Photon statistics divides light sources into three different categories, characterized by bunched, antibunched, or uncorrelated photon arrival times. Single atoms, ions, molecules, or solid state emitters display antibunching of photons, while classical thermal sources exhibit photon bunching. Here we demonstrate a light source in free space, where the photon statistics depends on the direction of observation, undergoing a continuous crossover between photon bunching and antibunching. We employ two trapped ions, observe their fluorescence under continuous laser light excitation, and record spatially resolved the autocorrelation function $g^{(2)}(\tau )$ with a movable Hanbury Brown and Twiss detector. Varying the detector position we find a minimum value for antibunching, $g^{(2)}(0)=0.60(5)$ and a maximum of $g^{(2)}(0)=1.46(8)$ for bunching, demonstrating that this source radiates fundamentally different types of light alike. The observed variation of the autocorrelation function is understood in the Dicke model from which the observed maximum and minimum values can be modeled, taking independently measured experimental parameters into account.

Strong coupling of two-dimensional semiconductor excitons with plasmonic resonators enables control of light-matter interaction at the subwavelength scale. Here we develop such strong coupling in plasmonic nanogap resonators, which allows modification of exciton strength by altering electromagnetic environments in nearby semiconductor monolayers. Using this system, we not only demonstrate a large vacuum Rabi splitting up to 163 meV and splitting features in photoluminescence spectra but also reveal that the effective exciton number contributing to the coupling can be reduced down to the single-digit level ($N<10$), which is 2 orders lower than that of previous systems, close to single-exciton based strong coupling. In addition, we prove that the strong coupling process is not affected by the large exciton coherence size that was previously believed to be detrimental to the formation of plasmon-exciton interaction. We provide a deeper understanding of strong coupling in two-dimensional semiconductors, paving the way for room-temperature quantum optics applications.

Improving our experimental and theoretical knowledge of electric potentials at liquid-solid boundaries is essential to achieve a deeper understanding of the driving forces behind interfacial processes. Electron holography has proved successful in probing solid-solid interfaces but requires knowledge of the materials’ mean inner potential (MIP, $V_0$), which is a fundamental bulk material property. Combining off-axis electron holography with liquid phase transmission electron microscopy (LPTEM), we provide the first quantitative MIP determination of liquid water $V_0=+4.48\pm 0.19\mathrm{V}$. This value is larger than most theoretical predictions, and to explain the disagreement we assess the dominant factors needed in quantum simulations of liquid water. A precise MIP lays the foundations for nanoscale holographic potential measurements in liquids, and provides a benchmark to improve quantum mechanical descriptions of aqueous systems and their interfaces in, e.g., electrochemistry, solvation processes, and spectroscopy.

Non-Bloch-band theory describes bulk energy spectra and topological invariants in non-Hermitian crystals with open boundaries, where the bulk eigenstates are squeezed toward the edges (skin effect). However, the interplay of non-Bloch-band theory, skin effect, and coherent Bloch dynamics is so far unexplored. In two-band non-Hermitian lattices, it is shown here that collapse of non-Bloch bands and skin modes deeply changes the Bloch dynamics under an external force. In particular, for resonance forcing non-Bloch-band collapse results in Wannier-Stark ladder coalescence and chiral Zener tunneling between the two dispersive Bloch bands.

Understanding many-body physics of elementary excitations has advanced our control over material properties. Here, we study spin-flip excitations in NiO using Ni $L_3$-edge resonant inelastic x-ray scattering (RIXS) and present a strikingly different resonant energy behavior between single and double spin-flip excitations. Comparing our results with single-site full-multiplet ligand field theory calculations we find that the spectral weight of the double-magnon excitations originates primarily from the double spin-flip transition of the quadrupolar RIXS process within a single magnetic site. Quadrupolar spin-flip processes are among the least studied excitations, despite being important for multiferroic or spin-nematic materials due to their difficult detection. We identify intermediate state multiplets and intra-atomic core-valence exchange interactions as the key many-body factors determining the fate of such excitations. RIXS resonant energy dependence can act as a convincing proof of existence of nondipolar higher-ranked magnetic orders in systems for which, only theoretical predictions are available.

We analyze a new model for growing networks, the constrained Leath invasion percolation model. Cluster dynamics are characterized by bursts in space and time. The model quantitatively reproduces the observed frequency-magnitude scaling of earthquakes in the limit that the occupation probability approaches the critical bond percolation probability in $d=2$. The model may have application to other systems characterized by burst dynamics.

Evidence for the presence of extra fields during inflation may be found in the anisotropies of the scalar and tensor spectra across a vast range of scales. Indeed, beyond the single-field slow-roll paradigm, a long tensor mode modulating the power spectrum can induce a sizable quadrupolar anisotropy. We investigate how these dynamics play out for the tensor two-point correlator. The resulting quadrupole stores information on squeezed tensor non-Gaussianities, including those sourced by extra field content and responsible for the breaking of so-called consistency relations. We underscore the potential of anisotropies as a probe of new physics: testable at cosmic microwave background scales through the detection of $B$ modes, they are accessible at smaller scales via pulsar timing arrays and interferometers. Our findings are particularly relevant in that recent studies show a considerable suppression for tensor non-Gaussianities if all modes are well inside the horizon. Quadrupolar anisotropies instead probe an unsuppressed ultrasqueezed bispectrum where the long mode can be horizon size.

The low-lying energy spectrum of the extremely neutron-deficient self-conjugate ($N=Z$) nuclide ${{}_{44}{}^{88}\mathrm{Ru}}_{44}$ has been measured using the combination of the Advanced Gamma Tracking Array (AGATA) spectrometer, the NEDA and Neutron Wall neutron detector arrays, and the DIAMANT charged particle detector array. Excited states in ${}^{88}\mathrm{Ru}$ were populated via the ${}^{54}\mathrm{Fe}({}^{36}\mathrm{Ar},2n\gamma ){{}^{88}\mathrm{Ru}}^{*}$ fusion-evaporation reaction at the Grand Accélérateur National d’Ions Lourds (GANIL) accelerator complex. The observed $\gamma$-ray cascade is assigned to ${}^{88}\mathrm{Ru}$ using clean prompt $\gamma \text{−}\gamma$-2-neutron coincidences in anticoincidence with the detection of charged particles, confirming and extending the previously assigned sequence of low-lying excited states. It is consistent with a moderately deformed rotating system exhibiting a band crossing at a rotational frequency that is significantly higher than standard theoretical predictions with isovector pairing, as well as observations in neighboring $N>Z$ nuclides. The direct observation of such a “delayed” rotational alignment in a deformed $N=Z$ nucleus is in agreement with theoretical predictions related to the presence of strong isoscalar neutron-proton pair correlations.

We provide a unified semiclassical theory for thermoelectric responses of any observable represented by an operator $\widehat{\mathit{\theta }}$ that is well defined in periodic crystals. The Einstein and Mott relations are established generally in the presence of Berry phase effects for various physical realizations of $\widehat{\mathit{\theta }}$ in electronic systems, including the familiar case of the electric current as well as the currently controversial cases of the spin polarization and spin current. The magnetization current, which has been proven indispensable in the thermoelectric response of electric current, is generalized to the cases of various $\widehat{\mathit{\theta }}$. In our theory the dipole density of a physical quantity emerges and plays a vital role, which contains not only the statistical sum of the dipole moment of $\widehat{\mathit{\theta }}$ but also a Berry phase correction.

The toughening of sparse elastic networks, such as hydrogels, foams, or meshes against fracture is one of the most important problems in materials science. However, the principles of toughening have not yet been established despite urgent engineering requirements and several efforts made by materials scientists. Here we address the above-mentioned problem by focusing on the topology of a network. We perform fracture experiments for two-dimensional periodic lattices fabricated from rubber strings and connecters with well-defined topological structures. We find that systematic increase in the largest coordination number while maintaining the average coordination number ($=4$) as constant leads to significant improvement in toughness. We reproduce the observed toughening behavior through numerical simulations and confirm that the stress concentration in the vicinity of a crack tip can be controlled by the topology of the network. This provides a new strategy for creating tough sparse elastic networks, especially hydrogels.

Quantum teleportation is a fundamental building block of quantum communications and quantum computations, transferring quantum states between distant physical entities. In the context of quantum secret sharing, the teleportation of quantum information shared by multiple parties without concentrating the information at any place is essential, and this cannot be realized by any previous scheme. We propose and experimentally demonstrate a novel teleportation protocol that enables one to perform this task. It is jointly performed by distributed participants, while none of them can fully access the information. Our scheme can be extended to arbitrary numbers of senders and receivers and to fault-tolerant quantum networks by incorporating error-correction codes.

We report on the realization of long-range Ising interactions in a cold gas of cesium atoms by Rydberg dressing. The interactions are enhanced by coupling to Rydberg states in the vicinity of a Förster resonance. We characterize the interactions by measuring the mean-field shift of the clock transition via Ramsey spectroscopy, observing one-axis twisting dynamics. We furthermore emulate a transverse-field Ising model by periodic application of a microwave field and detect dynamical signatures of the paramagnetic-ferromagnetic phase transition. Our results highlight the power of optical addressing for achieving local and dynamical control of interactions, enabling prospects ranging from investigating Floquet quantum criticality to producing tunable-range spin squeezing.

We demonstrate photon-mediated interactions between two individually trapped atoms coupled to a nanophotonic cavity. Specifically, we observe collective enhancement when the atoms are resonant with the cavity and level repulsion when the cavity is coupled to the atoms in the dispersive regime. Our approach makes use of individual control over the internal states of the atoms and their position with respect to the cavity mode, as well as the light shifts to tune atomic transitions individually, allowing us to directly observe the anticrossing of the bright and dark two-atom states. These observations open the door for realizing quantum networks and studying quantum many-body physics based on atom arrays coupled to nanophotonic devices.

Recently realized higher order topological insulators have taken a surge of interest among the theoretical and experimental condensed matter community. The two-dimensional second order topological insulators give rise to zero-dimensional localized corner modes that reside within the band gap of the system along with edge modes that inhabit a band edge next to bulk modes. Thanks to the topological nature, information can be trapped at the corners of these systems, which will be unhampered even in the presence of disorder. Being localized at the corners, the exchange of information among the corner states is an issue. Here we show that the nonlinearity in an exciton polariton system can allow the coupling between the different corners through the edge states based on optical parametric scattering, realizing a system of multiple connectible topological modes.

The propagation of a crack front in disordered materials is jerky and characterized by bursts of activity, called avalanches. These phenomena are the manifestation of an out-of-equilibrium phase transition originated by the disorder. As a result avalanches display universal scalings which are, however, difficult to characterize in experiments at a finite drive. Here, we show that the correlation functions of the velocity field along the front allow us to extract the critical exponents of the transition and to identify the universality class of the system. We employ these correlations to characterize the universal behavior of the transition in simulations and in an experiment of crack propagation. This analysis is robust, efficient, and can be extended to all systems displaying avalanche dynamics.

Understanding the wetting properties of premelting films requires knowledge of the film’s equation of state, which is not usually available. Here we calculate the disjoining pressure curve of premelting films and perform a detailed thermodynamic characterization of premelting behavior on ice. Analysis of the density profiles reveals the signature of weak layering phenomena, from one to two and from two to three water molecular layers. However, disjoining pressure curves, which closely follow expectations from a renormalized mean field liquid state theory, show that there are no layering phase transitions in the thermodynamic sense along the sublimation line. Instead, we find that transitions at mean field level are rounded due to capillary wave fluctuations. We see signatures that true first order layering transitions could arise at low temperatures, for pressures between the metastable line of water-vapor coexistence and the sublimation line. The extrapolation of the disjoining pressure curve above water-vapor saturation displays a true first order phase transition from a thin to a thick film consistent with experimental observations.

Repulsion of ligands is known as the key factor for hindering nanoparticle (NP) coalescence. Thus, during the past decade, it has generally accepted that the full removal of capping ligands of the contact surface is the first step for NP coalescence. Herein, using molecular dynamics simulations, we have identified a new mechanism for the coalescence of $\mathrm{S}{({\mathrm{CH}}_2)}_n\mathrm{COOH}$-coated Au NPs in water without ligand detachment. In contrast to the traditional mechanism, the aggregation of the NPs is induced by the twined hydrophobic chains of the ligands rather than the hydrophilic carboxyl tails as believed previously. Next, the exposed surface atoms attach to form the neck, and extend with the atomic rearrangement of the contact interface to merge the NPs, which do not need the removal of ligands as expected from traditional supposition. This finding refreshes the understanding of the atomic mechanism of the coalescence of NPs, which paves the way for the rational design and synthesis of NPs.