Environment is assumed to play a negative role in quantum mechanics, destroying the coherence in a quantum system and, thus, randomly changing its state. However, for a quantum system that is initially in a degenerate ground state, the situation could be different. In this case, the infinite manifold of ground state eigenfunctions can contain a few states of zero entanglement, which can be demonstrated based on the minimization of the von Neumann entropy. Then, following quantum Darwinism, these “classical” combinations are selected and promoted by the quantum environment, which means that different independent observers find them in experiments. In this work, we find and explore such classical states in the eigenspectra of skyrmionic and antiferromagnetic quantum systems starting from a numerical realization of Anderson’s tower of states. The degeneracy of the quantum ground state is shown to be the key for explaining nontrivial properties of magnetic matter in the classical world including topological protection arising in the classical limit.

Twisted bilayer ${\mathrm{MoTe}}_2$ is a promising platform to investigate the interplay between band topology and many-body interactions. We present a theoretical study of its interaction-driven quantum phase diagrams based on a three-orbital model, which can be viewed as a generalization of the Kane-Mele-Hubbard model with one additional orbital and long-range Coulomb repulsion. We predict a cascade of phase transitions tuned by the twist angle $\theta$. At the hole-filling factor $\nu =1$ (one hole per moiré unit cell), the ground state can be in the multiferroic phase, with coexisting spontaneous layer polarization and magnetism; the quantum anomalous Hall phase; and finally, the topologically trivial magnetic phases, as $\theta$ increases from 1.5° to 5°. At $\nu =2$, the ground state can have a second-order phase transition between an antiferromagnetic phase and the quantum spin Hall phase as $\theta$ passes through a critical value. The dependence of the phase boundaries on model parameters, such as the gate-to-sample distance, the dielectric constant, and the moiré potential amplitude, is examined. The predicted phase diagrams can guide the search for topological phases in twisted transition metal dichalcogenide homobilayers.

The quantum Jarzynski equality and the Crooks relation are fundamental laws connecting equilibrium processes with nonequilibrium fluctuations. They are promising tools to benchmark quantum devices and measure free energy differences. While they are well established theoretically and also experimental realizations for few-body systems already exist, their experimental validity in the quantum many-body regime has not been observed so far. Here, we present results for nonequilibrium protocols in systems with up to 16 interacting degrees of freedom obtained on trapped ion and superconducting qubit quantum computers, which test the quantum Jarzynski equality and the Crooks relation in the many-body regime. To achieve this, we overcome present-day limitations in the preparation of thermal ensembles and in the measurement of work distributions on noisy intermediate-scale quantum devices. We discuss the accuracy to which the Jarzynski equality holds on different quantum computing platforms subject to platform-specific errors. The analysis reveals the validity of Jarzynski’s equality in a regime with energy dissipation, compensated for by a fast unitary drive. This provides new insights for analyzing errors in many-body quantum simulators.

Topological superconductivity is a long-sought state of matter in bulk materials, and the odd-parity superconductor ${\mathrm{UTe}}_2$ is a prime candidate. The recent observation of a field-trainable spontaneous Kerr signal in ${\mathrm{UTe}}_2$ at the onset of superconductivity provides strong evidence that the superconducting order parameter is multicomponent and breaks time-reversal symmetry. Here, we perform Kerr effect measurements on a number of ${\mathrm{UTe}}_2$ samples—grown via both chemical vapor transport and the molten-salt-flux methods—that show a single superconducting transition between 1.6 K and 2.1 K. Our results show no evidence for a spontaneous Kerr signal in zero-field measurements. This implies that the superconducting state of ${\mathrm{UTe}}_2$ does not intrinsically break time-reversal symmetry. Instead, we observe a field-trainable signal that varies in magnitude between samples and between different locations on a single sample, which is a sign of inhomogeneous magnetic regions. Our results provide an examination of representative ${\mathrm{UTe}}_2$ samples and place strong constraints on the superconducting order parameter of ${\mathrm{UTe}}_2$.

Quantum materials display rich and myriad types of magnetic, electronic, and structural ordering, often with these ordering modes either competing with one another or “intertwining,” that is, reinforcing one another. Low-dimensional quantum materials influenced strongly by competing interactions and/or geometric frustration are particularly susceptible to such ordering phenomena and thus offer fertile ground for understanding the consequent emergent collective quantum phenomena. Such is the case of the quasi-2D materials ${\mathrm{R}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($R=\mathrm{La}$, Pr), in which intertwined charge- and spin-density waves (CDW and SDW) on the Ni sublattice have been identified and characterized. Not unexpectedly, these density waves are largely quasi-2D as a result of weak coupling between planes, compounded with magnetic frustration. In the case of $R=\mathrm{Pr}$, however, we show here that exchange coupling between the transition-metal and rare-earth sublattices upon cooling overcomes both obstacles, leading to a dimensional crossover into a fully 3D-ordered and coupled SDW state on both sublattices, as an induced moment on notionally nonmagnetic ${\mathrm{Pr}}^{3+}$ opens exchange pathways in the third dimension. In the process, the structure of the SDW on the Ni sublattice is irreversibly altered, an effect that survives reheating of the material until the underlying CDW melts. This “bootstrapping” mechanism linking incommensurate SDWs on the two sublattices illustrates a new member of the multitude of quantum states that low-dimensional magnets can express, driven by coupled orders and modulated by frustrated exchange pathways.

A time crystal is a state of periodically driven matter that breaks discrete time-translation symmetry. Time crystals have been demonstrated experimentally in various programmable quantum simulators, and they exemplify how nonequilibrium, driven quantum systems can exhibit intriguing and robust properties absent in systems at equilibrium. These robust driven states need to be stabilized by some mechanism, with the preeminent candidates being many-body localization and prethermalization. This introduces additional constraints that make it challenging to experimentally observe time crystallinity in naturally occurring systems. Recent theoretical work has developed the notion of prethermalization without temperature, expanding the class of time-crystal systems to explain time-crystalline observations at (or near) infinite temperature. In this work, we conclusively observe the emergence of a prethermal $U(1)$ time-crystalline state at quasi-infinite temperature in a solid-state NMR quantum emulator by verifying the requisites of prethermalization without temperature. In addition to observing the signature period-doubling behavior, we show the existence of a long-lived prethermal regime whose lifetime is significantly enhanced by strengthening an emergent $U(1)$ conservation law. Not only do we measure this enhancement through the global magnetization, but we also exploit on-site disorder to measure local observables, ruling out the possibility of many-body localization and confirming the emergence of long-range correlations.

As spin-based quantum processors grow in size and complexity, maintaining high fidelities and minimizing crosstalk will be essential for the successful implementation of quantum algorithms and error-correction protocols. In particular, recent experiments have highlighted pernicious transient qubit frequency shifts associated with microwave qubit driving. Work-arounds for small devices, including prepulsing with an off-resonant microwave burst to bring a device to a steady state, wait times prior to measurement, and qubit-specific calibrations all bode ill for device scalability. Here, we make substantial progress in understanding and overcoming this effect. We report a surprising nonmonotonic relation between mixing chamber temperature and spin Larmor frequency which is consistent with observed frequency shifts induced by microwave and baseband control signals. We find that purposefully operating the device at 200 mK greatly suppresses the adverse heating effect while not compromising qubit coherence or single-qubit fidelity benchmarks. Furthermore, systematic non-Markovian crosstalk is greatly reduced. Our results provide a straightforward means of improving the quality of multispin control while simplifying calibration procedures for future spin-based quantum processors.

Electronic Fabry-Pérot interferometry is a powerful method to probe quasiparticle charge and anyonic braiding statistics in the fractional quantum Hall regime. We extend this technique to the hierarchy $\nu =2/5$ fractional quantum Hall state, possessing two edge modes that in our device can be interfered independently. The outer edge mode exhibits interference similar to the behavior observed at the $\nu =1/3$ state, indicating that the outer edge mode at $\nu =2/5$ has properties similar to the single mode at $\nu =1/3$. The inner mode shows an oscillation pattern with a series of discrete phase jumps indicative of distinct anyonic braiding statistics. After taking into account the impact of bulk-edge coupling, we extract an interfering quasiparticle charge $e^{*}=0.17\pm 0.02$ and anyonic braiding phase ${\theta }_a=(-0.43\pm 0.05)\times 2\pi$, which serve as experimental verification of the theoretically predicted values of $e^{*}=\frac{1}{5}$ and ${\theta }_a=-(4\pi /5)$.

First-principles calculations of phonons are often based on the adiabatic approximation and on Brillouin-zone samplings that might not always be sufficient to capture the subtleties of Kohn anomalies. These shortcomings can be addressed through corrections to the phonon self-energy arising from the low-energy electrons. The exact self-energy involves a product of a bare and a screened electron-phonon vertex [Rev. Mod. Phys. 89, 015003 (2017)]; still, calculations often employ two adiabatically screened vertices, which have been proposed as a reliable approximation for self-energy differences [Phys. Rev. B 82, 165111 (2010)]. We assess the accuracy of both approaches in estimating the phonon spectral functions of model Hamiltonians and the adiabatic low-temperature phonon dispersions of monolayer ${\mathrm{TaS}}_2$ and doped ${\mathrm{MoS}}_2$. We find that the approximate method yields excellent corrections at low computational cost, due to its designed error cancellation to first order, while using a bare vertex could in principle improve these results but is challenging in practice. We offer an alternative strategy based on downfolding to partially screened phonons and interactions [Phys. Rev. B 92, 245108 (2015)]. This is a natural scheme to include electron-electron interactions and tackle phonons in strongly correlated materials and the frequency dependence of the electron-phonon vertex.

We present comprehensive theoretical studies on the lattice relaxation and the electronic structures in general nonsymmetric twisted trilayer graphenes. By using an effective continuum model, we show that the relaxed lattice structure forms a patchwork of moiré-of-moiré domains, where a moiré pattern given by layer 1 and 2 and another pattern given by layer 2 and 3 become locally commensurate. The atomic configuration inside the domain exhibits a distinct contrast between chiral and alternating stacks, which are determined by the relative signs of the two twist angles. In the chiral case, the electronic band calculation reveals a wide energy window ($>50\mathrm{meV}$) with low density of states, featuring sparsely distributed highly one-dimensional electron bands. These one-dimensional states exhibit a sharp localization at the boundaries between supermoiré domains, and they are identified as a topological boundary state between distinct Chern insulators. The alternating trilayer exhibits a coexistence of the flat bands and a monolayerlike Dirac cone, and it is attributed to the formation of moiré-of-moiré domains equivalent to the mirror-symmetric twisted trilayer graphene.

We present an extensive study of resonant two-dimensional (2D) plasmon excitations in grating-gated quantum well heterostructures, which enable an electrical control of periodic charge carrier density profile. Our study combines theoretical and experimental investigations of nanometer-scale $\mathrm{AlGaN}/\mathrm{GaN}$ grating-gate structures and reveals that all terahertz (THz) plasmonic resonances in these structures can be explained only within the framework of the plasmonic crystal model. We identify two different plasmonic crystal phases. The first is the delocalized phase, where THz radiation is absorbed with the entire grating-gate structure that is realized at a weakly modulated 2D electron gas (2DEG) regime. In the second, the localized phase, THz radiation interacts only with the ungated portions of the structure. This phase is achieved by fully depleting the gated regions, resulting in strong modulation. By gate controlling of the modulation degree, we observe a continuous transition between these phases. We also discover that, unexpectedly, the resonant plasma frequencies of ungated parts (in the localized phase) still depend on the gate voltage. We attribute this phenomenon to the specific depletion of the conductive profile in the ungated region of the 2DEG, the so-called edge gating effect. Although we study a specific case of plasmons in $\mathrm{AlGaN}/\mathrm{GaN}$ grating-gate structures, our results have a general character and are applicable to any other semiconductor-based plasmonic crystal structures. Our work represents the first demonstration of an electrically tunable transition between different phases of THz plasmonic crystals, which is a crucial step toward a deeper understanding of THz plasma physics and the development of all-electrically tunable devices for THz optoelectronics.

We present a method for deriving bulk and edge invariants for interacting, many-body localized Floquet systems in two spatial dimensions. This method is based on a general mathematical object which we call a flow. As an application of our method, we derive bulk invariants for Floquet systems without symmetry, as well as for systems with U(1) symmetry. We also derive new formulations of previously known single-particle and many-body invariants. For bosonic systems without symmetry, our invariant gives a bulk counterpart of the rational-valued Gross-Nesme-Vogts-Werner index $p/q$ quantifying transport of quantum information along the edge.

The interplay between strong correlations and topology can lead to the emergence of intriguing quantum states of matter. One well-known example is the fractional quantum Hall effect, where exotic electron fluids with fractionally charged excitations form in partially filled Landau levels. The emergence of topological moiré flat bands provides exciting opportunities to realize the lattice analogs of both the integer and fractional quantum Hall effects without the need for an external magnetic field. These effects are known as the integer and fractional quantum anomalous Hall (IQAH and FQAH) effects. Here, we present direct transport evidence of the existence of both IQAH and FQAH effects in small-angle-twisted bilayer ${\mathrm{MoTe}}_2$. At zero magnetic field, we observe well-quantized Hall resistance of $h/e^2$ around moiré filling factor $\nu =-1$ (corresponding to one hole per moiré unit cell), and nearly quantized Hall resistance of $3h/2e^2$ around $\nu =-2/3$, respectively. Concomitantly, the longitudinal resistance exhibits distinct minima around $\nu =-1$ and $-2/3$. The application of an electric field induces topological quantum phase transition from the IQAH state to a charge transfer insulator at $\nu =-1$, and from the FQAH state to a topologically trivial correlated insulator, further transitioning to a metallic state at $\nu =-2/3$. Our study paves the way for the investigation of fractionally charged excitations and anyonic statistics at zero magnetic field based on semiconductor moiré materials.

The interaction between a mobile quantum impurity and a bosonic bath leads to the formation of quasiparticles, termed Bose polarons. The elementary properties of Bose polarons, such as their mutual interactions, can differ drastically from those of the bare impurities. Here, we explore Bose polaron physics in a two-dimensional nonequilibrium setting by injecting ${\sigma }^{-}$ polarized exciton-polariton impurities into a bath of coherent ${\sigma }^{+}$ polarized polaritons generated by resonant laser excitation of monolayer ${\mathrm{MoSe}}_2$ embedded in an optical cavity. By exploiting a biexciton Feshbach resonance between the impurity and the bath polaritons, we tune the interacting system to the strong-coupling regime and demonstrate the coexistence of two new quasiparticle branches. Using time-resolved pump-probe measurements, we observe how polaron dressing modifies the interaction between impurity polaritons. Remarkably, we find that the interactions between high-energy polaron quasiparticles, which are repulsive for small bath occupancy, can become attractive in the strong impurity-bath coupling regime. Our experiments provide the first direct measurement of Bose polaron-polaron interaction strength in any physical system and pave the way for exploration and control of many-body correlations in driven-dissipative settings.

A short superconducting segment can couple attached quantum dots via elastic cotunneling (ECT) and crossed Andreev reflection (CAR). Such coupled quantum dots can host Majorana bound states provided that the ratio between CAR and ECT can be controlled. Metallic superconductors have so far been shown to mediate such tunneling phenomena, albeit with limited tunability. Here, we show that Andreev bound states formed in semiconductor-superconductor heterostructures can mediate CAR and ECT over mesoscopic length scales. Andreev bound states possess both an electron and a hole component, giving rise to an intricate interference phenomenon that allows us to tune the ratio between CAR and ECT deterministically. We further show that the combination of intrinsic spin-orbit coupling in InSb nanowires and an applied magnetic field provides another efficient knob to tune the ratio between ECT and CAR and optimize the amount of coupling between neighboring quantum dots.

The kagome superconductor family $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{Cs}$, K, Rb) emerged as an exciting platform to study exotic Fermi surface instabilities. Here, we use spectroscopic-imaging scanning tunneling microscopy (SI-STM) and angle-resolved photoemission spectroscopy (ARPES) to reveal how the surprising cascade of higher- and lower-dimensional density waves in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ is intimately tied to a set of small reconstructed Fermi pockets. ARPES measurements visualize the formation of these pockets generated by a 3D charge density wave transition. The pockets are connected by dispersive $q^{*}$ wave vectors observed in Fourier transforms of STM differential conductance maps. As the additional 1D charge order emerges at a lower temperature, $q^{*}$ wave vectors become substantially renormalized, signaling further reconstruction of the Fermi pockets. Remarkably, in the superconducting state, the superconducting gap modulations give rise to an in-plane Cooper pair density wave at the same $q^{*}$ wave vectors. Our work demonstrates the intrinsic origin of the charge stripes and the pair density wave in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ and their relationship to the Fermi pockets. These experiments uncover a unique scenario of how Fermi pockets generated by a parent charge density wave state can provide a favorable platform for the emergence of additional density waves.

Unveiling nonequilibrium dynamics of solitonic and topological defect structures in a multidimensional nonlinear medium is a current frontier across diverse fields. One of the quintessential objects is a ring dark soliton (RDS), whose dynamics are expected to display remarkable interplay between symmetry and self-patterned topological defect formation from a transverse (snake) instability, but it has thus far evaded full experimental observations. Here, we report an experimental realization of RDS generation in a two-dimensional atomic superfluid trapped in a circular box. By quenching the confining box potential, we observe an RDS emitted from the edge and its peculiar signature in the radial motion. As an RDS evolves, we observe transverse modulations at discrete azimuthal angles, which clearly result in a patterned formation of a circular vortex dipole array. Through collisions of the vortex dipoles with the box trap, we observe vortex unbinding, vortex pinning to the edge, and emission of rarefaction pulses. Our box-quench protocol opens a new way to study multidimensional dark solitons, structured formation of topological defects, and potentially the dynamics of ordered quantum vortex matter.

We develop a theory of the nonlocal transport of two counterpropagating $\nu =1$ quantum Hall edges coupled via a narrow disordered superconductor. Contrary to the predictions developed for the clean case, the edge states proximitized in this way do not turn into a topological superconductor. Instead, they are naturally tuned to the critical point between trivial and topological phases. This occurs due to the competition between tunneling processes with and without particle-hole conversion. The critical conductance is a random, sample-specific quantity with a zero average and unusual bias dependence. The negative values of conductance are relatively stable against variations of the carrier density, which may make the critical state appear as a topological one. Our results offer an interpretation of recent experiments [G.-H. Lee , Nat. Phys. 13, 693 (2017); O. Gül , Phys. Rev. X 12, 021057 (2022)].

In this work, we lay down the ab initio many-body quantum theory of electrons and phonons in equilibrium as well as in steady-state or time-varying settings. Our focus is on the harmonic approximation, but the developed tools can readily incorporate anharmonic effects. We begin by showing the necessity of determining the ab initio Hamiltonian in a self-consistent manner to ensure the existence of an equilibrium state. We then identify the correct partitioning into a “noninteracting” and an “interacting” part to carry out diagrammatic expansions in terms of dressed propagators and screened interactions. The final outcome is the finite-temperature nonequilibrium extension of the Hedin equations, showcasing the emergence of the coupling between electrons and coherent phonons through the time-local Ehrenfest diagram. The Hedin equations have limited practical utility for real-time simulations of systems driven out of equilibrium by external fields. To overcome this limitation, we leverage the versatility of the diagrammatic approach to generate a closed system of differential equations for the dressed propagators and nuclear displacements. These are the Kadanoff-Baym equations for electrons and phonons. The formalism naturally merges with the theory of conserving approximations, which guarantee the satisfaction of the continuity equation and the conservation of total energy during time evolution. As an example, we show that the popular Born-Oppenheimer approximation is not conserving whereas its dynamical extension is conserving, provided that the electrons are treated in the Fan-Migdal approximation with a dynamically screened electron-phonon coupling. We also derive the formal solution of the Kadanoff-Baym equations for nonequilibrium steady states, which is useful for studies in photovoltaics and optoelectronics. Interestingly, the expansion of the phononic Green’s function around the quasiphonon energies points to a correlation-induced splitting of the phonon dispersion in materials with no time-reversal invariance.

The high-pressure (HP) behavior of dioxides is of interest due to their extensive polymorphism and role as analogs for ${\mathrm{SiO}}_2$, a phase expected to be important in the deep mantles of Earth and terrestrial exoplanets. Here we report on dynamic ramp compression of quartz-type germanium dioxide ${\mathrm{GeO}}_2$ to stresses up to 882 GPa, a higher peak stress than previous studies by a factor of 5. X-ray diffraction data show that $\mathrm{HP}\text{−}{\mathrm{PdF}}_2$-type ${\mathrm{GeO}}_2$ occurs under ramp loading from 154 to 440 GPa, and this phase persists to higher pressure than predicted by theory. Above 440 GPa, we observe evidence for transformation to a new phase of ${\mathrm{GeO}}_2$. Based on the diffraction data, the best candidate for this new phase is the cotunnite-type structure which has been predicted to be a stable phase of ${\mathrm{GeO}}_2$ above 300 GPa. The $\mathrm{HP}\text{−}{\mathrm{PdF}}_2$-type and cotunnite-type structures are important phases in a wide range of $A{X}_2$ compounds, including ${\mathrm{SiO}}_2$, at multihundred GPa stresses. Our results demonstrate that ramp compression can be an effective technique for synthesizing and characterizing such phases in oxides. In addition, we show that pulsed x-ray diffraction under ramp compression can be used to examine lower-symmetry phases in oxide materials.