condensation in living systems at ambient temperatures. This prompts general questions relating to fundamental differences between coherence phenomena in open and isolated quantum systems. To that end, we introduce a simple model of bosonic condensation in an open quantum system (OQS) formulation, wherein bosons dissipatively interact with an oscillator (phonon) bath. Our derived equations of motion for expected boson occupations turn out to be similar in form to the rate equations governing Fröhlich condensation. Provided that specific system parameters result in correlations that amplify or diminish the condensation effects, we thereby posit that our treatment offers a better description of high-temperature condensation compared to traditional formulations obtained using equilibrium thermodynamics. By comparing our OQS derivation with the original uncorrelated and previous semiclassical rate equations, we furthermore highlight how both classical anticorrelations and quantum correlations alter the bosonic occupation distribution.