Science China Physics, Mechanics & Astronomy最新文献

The nonmagnetic kagome metal ScV₆Sn₆ displays an unconventional charge order (CO) accompanied by signatures of an anomalous Hall effect, hidden magnetism, and multiple lattice instabilities. In this study, we report the observation of unconventional anomalous thermoelectric properties. Notably, unexpected anomalous transverse Nernst signals reach a peak value of ∼4 µV/K near the T_CDW ∼92 K in ScV₆Sn₆, and these signals persist in the charge-ordered state as the temperature decreases to 10 K. Furthermore, both thermopower and thermal conductivity exhibit significant changes under magnetic fields, even in the nonmagnetic ground state. These observations strongly suggest the emergence of time-reversal symmetry breaking in ScV₆Sn₆, as supported by muon spin relaxation (µSR) measurements. While hidden magnetism represents the most plausible origin, alternative mechanisms involving orbital currents and chiral charge order remain possible.

Gravitational waves (GWs) originating from cosmological sources offer direct insights into the physics of the primordial Universe, the fundamental nature of gravity, and the cosmic expansion of the Universe. In this review paper, we present a comprehensive overview of our recent advances in GW cosmology, supported by the national key research and development program of China, focusing on cosmological GW sources and their implications for fundamental physics and cosmology. We first discuss the generation mechanisms and characteristics of stochastic gravitational wave backgrounds generated by physical processes that occurred in the early Universe, including those from inflation, phase transitions, and topological defects, and summarize current and possible future constraints from pulsar timing arrays and space-based detectors. Next, we explore the formation and observational prospects of primordial black holes as GW sources and their potential connection to dark matter. We then analyze how GWs are affected by large-scale structure, cosmological perturbations, and possible modifications of gravity on GW propagation, and how these effects can be used to test fundamental symmetry of gravity. Finally, we discuss the application of GW standard sirens in measuring the Hubble constant, the expansion history, and dark energy parameters, including their combination with electromagnetic observations. These topics together show how GW observations, especially with upcoming space-based detectors, such as LISA, Taiji, and TianQin, can provide new information about the physics of the early Universe, cosmological evolution, and the nature of gravity.

Degenerate states including exceptional points (EPs) and diabolic points (DPs) arise due to the underlying symmetry in physical systems. The interplay between different symmetry breakings opens a promising route for exceptional wave manipulation. Here, we conceptually demonstrate and experimentally prove that breaking parity symmetry and time-reversal symmetry through spatial perturbation and non-Hermitian perturbation, respectively, result in the evolution of EPs in pairs in a scattering system. These pairwise scattering EPs, which are orthogonal to each other and can be interconverted by mirror inversion, evolve continuously in the perturbation space and ultimately merge into a special non-Hermitian degenerate state—a non-Hermitian DP. The EPs and DP observed here exhibit distinct topological structures from different planes in the perturbation space, thus both carrying hybrid topological charges. Based on these findings, we show that metasurfaces at EPs can encode differences in scattering asymmetry, allowing for a complete yet arbitrary wave manipulation beyond previously reported non-Hermitian scattering metasurfaces. Our findings establish a general framework for exploring extreme wave scattering through combined-perturbation-driven degeneration evolution.

In the noisy intermediate-scale quantum era, emerging classical-quantum hybrid optimization algorithms, such as variational quantum algorithms (VQAs), can leverage the unique characteristics of quantum devices to accelerate computations tailored to specific problems with shallow circuits. However, these algorithms encounter biases and iteration difficulties due to significant noise in quantum processors. These difficulties can only be partially addressed without error correction by optimizing hardware, reducing circuit complexity, or fitting and extrapolating. A compelling solution is applying probabilistic error cancellation (PEC), a quantum error mitigation technique that enables unbiased results without full error correction. Traditional PEC is challenging to apply in VQAs due to its variance amplification, contradicting iterative process assumptions. This paper proposes a novel noise-adaptable strategy that combines PEC with the quantum approximate optimization algorithm (QAOA). It is implemented through invariant sampling circuits (invariant-PEC, or IPEC) and substantially reduces iteration variance. This strategy marks the first successful integration of PEC and QAOA, resulting in efficient convergence. Moreover, we introduce adaptive partial PEC (APPEC), which modulates the error cancellation proportion of IPEC during iteration. We experimentally validate this technique on a superconducting quantum processor, cutting sampling cost by 90.1%. Notably, we find that dynamic adjustments of error levels via APPEC can enhance the ability to escape from local minima and reduce sampling costs. These results open promising avenues for executing VQAs with large-scale, low-noise quantum circuits, paving the way for practical quantum computing advancements.

We unveil a unique critical phenomenon of topological edge modes in non-Hermitian systems, dubbed the critical non-Hermitian edge modes (CNHEM). Specifically, in the thermodynamic limit, the eigenvectors of edge modes jump discontinuously under infinitesimal on-site staggered perturbations. The CNHEM arises from the competition between the introduced on-site staggered potentials and size-dependent non-reciprocal coupling between edge modes, and are closely connected to the exceptional point (EP). As the system size increases, the coupling between edge modes decreases while the non-reciprocity is enhanced, causing the eigenvectors to gradually collapse toward the EP. However, when the on-site potentials dominate, this weakened coupling assists the eigenvectors to stay away from the EP. Such a critical phenomenon is absent in Hermitian systems, where the coupling between edge modes is reciprocal.

This work investigates quantum speedups for the popular game named Mastermind, in which there are two participants: the codemaker who selects a secret string, and the codebreaker who submits query strings and receives answers from the codemaker. The codebreaker’s objective is to learn the secret string in as few queries as possible. This work focuses on playing the Mastermind game on quantum computers using different types of codemaker’s answers such as black count, ℓ_p distance, and separable distance. We show that the codebreaker can learn the secret with certainty by using quantum algorithms which exhibit a sharp reduction in query numbers compared with their classical counterparts. Specifically, our quantum algorithms require ({cal O}(k log k)) black-count queries, ({cal O}(log k)) ℓ_p-distance queries, and ({cal O}(log M)) separable-distance queries to learn the secret s ∈ [k]ⁿ, respectively, where M is completely determined by k. Thus, the quantum query complexity is independent of the length n of the secret s, as opposed to the query complexity linear in n of classical algorithms.

Kagome metal CsV₃Sb₅ has attracted much recent attention due to the coexistence of multiple exotic orders and the associated proposals to mimic unconventional high temperature superconductors. Nevertheless, magnetism and strong electronic correlations—two essential ingredients for unconventional superconductivity, are absent in this V-based Kagome metal. CsCr₃Sb₅ is a newly discovered Cr-based parallel of CsV₃Sb₅, in which magnetism appears with charge density wave and superconductivity at different temperature and pressure regions. Enhanced electronic correlations are also suggested by theoretical proposals due to the calculated flat bands. Here, we report angle-resolved photoemission measurements and first-principles calculations on this new material system. Electron energy bands and the associated orbitals are resolved. Flat bands are observed near the Fermi level. Doping dependent measurements on Cs(V_1−xCr_x)₃Sb₅ reveal a gradually enhanced band renormalization from CsV₃Sb₅ to CsCr₃Sb₅, accompanied by distinct spatial symmetry breaking states in the phase diagram.

Symmetry-protected topological edge states of acoustic metamaterials offer an unprecedented prospect for guiding and manipulating sound waves. However, these conventional boundaries—conventionally constructed between crystals of distinct topological phases—are inherently limited by crystalline symmetry requirements (e.g., spin-momentum-locked edge states). Here, by randomly doping an acoustic semimetal with topological or ordinary phases at varying concentrations, we realize a class of aperiodic metamaterials, termed acoustic topological (ordinary) alloys in time-reversal symmetric systems. Such an alloy inherits the topological (ordinary) property of doped elements, where the native topological invariant and acoustic pseudospins remain unchanged. We demonstrate that the boundaries formed by such topological alloys are no longer constrained by crystalline symmetry, enabling robust transmission along arbitrary paths. Finally, we demonstrate a tunable multi-port power divider and an acoustic topological logic gate based on acoustic alloys. Our findings eliminate the geometric constraint on the topological boundary shape, offering a versatile platform for on-demand wave control along irregular and arbitrary paths.