We have measured the spin polarization of a slow positron beam via state-selective depopulation of $2{}^3{\mathrm{S}}_1$ positronium atoms, generated by passing the beam through a gas cell. Our method employs circularly polarized microwave radiation to drive $2{}^3{\mathrm{S}}_1\to 2{}^3{\mathrm{P}}_1$ transitions, for which either $\mathrm{\Delta }{m}_J=+1$ or $\mathrm{\Delta }{m}_J=-1$, and relies on the fact that asymmetries between the two cases yield the underlying positron beam polarization. Using this technique we show that the polarization of a positron beam derived from a solid neon moderator may be increased from 30% to 90% by increasing the moderator thickness, with an associated reduction in beam intensity of 60%.

We make absolute frequency measurements of Cs Rydberg transitions, $|6S_{1/2},F=3⟩\to |n{S}_{1/2}(n=23–90)⟩$ and $|n{D}_{3/2,5/2}(n=21–90)⟩$, with an accuracy of less than 72 kHz. The quantum defect parameters for the measured Rydberg series are the most precise obtained to date. The quantum defect series is terminated at ${\delta }_4$, showing that prior fits requiring higher order quantum defects reflect uncertainties in the observations. The precision of the measured quantum defects allow for the calculation of Rydberg electric-dipole transitions and fine-structure intervals extrapolated from high principal quantum numbers, to rival that of sophisticated many-body relativistic calculations carried out at low Rydberg principal quantum numbers. We quantitatively predict the contributions to the quantum defect parameters from core polarization and core penetration of Cs inner shell electrons. A new value for the ionization energy, consistent across the $n{S}_{1/2}$ and $n{D}_{3/2,5/2}$ Rydberg series, is reported at $31406.46775148(14){\mathrm{cm}}^{-1}$.

The hybrid quantum system of cold atomic gas and optical cavity can host many exotic phenomena including phase transitions and multistabilities. In this Letter, we investigate the effect of photon hopping between two Dicke cavities and show rich quantum phases for steady states and dynamic processes. Starting from a generic dimer system where the two cavities are not necessarily identical, we analytically obtain all possible steady-state phases and confirm their existence by numerical calculations. We then focus on the special case where the two cavities are identical, where exact solutions of all phases are obtained. Our results suggest that photon hopping is a convenient and powerful tool to manipulate the quantum phases and induce multistable behavior in this system.

Harnessing chiral optical forces facilitates numerous applications in enantioselective sorting and sensing. To date, significant challenges persist in substantiating the holistic complex theorem of these forces as experimental demonstrations employ common light waves (e.g., Gaussian beams and evanescent waves), showcasing the reversal of optical force against handedness (light’s or particle’s). Here, we elucidate the cooperative significance of incorporating multiple optical forces to generate an intricate chiral optical force in a complex spin-curl field. The interplay of distinct light properties (the transverse spin, Belinfante spin momentum, and energy vortex) is found to induce abnormal phenomena. For instance, this intriguing total force not only reverses its sign with particle handedness but also correlates strongly with particle size and light helicity. Experimental measurements of quasi-achiral and chirality-determined forces manifest our research as a paradigm for testing the holistic optical-force theorem on chiral particles, which is unattainable by previous reports that only affirm a specific type of chiral optical forces. Our study offers wide applications in chiral sensing, sorting, spintronics, metamaterials, etc.

Chiral objects are abundant in nature, and although the enantiomers have almost identical physical properties apart from their handedness, they can exhibit significantly different chemical properties and biological functions. This underscores the importance of sorting chiral substances. In this Letter, we demonstrate that chirality-sorting optical force pairs can be inversely generated in a tightly focused Gaussian beam by tailoring the input polarization state. We provide a detailed method for constructing the polarization state of the incident light to create the desired chiral optical field that generates the chirality-sorting optical force pairs. These force pairs precisely trap two opposite enantiomers at distinct predetermined positions within the same equilibrium plane, enabling their simultaneous identification and separation. Notably, the trapping positions and separation distances can be freely adjusted by altering the incident polarization parameters.

We present a high-resolution single crystal x-ray diffraction study of kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, exploring its response to variations in pressure and temperature. We discover that at low temperatures, the structural modulations of the electronic superlattice, commonly associated with charge-density-wave order, undergo a transformation around $p\sim 0.7\mathrm{GPa}$ from the familiar $2\times 2$ pattern to a long-range-ordered modulation at wave vector $q=(0,3/8,1/2)$. Our observations align with inferred changes in the charge-density-wave pattern from prior transport and nuclear-magnetic-resonance studies, providing new insights into these transitions. Interestingly, the pressure-induced variations in the electronic superlattice correlate with two peaks in the superconducting transition temperature as pressure changes, hinting that fluctuations within the electronic superlattice could be key to stabilizing superconductivity. However, our findings contrast with the minimal pressure dependency anticipated by ab initio calculations of the electronic structure. They also challenge prevailing scenarios based on a Peierls-like nesting mechanism involving Van Hove singularities.

Ordered media often support vortex structures with intriguing topological properties. Here, we investigate non-Abelian vortices in tetrahedral order using the mathematical formalism of colored links. Due to the generality of our methods, the results apply to all physical systems governed by tetrahedral order, such as the cyclic phase of spin-2 Bose–Einstein condensates and the tetrahedratic phase of bent-core nematic liquid crystals. Using these vortices, we construct topologically protected knots in the sense that they cannot decay into unlinked simple loop defects through vortex crossings and reconnections without destroying the phase. The discovered structures are the first examples of knots bearing such topological protection in a known experimentally realizable system.

The frustrated honeycomb spin model can stabilize a subextensively degenerate spiral spin liquid with nontrivial topological excitations and defects, but its material realization remains rare. Here, we report the experimental realization of this model in the structurally disorder-free compound GdZnPO. Using a single-crystal sample, we find that spin-$7/2$ rare-earth ${\mathrm{Gd}}^{3+}$ ions form a honeycomb lattice with dominant second-nearest-neighbor antiferromagnetic and first-nearest-neighbor ferromagnetic couplings, along with easy-plane single-site anisotropy. This frustrated model stabilizes a unique spiral spin liquid with a degenerate contour around the $\mathrm{K}\{1/3,1/3\}$ point in reciprocal space, consistent with our experiments down to 30 mK, including the observation of a giant residual specific heat. Our results establish GdZnPO as an ideal platform for exploring the stability of spiral spin liquids and their novel properties, such as the emergence of low-energy topological defects on the sublattices.

Electron-hole exchange interaction in two-dimensional transition metal dichalcogenides is extremely strong due to the dimension reduction, which promises valley-superposed excitonic states with linearly polarized optical emissions. However, strong circular polarization reflecting valley-polarized excitonic states is commonly observed in helicity-resolved optical experiments. Here, we present a non-Hermitian theory of valley excitons by incorporating optical pumping and intrinsic decay, which unveils an anomalous valley-polarized excitonic state with elliptically polarized optical emission. This novel state arises from the non-Hermiticity induced parity-time ($\mathcal{PT}$) symmetry breaking, which impedes the experimental observation of intervalley excitonic coherence effect. At large excitonic center-of-mass momenta, the $\mathcal{PT}$ symmetry is restored and the excitonic states recover their valley coherence. Interestingly, the linear polarization directions in optical emissions from these valley-superposed excitonic states are nonorthogonal and even become parallel at exceptional points. Our non-Hermitian theory also predicts a nonzero Berry curvature for valley excitons, which admits a topological excitonic Hall transport beyond the Hermitian predictions.

We introduce protocols to prepare many-body quantum states with quantum circuits assisted by local operations and classical communication. We show that by lifting the requirement of exact preparation, one can substantially save resources. In particular, the so-called $W$ and, more generally, Dicke states require a circuit depth and number of ancillas per site that are independent of the system size. As a by-product of our work, we introduce an efficient scheme to implement certain nonlocal, non-Clifford unitary operators. We also discuss how similar ideas may be applied in the preparation of eigenstates of well-known spin models, both free and interacting.

Efficient simulation of quantum computers relies on understanding and exploiting the properties of quantum states. This is the case for methods such as tensor networks, based on entanglement, and the tableau formalism, which represents stabilizer states. In this Letter, we integrate these two approaches to present a generalization of the tableau formalism used for Clifford circuit simulation. We explicitly prove how to update our formalism with Clifford gates, non-Clifford gates, and measurements, enabling universal circuit simulation. We also discuss how the framework allows for efficient simulation of more states, raising some interesting questions on the representation power of tensor networks and the quantum properties of resources such as entanglement and magic, and support our claims with simulations.

We investigate the driven-dissipative dynamics of multilevel atomic arrays interacting via dipolar interactions at subwavelength spacings. Unlike two-level atoms in the weakly excited regime, multilevel atoms can become strongly entangled. The entanglement manifests as the growth of spin waves in the ground-state manifold and survives after turning off the drive. We propose the $2.9\mathrm{\mu }\mathrm{m}$ transition between ${{}^{3}P}_2\leftrightarrow {{}^{3}D}_3$ in ${}^{88}\mathrm{Sr}$ with 389 nm trapping light as a platform to test our predictions and explore many-body physics with light-matter interactions.

We explore the emergence of universal dynamic scaling in an interacting Bose gas around the condensation transition, under the combined influence of an external driving force and spatial disorder. As time progresses, we find that the Bose gas crosses over three distinct dynamical regimes: (i) an inverse turbulent cascade where interactions dominate the drive, (ii) a stationary regime where the inverse cascade and the drive counterbalance one other, and (iii) a sub-diffusive cascade in energy space governed by the drive and disorder, a phenomenon recently observed experimentally. We show that all three dynamical regimes can be described by self-similar scaling laws.

Optical binding refers to the light-induced interaction between two or more objects illuminated by laser fields. The high tunability of the strength, sign, and reciprocity of this interaction renders it highly attractive for controlling nanoscale mechanical motion. Here, we discuss the quantum theory of optical binding and identify unique signatures of this interaction in the quantum regime. We show that these signatures are observable in near-future experiments with levitated nanoparticles. In addition, we prove the impossibility of entanglement induced by far-field optical binding in free space and identify strategies to circumvent this no-go theorem.

The ability to generate Landau levels using a pseudomagnetic field (PMF), also called an artificial gauge field, opens up new pathways for exploring fundamental physics and developing novel applications based on topological protection. In this Letter, we simultaneously realize a PMF and a pseudoelectric field (PEF) on a photonic crystal platform and observe a rainbow effect of the Landau zeroth modes. While a PMF induces a series of discretized Landau levels of photons in a similar way as the quantum Hall effect for electrons, a PEF breaks the degeneracy of the flat band of Landau levels over a broad range. Thus, different frequencies of the Landau modes can be dispersed into different spatial positions, leading us to term this phenomenon the “Landau rainbow.” The Landau rainbow, induced by the joint action of PMFs and PEFs, features robustness, broadband capability, and scalability, and may find applications in areas such as slow-light effect, multifrequency divider, and optical information multiplexing.

The development of two-dimensional (2D) semiconductors is limited by the lack of doping methods. We propose surface isovalent substitution as an efficient doping mechanism for 2D semiconductors by revealing the evolution of the structure and electronic properties of 2D $\mathrm{Se}/\mathrm{Te}$. Because of the different electronegativity of Se and Te, Se substitution for Te at the specific lattice sites introduces electric dipoles and leads to charge redistribution, which lowers the work function and tunes the Te films from $p$-type to $n$-type semiconductors. This differs from the gap enlargement of alloy films with random Se-Te substitutions. This doping method minimizes the change of lattice structure and surface roughness, which benefits structure stacking. Further increasing the Se content leads to the formation of two types of 2D semiconducting Se polymorphs.

Ultrafast photoexcitation offers a novel approach to manipulating quantum materials. One of the long-standing goals in this field is to achieve optical control over topological properties. However, the impact on their electronic structures, which host gapless surface states, has yet to be directly observed. Here, using time- and angle-resolved photoemission spectroscopy, we visualize the photoinduced evolution of the band structure in ${\mathrm{Bi}}_y{({\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x)}_{1-y}\mathrm{Se}(111)$ films from topological to trivial insulators. Following near-infrared ultrafast laser excitation, we observe that the topological surface state opens a substantial gap of up to 0.1 eV. Considering the topological phase diagram associated with lattice distortion and atomic displacement, we show that a uniaxial strain generated by the ultrafast optical pulse is sufficiently effective and strong for the observed topological phase transition. Our Letter highlights the potential of optical tuning of materials through laser excitation to control topological properties on ultrafast timescales.