We present a new paradigm for achieving thermal relic dark matter. The mechanism arises when a nearly secluded dark sector is thermalized with the standard model after reheating. The freeze-out process is a number-changing $3\to 2$ annihilation of strongly interacting massive particles (SIMPs) in the dark sector, and points to sub-GeV dark matter. The couplings to the visible sector, necessary for maintaining thermal equilibrium with the standard model, imply measurable signals that will allow coverage of a significant part of the parameter space with future indirect- and direct-detection experiments and via direct production of dark matter at colliders. Moreover, $3\to 2$ annihilations typically predict sizable $2\to 2$ self-interactions which naturally address the “core versus cusp” and “too-big-to-fail” small-scale structure formation problems.

Harnessing nonlinearities strong enough to allow single photons to interact with one another is not only a fascinating challenge but also central to numerous advanced applications in quantum information science. Here we report the nonlinear interaction between two single photons. Each photon is generated in independent parametric down-conversion sources. They are subsequently combined in a nonlinear waveguide where they are converted into a single photon of higher energy by the process of sum-frequency generation. Our approach results in the direct generation of photon triplets. More generally, it highlights the potential for quantum nonlinear optics with integrated devices and, as the photons are at telecom wavelengths, it opens the way towards novel applications in quantum communication such as device-independent quantum key distribution.

Friction, the resistive force between two surfaces sliding past each other, is at the core of a wide diversity of locomotion schemes. While such schemes are well described for homogeneous environments, locomotion based on friction in inhomogeneous environments has not received much attention. Here we introduce and demonstrate the concept of tribotaxis, a motion that is guided by gradients in the friction coefficient. Our system is composed of microwalkers that undergo an effective frictional interaction with biological receptors on the substrate, which is regulated by the density of such receptors. When actuated stochastically, microwalkers migrate to regions of higher friction, much like a chemotactic cell migrates to regions of higher chemoattractant concentration. Simulations and theory based on biased random walks are in excellent agreement with experiments. We foresee important implications for tribotaxis in artificial and natural locomotion in biological environments.

We present new constraints on sub-GeV dark matter and dark photons from the electron beam-dump experiment E137 conducted at SLAC in 1980–1982. Dark matter interacting with electrons (e.g., via a dark photon) could have been produced in the electron-target collisions and scattered off electrons in the E137 detector, producing the striking, zero-background signature of a high-energy electromagnetic shower that points back to the beam dump. E137 probes new and significant ranges of parameter space and constrains the well-motivated possibility that dark photons that decay to light dark-sector particles can explain the $\sim 3.6\sigma$ discrepancy between the measured and standard model value of the muon anomalous magnetic moment. It also restricts the parameter space in which the relic density of dark matter in these models is obtained from thermal freeze-out. E137 also convincingly demonstrates that (cosmic) backgrounds can be controlled and thus serves as a powerful proof of principle for future beam-dump searches for sub-GeV dark-sector particles scattering off electrons in the detector.

Thirty years ago, theorists showed that a properly designed combination of incident waves could be fully transmitted through (or reflected by) a disordered medium, based on the existence of propagation channels which are essentially either closed or open (bimodal law). In this Letter, we study elastic waves in a disordered waveguide and present direct experimental evidence of the bimodal law. Full transmission and reflection are achieved. The wave field is monitored by laser interferometry and highlights the interference effects that take place within the scattering medium.

Using a combination of the Raman spectroscopy and density functional theory calculations on dense hydrogen-deuterium mixtures of various concentrations, we demonstrate that, at 300 K and above 200 GPa, they transform into phase IV, forming a disordered binary alloy with six highly localized intramolecular vibrational (vibrons) and four delocalized low-frequency ($<1200{\mathrm{cm}}^{-1}$) modes. Hydrogen-deuterium mixtures are unique in showing a purely mass-induced localization effect in the quantum solid: chemical bonding is isotope-independent while the mass varies by a factor of 2.

We report in-plane resistivity ($\rho$) and transverse magnetoresistance (MR) measurements for underdoped ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ (Hg1201). Contrary to the long-standing view that Kohler’s rule is strongly violated in underdoped cuprates, we find that it is in fact satisfied in the pseudogap phase of Hg1201. The transverse MR shows a quadratic field dependence, $\delta \rho /{\rho }_0=a{H}^2$, with $a(T)\propto T^{-4}$. In combination with the observed $\rho \propto T^2$ dependence, this is consistent with a single Fermi-liquid quasiparticle scattering rate. We show that this behavior is typically masked in cuprates with lower structural symmetry or strong disorder effects.

Up-conversion luminescence and long-persistent luminescence are two well-studied, special luminescence processes. By combining the unique features of these two luminescence processes, here we design a new luminescence process called up-converted persistent luminescence (UCPL), which enables us to generate persistent luminescence having an emission energy higher than the excitation energy. Guided by the UCPL concept, we create the first UCPL phosphor ${\mathrm{Zn}}_3{\mathrm{Ga}}_2{\mathrm{GeO}}_8:1\%{\mathrm{Cr}}^{3+}$, $5\%{\mathrm{Yb}}^{3+}$, $0.5\%{\mathrm{Er}}^{3+}$ by incorporating an up-converting ion pair ${\mathrm{Yb}}^{3+}/{\mathrm{Er}}^{3+}$ into a ${\mathrm{Zn}}_3{\mathrm{Ga}}_2{\mathrm{GeO}}_8:1\%{\mathrm{Cr}}^{3+}$ near-infrared persistent phosphor. After being excited by a 980 nm laser, the phosphor emits long-lasting ($>24\mathrm{h}$) near-infrared persistent emission peaking at 700 nm. The UCPL concept and the associated phosphors are expected to have important implications for several fields such as biomedical imaging.

We demonstrate the instability of the free surface of a soft elastic solid facing downwards. Experiments are carried out using a gel of constant density $\rho$, shear modulus $\mu$, put in a rigid cylindrical dish of depth $h$. When turned upside down, the free surface of the gel undergoes a normal outgoing acceleration $g$. It remains perfectly flat for $\rho gh/\mu <{\alpha }^{*}$ with ${\alpha }^{*}\simeq 6$, whereas a steady pattern spontaneously appears in the opposite case. This phenomenon results from the interplay between the gravitational energy and the elastic energy of deformation, which reduces the Rayleigh waves celerity and vanishes it at the threshold.

We present a compact formula for the complete tree-level $S$-matrix of pure Yang-Mills and gravity theories in arbitrary spacetime dimensions. The new formula for the scattering of $n$ particles is given by an integral over the positions of $n$ points on a sphere restricted to satisfy a dimension-independent set of equations. The integrand is constructed using the reduced Pfaffian of a $2n\times 2n$ matrix, $\mathrm{\Psi }$, that depends on momenta and polarization vectors. In its simplest form, the gravity integrand is a reduced determinant which is the square of the Pfaffian in the Yang-Mills integrand. Gauge invariance is completely manifest as it follows from a simple property of the Pfaffian.

We consider classical models of the kicked rotor type, with piecewise linear kicking potentials designed so that momentum changes only by multiples of a given constant. Their dynamics display quasilocalization of momentum, or quadratic growth of energy, depending on the arithmetic nature of the constant. Such purely classical features mimic paradigmatic features of the quantum kicked rotor, notably dynamical localization in momentum, or quantum resonances. We present a heuristic explanation, based on a classical phase space generalization of a well-known argument, that maps the quantum kicked rotor on a tight-binding model with disorder. Such results suggest reconsideration of generally accepted views that dynamical localization and quantum resonances are a pure result of quantum coherence.

Data compression is a ubiquitous aspect of modern information technology, and the advent of quantum information raises the question of what types of compression are feasible for quantum data, where it is especially relevant given the extreme difficulty involved in creating reliable quantum memories. We present a protocol in which an ensemble of quantum bits (qubits) can in principle be perfectly compressed into exponentially fewer qubits. We then experimentally implement our algorithm, compressing three photonic qubits into two. This protocol sheds light on the subtle differences between quantum and classical information. Furthermore, since data compression stores all of the available information about the quantum state in fewer physical qubits, it could allow for a vast reduction in the amount of quantum memory required to store a quantum ensemble, making even today’s limited quantum memories far more powerful than previously recognized.

The quantum statistics of bosons or fermions are manifest through the even or odd relative angular momentum of a pair. We show theoretically that, under certain conditions, a pair of certain test particles immersed in a fractional quantum Hall state possesses, effectively, a fractional relative angular momentum, which can be interpreted in terms of fractional braid statistics. We propose that the fractionalization of the angular momentum can be detected directly through the measurement of the pair correlation function in rotating ultracold atomic systems in the fractional quantum Hall regime. Such a measurement will also provide direct evidence for the effective magnetic field resulting from Berry phases arising from attached vortices, and of excitations with a fractional particle number, analogous to the fractional charge of the electron fractional quantum Hall effect.

The classical Kepler problem, as well as its quantum mechanical version, the hydrogen atom, enjoys a well-known hidden symmetry, the conservation of the Laplace-Runge-Lenz vector, which makes these problems superintegrable. Is there a relativistic quantum field theory extension that preserves this symmetry? In this Letter we show that the answer is positive: in the nonrelativistic limit, we identify the dual conformal symmetry of planar $\mathcal{N}=4$ super Yang-Mills theory with the well-known symmetries of the hydrogen atom. We point out that the dual conformal symmetry offers a novel way to compute the spectrum of bound states of massive $W$ bosons in the theory. We perform nontrivial tests of this setup at weak and strong coupling and comment on the possible extension to arbitrary values of the coupling.

Atomic comagnetometers are widely used in precision measurements searching for spin interactions beyond the standard model. We describe a new ${}^3\mathrm{He}\text{−}{}^{129}\mathrm{Xe}$ comagnetometer probed by Rb atoms and use it to identify two general classes of systematic effects in gas comagnetometers, one associated with diffusion in second-order magnetic-field gradients and another due to temperature gradients. We also develop and confirm experimentally a general and practical approach for calculating spin relaxation and frequency shifts due to arbitrary magnetic-field gradients.

Optical excitation typically enhances electrical conduction and low-frequency radiation absorption in semiconductors. We, however, observe a pronounced transient decrease of conductivity in doped monolayer molybdenum disulfide (${\mathrm{MoS}}_2$), a two-dimensional (2D) semiconductor, using ultrafast optical-pump terahertz-probe spectroscopy. In particular, the conductivity is reduced to only 30% of its equilibrium value at high pump fluence. This anomalous phenomenon arises from the strong many-body interactions in the 2D system, where photoexcited electron-hole pairs join the doping-induced charges to form trions, bound states of two electrons and one hole. The resultant increase of the carrier effective mass substantially diminishes the conductivity.

We demonstrate a way to prepare single photons with a temporal envelope that resembles the time reversal of photons from the spontaneous decay process. We use the photon pairs generated from a time-ordered cascade decay: the detection of the first photon of the cascade is used as a herald for the ground-state transition resonant second photon. We show how the interaction of the heralding photon with an asymmetric Fabry-Perot cavity reverses the temporal shape of its twin photon from a decaying to a rising exponential envelope. This single photon is expected to be ideal for interacting with two-level systems.

The Letter draws the attention to the spatiotemporal symmetry of various vectorlike physical quantities. The symmetry is specified by their invariance under the action of symmetry operations of the nonrelativistic space-time rotation group $\mathrm{O}(3)\times \{1,1^{\prime }\}={\mathrm{O}}^{\prime }(3)$, where $1^{\prime }$ is a time-reversal operation, the $\text{symbol}\times \text{stands}$ for the group direct product, and O(3) is a group of proper and improper rotations. It is argued that along with the canonical polar vector, there are another seven symmetrically distinct classes of stationary physical quantities, which can be—and often are—denoted as standard three-component vectors, even though they do not transform as a static polar vector under all operations of ${\mathrm{O}}^{\prime }(3)$. The octet of symmetrically distinct “directional quantities” can be exemplified by two kinds of polar vectors (electric dipole moment $\mathbf{P}$ and magnetic toroidal moment $\mathbf{T}$), two kinds of axial vectors (magnetization $\mathbf{M}$ and electric toroidal moment $\mathbf{G}$), two kinds of chiral “bidirectors” $\mathbf{C}$ and $\mathbf{F}$ (associated with the so-called true and false chirality, respectively) and still another two bidirectors $\mathbf{N}$ and $\mathbf{L}$, achiral ones, transforming as the nematic liquid crystal order parameter and as the antiferromagnetic order parameter of the hematite crystal $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, respectively.

The resonant substructure of $B_s^0\to {\overline{D}}^0{K}^{-}{\pi }^{+}$ decays is studied using a data sample corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$ of $pp$ collision data recorded by the LHCb detector. An excess at $m({\overline{D}}^0{K}^{-})\approx 2.86\mathrm{GeV}/c^2$ is found to be an admixture of spin-1 and spin-3 resonances. Therefore, the $D_{sJ}^{*}(2860{)}^{-}$ state previously observed in inclusive $e^{+}{e}^{-}\to {\overline{D}}^0{K}^{-}X$ and $pp\to {\overline{D}}^0{K}^{-}X$ processes consists of at least two particles. This is the first observation of a heavy flavored spin-3 resonance, and the first time that any spin-3 particle has been seen to be produced in $B$ decays. The masses and widths of the new states and of the $D_{s2}^{*}(2573{)}^{-}$ meson are measured, giving the most precise determinations to date.

We present an upper bound on the branching fraction of the Higgs boson to invisible particles by recasting a CMS Collaboration search for stop quarks decaying to $t\overline{t}+E_{\mathrm{T}}^{\text{miss}}$. The observed (expected) bound, $\mathrm{BF}(H\to \text{inv}.)<0.40(0.65)$ at 95% C.L., is the strongest direct limit to date, benefiting from a downward fluctuation in the CMS data in that channel. In addition, we combine this new constraint with existing published constraints to give an observed (expected) bound of $\mathrm{BF}(H\to \text{inv}.)<0.40(0.40)$ at 95% C.L., and we show some of the implications for theories of dark matter which communicate through the Higgs portal.