Starting with the solution to the Bethe-Salpeter equation for the pion, in a beyond rainbow-ladder truncation to QCD’s Dyson-Schwinger equations, we determine the pion’s $l_z=0$ and $|l_z|=1$ leading Fock-state light-front wave functions (LFWFs) [labeled by ${\psi }_{l_z}(x,{\mathit{k}}_T^2)$]. The leading-twist time-reversal even transverse momentum dependent parton distribution function (TMD) of the pion is then directly obtained using these LFWFs. A key characteristic of the LFWFs, which is driven by dynamical chiral symmetry breaking, is that at typical hadronic scales they are broad functions in the light-cone momentum fraction $x$. The LFWFs have a nontrivial $(x,{\mathit{k}}_T^2)$ dependence and in general do not factorize into separate functions of each variable. For ${\mathit{k}}_T^2\lesssim 1{\mathrm{GeV}}^2$ the ${\mathit{k}}_T^2$ dependence of the LFWFs is well described by a Gaussian; however for ${\mathit{k}}_T^2\gtrsim 10{\mathrm{GeV}}^2$ these LFWFs behave as ${\psi }_0\propto x(1-x)/{\mathit{k}}_T^2$ and ${\psi }_1\propto x(1-x)/{\mathit{k}}_T^4$, and therefore exhibit the power-law behavior predicted by perturbative QCD. The pion’s TMD naturally inherits many features from the LFWFs. The TMD evolution of our result is studied using both the $b^{*}$ and $\zeta$ prescriptions which allows a qualitative comparison with Drell-Yan data.

We present a direct determination of photofragment alignment produced by circularly polarized light in photolysis of a planar polyatomic molecule. This alignment arises via a new mechanism involving coherent excitation of two mutually perpendicular in-plane transition dipole moment components. The alignment is described by a new anisotropy parameter ${\gamma }_2^{\prime }$ that was isolated by a unique laser polarization geometry. The determination of the parameter ${\gamma }_2^{\prime }$ was realized in ozone photolysis at 266 nm where dc slice images of $\mathrm{O}({}^1{D}_2)$ atomic fragments were acquired. A model developed for interpretation of the photolysis mechanism shows that it can exist only in case of failure of the Born-Oppenheimer approximation when electronic and vibrational (vibronic) interactions have to be taken into account. This finding suggests that determination of the alignment parameter ${\gamma }_2^{\prime }$ can be used as a key for direct insight into vibronic interactions in photolysis of polyatomic molecules. The results obtained for ozone photolysis via the Hartley band showed significant ${\gamma }_2^{\prime }$ alignment but little recoil speed dependence, consistent with the notion that, as opposed to the situation for derivative coupling, under our experimental conditions, the vibronic contributions to the nonadiabatic dynamics are not dependent on recoil speed.

We present the effects of resonator birefringence on the cavity-enhanced interfacing of quantum states of light and matter, including the first observation of single photons with a time-dependent polarization state that evolves within their coherence time. A theoretical model is introduced and experimentally verified by the modified polarization of temporally long single photons emitted from a ${}^{87}\mathrm{Rb}$ atom coupled to a high-finesse optical cavity by a vacuum-stimulated Raman adiabatic passage process. Further theoretical investigation shows how a change in cavity birefringence can both impact the atom-cavity coupling and engender starkly different polarization behavior in the emitted photons. With polarization a key resource for encoding quantum states of light and modern micron-scale cavities particularly prone to birefringence, the consideration of these effects is vital to the faithful realization of efficient and coherent emitter-photon interfaces for distributed quantum networking and communications.

Detecting nanomechanical motion has become an important challenge in science and technology. Recently, electromechanical coupling to focused electron beams has emerged as a promising method adapted to ultralow scale systems. However the fundamental measurement processes associated with such complex interaction remain to be explored. Here we report a highly sensitive detection of the Brownian motion of $\mu \mathrm{m}$-long semiconductor nanowires (InAs). The measurement imprecision is found to be set by the shot noise of the secondary electrons generated along the electromechanical interaction. By carefully analyzing the nanoelectromechanical dynamics, we demonstrate the existence of a radial backaction process that we identify as originating from the momentum exchange between the electron beam and the nanomechanical device, which is also known as radiation pressure.

We explore a single degenerate optical cavity supporting a synthetic two-dimensional space, which includes the frequency and the orbital angular momentum (OAM) axes of light. We create the effective gauge potential inside this synthetic space and show that the system exhibits topologically protected one-way edge states along the OAM axis at the boundaries of the frequency dimension. In this synthetic space, we present a robust generation and manipulation of entanglement between the frequency and OAM of photons. Our Letter shows that a higher-dimensional synthetic space involving multiple degrees of freedom of light can be achieved in a “zero-dimensional” spatial structure, pointing towards a unique platform to explore topological photonics and to realize potential applications in optical communications and quantum information processing.

We provide evidence of an extremely slow thermalization occurring in the discrete nonlinear Schrödinger model. At variance with many similar processes encountered in statistical mechanics—typically ascribed to the presence of (free) energy barriers—here the slowness has a purely dynamical origin: it is due to the presence of an adiabatic invariant, which freezes the dynamics of a tall breather. Consequently, relaxation proceeds via rare events, where energy is suddenly released towards the background. We conjecture that this exponentially slow relaxation is a key ingredient contributing to the nonergodic behavior recently observed in the negative-temperature region of the discrete nonlinear Schrödinger equation.

The electrical $\sigma$ and thermal conductivity $\lambda$ of liquid iron are calculated with spin-polarized density-functional-theory-based simulations over a significant pressure and temperature range using the Kubo-Greenwood formalism. We show that a paramagnetic state is stable in the liquid up to high temperatures at ambient pressure and that the discrepancy between experimental results and spin-degenerate simulations for $\sigma$ and $\lambda$ of more than 30% are reduced to within 10% with lower values resulting from the spin-polarized simulations. Along the 3700 K isotherm, we explore the persistence of magnetic fluctuations toward high densities, and beyond 20–50 GPa the liquid becomes diamagnetic, which suggests the existence of a continuous paramagnetic-to-diamagnetic transition. This transition exerts a significant influence on the physical properties of liquid iron, especially on $\sigma$ and $\lambda$, and is potentially of high relevance for dynamo processes in Mercury and Mars.

A topological electric quadrupole is a recently proposed concept that extends the theory of electric polarization of crystals to higher orders. Such a quadrupole phase supports topological states localized on both edges and corners. In this work, we show that in a quadrupole phase of a honeycomb lattice, topological helical edge states and pseudospin-polarized corner states appear by making use of a pseudospin degree of freedom related to point group symmetry. Furthermore, we argue that a general condition for the emergence of helical edge states in a (pseudo)spinful quadrupole phase is the existence of either mirror or time-reversal symmetry. Our results offer a way of generating topological helical edge states without spin-orbital couplings.

We study the Kondo physics of a quantum magnetic impurity in two-dimensional topological superconductors (TSCs), either intrinsic or induced on the surface of a bulk topological insulator, using a numerical renormalization group technique. We show that, despite sharing the $p+ip$ pairing symmetry, intrinsic and extrinsic TSCs host different physical processes that produce distinct Kondo signatures. Extrinsic TSCs harbor an unusual screening mechanism involving both electron and orbital degrees of freedom that produces rich and prominent Kondo phenomena, especially an intriguing pseudospin Kondo singlet state in the superconducting gap and a spatially anisotropic spin correlation. In sharp contrast, intrinsic TSCs support a robust impurity spin doublet ground state and an isotropic spin correlation. These findings advance fundamental knowledge of novel Kondo phenomena in TSCs and suggest experimental avenues for their detection and distinction.

A salient feature of skeletal muscles is their ability to take up an applied slack in a microsecond timescale. Behind this fast adaptation is a collective folding in a bundle of elastically interacting bistable elements. Since this interaction has a long-range character, the behavior of the system in force and length controlled ensembles is different; in particular, it can have two distinct order-disorder–type critical points. We show that the account of the disregistry between myosin and actin filaments places the elementary force-producing units of skeletal muscles close to both such critical points. The ensuing “double criticality” contributes to the system’s ability to perform robustly and suggests that the disregistry is functional.

During the spreading of epithelial tissues, the advancing tissue front often develops fingerlike protrusions. Their resemblance to traditional viscous fingering patterns in driven fluids suggests that epithelial fingers could arise from an interfacial instability. However, the existence and physical mechanism of such a putative instability remain unclear. Here, based on an active polar fluid model for epithelial spreading, we analytically predict a generic instability of the tissue front. On the one hand, active cellular traction forces impose a velocity gradient that leads to an accelerated front, which is, thus, unstable to long-wavelength perturbations. On the other hand, contractile intercellular stresses typically dominate over surface tension in stabilizing short-wavelength perturbations. Finally, the finite range of hydrodynamic interactions in the tissue selects a wavelength for the fingering pattern, which is, thus, given by the smallest between the tissue size and the hydrodynamic screening length. Overall, we show that spreading epithelia experience an active fingering instability based on a simple kinematic mechanism. Moreover, our results underscore the crucial role of long-range hydrodynamic interactions in the dynamics of tissue morphology.

The no-programing theorem prohibits the existence of a universal programmable quantum processor. This statement has several implications in relation to quantum computation but also to other tasks of quantum information processing, making this construction a central notion in this context. Nonetheless, it is well known that, even when the strict model is not implementable, it is possible to conceive of it in an approximate sense. Unfortunately, the minimal resources necessary for this aim are still not completely understood. Here, we investigate quantitative statements of the theorem, improving exponentially previous bounds on the resources required by such a hypothetical machine. The proofs exploit a new connection between quantum channels and embeddings between Banach spaces which allows us to use classical tools from geometric Banach space theory in a clean and simple way.

The nature of dark matter is one of the longest-standing puzzles in science. Axions or axionlike particles are a key possibility and arise in mechanisms to solve the strong $CP$ problem, but also in low-energy limits of string theory. Extensive experimental and observational efforts are actively looking for “axionic” imprints. Independent of their nature, abundance, and contribution to the dark matter problem, axions form dense clouds around spinning black holes, grown by superradiant mechanisms. It was recently suggested that once couplings to photons are considered, an exponential (quantum) stimulated emission of photons ensues at large enough axion number. Here we solve numerically the classical problem in different setups. We show that laserlike emission from clouds exists at the classical level, and we provide the first quantitative description of the problem.

We show for the first time that it is possible to realize laser beam focusing at the few-photon level in the four-wave-mixing process, and at the same time reduce the quantum uncertainty in width. The reduction in quantum uncertainty results directly from the strong suppression of local intensity fluctuations. This surprising effect of simultaneous focusing and reduction of width uncertainty is enabled by multi-spatial-mode (MSM) squeezing, and is not possible via any classical optical approach or single-spatial-mode squeezing. Our results open promising possibilities for quantum-enhanced imaging and metrology; as an example, the limit on the measurement of very small beam displacement can be enhanced within feasible experimental parameters because of beam focusing and the noiseless amplification in the MSM squeezing process.

We show that moiré bands of twisted homobilayers can be topologically nontrivial, and illustrate the tendency by studying valence band states in $\pm K$ valleys of twisted bilayer transition metal dichalcogenides, in particular, bilayer ${\mathrm{MoTe}}_2$. Because of the large spin-orbit splitting at the monolayer valence band maxima, the low energy valence states of the twisted bilayer ${\mathrm{MoTe}}_2$ at the $+K$ ($-K$) valley can be described using a two-band model with a layer-pseudospin magnetic field $\mathbf{\Delta }(\mathit{r})$ that has the moiré period. We show that $\mathbf{\Delta }(\mathit{r})$ has a topologically nontrivial skyrmion lattice texture in real space, and that the topmost moiré valence bands provide a realization of the Kane-Mele quantum spin-Hall model, i.e., the two-dimensional time-reversal-invariant topological insulator. Because the bands narrow at small twist angles, a rich set of broken symmetry insulating states can occur at integer numbers of electrons per moiré cell.

Since the discovery of the fractional quantum Hall effect in 1982 there has been considerable theoretical discussion on the possibility of fractional quantization of conductance in the absence of Landau levels formed by a quantizing magnetic field. Although various situations have been theoretically envisaged, particularly lattice models in which band flattening resembles Landau levels, the predicted fractions have never been observed. In this Letter, we show that odd and even denominator fractions can be observed, and manipulated, in the absence of a quantizing magnetic field, when a low-density electron system in a GaAs based one-dimensional quantum wire is allowed to relax in the second dimension. It is suggested that such a relaxation results in formation of a zigzag array of electrons with ring paths which establish a cyclic current and a resultant lowering of energy. The behavior has been observed for both symmetric and asymmetric confinement but increasing the asymmetry of the confinement potential, to result in a flattening of confinement, enhances the appearance of new fractional states. We find that an in-plane magnetic field induces new even denominator fractions possibly indicative of electron pairing. The new quantum states described here have implications both for the physics of low dimensional electron systems and also for quantum technologies. This work will enable further development of structures which are designed to electrostatically manipulate the electrons for the formation of particular configurations. In turn, this could result in a designer tailoring of fractional states to amplify particular properties of importance in future quantum computation.