We report the first demonstration of a regime of operation in optical parametric oscillators (OPOs), in which the formation of temporal simultons produces stable femtosecond half-harmonic pulses. Simultons are simultaneous bright-dark solitons of a signal field at frequency $\omega$ and the pump field at $2\omega$, which form in a quadratic nonlinear medium. The formation of simultons in an OPO is due to the interplay of nonlinear pulse acceleration with the timing mismatch between the pump repetition period and the cold-cavity round-trip time and is evidenced by ${\mathrm{sech}}^2$ spectra with broad instantaneous bandwidths when the resonator is detuned to a slightly longer round-trip time than the pump repetition period. We provide a theoretical description of an OPO operating in a regime dominated by these dynamics, observe the distinct features of simulton formation in an experiment, and verify our results with numerical simulations. These results represent a new regime of operation in nonlinear resonators, which can lead to efficient and scalable sources of few-cycle frequency combs at arbitrary wavelengths.

Electron-ion collisional dynamics is of fundamental importance in determining plasma transport properties, nonequilibrium plasma evolution, and electron damage in diffraction imaging applications using bright x-ray free-electron lasers (FELs). Here we describe the first experimental measurements of ultrafast electron impact collisional ionization dynamics using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with the Linac Coherent Light Source x-ray FEL. By resonantly pumping the $1s\to 2p$ transition in highly charged ions within an optically thin plasma, we have measured how off-resonance charge states are populated via collisional processes on femtosecond time scales. We present a collisional cross section model that matches our results and demonstrates how the cross sections are enhanced by dense-plasma effects including continuum lowering. Nonlocal thermodynamic equilibrium collisional radiative simulations show excellent agreement with the experimental results and provide new insight on collisional ionization and three-body-recombination processes in the dense-plasma regime.

We investigate unconventional superconductivity in three-dimensional electronic systems with the chemical potential close to a quadratic band touching point in the band dispersion. Short-range interactions can lead to $d$-wave superconductivity, described by a complex tensor order parameter. We elucidate the general structure of the corresponding Ginzburg-Landau free energy and apply these concepts to the case of an isotropic band touching point. For a vanishing chemical potential, the ground state of the system is given by the superconductor analogue of the uniaxial nematic state, which features line nodes in the excitation spectrum of quasiparticles. In contrast to the theory of real tensor order in liquid crystals, however, the ground state is selected here by the sextic terms in the free energy. At a finite chemical potential, the nematic state has an additional instability at weak coupling and low temperatures. In particular, the one-loop coefficients in the free energy indicate that at weak coupling genuinely complex orders, which break time-reversal symmetry, are energetically favored. We relate our analysis to recent measurements in the half-Heusler compound YPtBi and discuss the role of cubic crystal symmetry.

The determination of the band gap and exciton energies of lead halide perovskites is very important from the viewpoint of fundamental physics and photonic device applications. By using photoluminescence excitation (PLE) spectra, we reveal the optical properties of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbCl}}_3$ single crystals in the near-band-edge energy regime. The one-photon PLE spectrum exhibits the $1s$ exciton peak at 3.11 eV. On the contrary, the two-photon PLE spectrum exhibits no peak structure. This indicates photon recycling of excitonic luminescence. By analyzing the spatial distribution of the excitons and photon recycling, we obtain 3.15 eV for the band gap energy and 41 meV for the exciton binding energy.

We use molecular simulations to probe the local viscoelasticity of an entangled polymer melt by tracking the motion of embedded nonsticky nanoparticles (NPs). As in conventional microrheology, the generalized Stokes-Einstein relation is employed to extract an effective stress relaxation function $G_{\mathrm{GSE}}(t)$ from the mean square displacement of NPs. $G_{\mathrm{GSE}}(t)$ for different NP diameters $d$ are compared with the stress relaxation function $G(t)$ of a pure polymer melt. The deviation of $G_{\mathrm{GSE}}(t)$ from $G(t)$ reflects the incomplete coupling between NPs and the dynamic modes of the melt. For linear polymers, a plateau in $G_{\mathrm{GSE}}(t)$ emerges as $d$ exceeds the entanglement mesh size $a$ and approaches the entanglement plateau in $G(t)$ for a pure melt with increasing $d$. For ring polymers, as $d$ increases towards the spanning size $R$ of ring polymers, $G_{\mathrm{GSE}}(t)$ approaches $G(t)$ of the ring melt with no entanglement plateau.

Evolution occurs in populations of reproducing individuals. Reproduction depends on the payoff a strategy receives. The payoff depends on the environment that may change over time, on intrinsic uncertainties, and on other sources of randomness. These temporal variations in the payoffs can affect which traits evolve. Understanding evolutionary game dynamics that are affected by varying payoffs remains difficult. Here we study the impact of arbitrary amplitudes and covariances of temporally varying payoffs on the dynamics. The evolutionary dynamics may be “unfair,” meaning that, on average, two coexisting strategies may persistently receive different payoffs. This mechanism can induce an anomalous coexistence of cooperators and defectors in the prisoner’s dilemma, and an unexpected selection reversal in the hawk-dove game.

Solving linear systems of equations is a frequently encountered problem in machine learning and optimization. Given a matrix $A$ and a vector $\mathbf{b}$ the task is to find the vector $\mathbf{x}$ such that $A\mathbf{x}=\mathbf{b}$. We describe a quantum algorithm that achieves a sparsity-independent runtime scaling of $\mathcal{O}\mathbf{(}{\kappa }^2\sqrt{n}\text{polylog}(n)/\epsilon \mathbf{)}$ for an $n\times n$ dimensional $A$ with bounded spectral norm, where $\kappa$ denotes the condition number of $A$, and $\epsilon$ is the desired precision parameter. This amounts to a polynomial improvement over known quantum linear system algorithms when applied to dense matrices, and poses a new state of the art for solving dense linear systems on a quantum computer. Furthermore, an exponential improvement is achievable if the rank of $A$ is polylogarithmic in the matrix dimension. Our algorithm is built upon a singular value estimation subroutine, which makes use of a memory architecture that allows for efficient preparation of quantum states that correspond to the rows of $A$ and the vector of Euclidean norms of the rows of $A$.

It has been conjectured that, for any distillation protocol for magic states for the $T$ gate, the number of noisy input magic states required per output magic state at output error rate $\epsilon$ is $\mathrm{\Omega }[\log (1/\epsilon )]$. We show that this conjecture is false. We find a family of quantum error correcting codes of parameters $⟦\sum _{i=w+1}^m(\genfrac{}{}{0ex}{}{m}{i}),\sum _{i=0}^w(\genfrac{}{}{0ex}{}{m}{i}),\sum _{i=w+1}^{r+1}(\genfrac{}{}{0ex}{}{r+1}{i})⟧$ for any integers $m>2r$, $r>w\ge 0$, by puncturing quantum Reed-Muller codes. When $m>\nu r$, our code admits a transversal logical gate at the $\nu$th level of Clifford hierarchy. In a distillation protocol for magic states at the level $\nu =3$ ($T$ gate), the ratio of input to output magic states is $O\mathbf{(}{\log }^{\gamma }(1/\epsilon )\mathbf{)}$, where $\gamma =\log (n/k)/\log (d)<0.678$ for some $m$, $r$, $w$. The smallest code in our family for which $\gamma <1$ is on $\approx 2^{58}$ qubits.

We show that a simple modification of the surface code can exhibit an enormous gain in the error correction threshold for a noise model in which Pauli $Z$ errors occur more frequently than $X$ or $Y$ errors. Such biased noise, where dephasing dominates, is ubiquitous in many quantum architectures. In the limit of pure dephasing noise we find a threshold of 43.7(1)% using a tensor network decoder proposed by Bravyi, Suchara, and Vargo. The threshold remains surprisingly large in the regime of realistic noise bias ratios, for example 28.2(2)% at a bias of 10. The performance is, in fact, at or near the hashing bound for all values of the bias. The modified surface code still uses only weight-4 stabilizers on a square lattice, but merely requires measuring products of $Y$ instead of $Z$ around the faces, as this doubles the number of useful syndrome bits associated with the dominant $Z$ errors. Our results demonstrate that large efficiency gains can be found by appropriately tailoring codes and decoders to realistic noise models, even under the locality constraints of topological codes.

We investigate the emergence of halos and Efimov states in nuclei by use of a newly designed model that combines self-consistent mean-field and three-body descriptions. Recent interest in neutron heavy calcium isotopes makes ${}^{72}\mathrm{Ca}$ (${}^{70}\mathrm{Ca}+n+n$) an ideal realistic candidate on the neutron dripline, and we use it as a representative example that illustrates our broadly applicable conclusions. By smooth variation of the interactions we simulate the crossover from well-bound systems to structures beyond the threshold of binding, and find that halo configurations emerge from the mean-field structure for three-body binding energy less than $\sim 100\mathrm{keV}$. Strong evidence is provided that Efimov states cannot exist in nuclei. The structure that bears the most resemblance to an Efimov state is a giant halo extending beyond the neutron-core scattering length. We show that the observable large-distance decay properties of the wave function can differ substantially from the bulk part at short distances, and that this evolution can be traced with our combination of few- and many-body formalisms. This connection is vital for interpretation of measurements such as those where an initial state is populated in a reaction or by a beta decay.

Temporal cavity solitons (CS) are optical pulses that can persist in passive resonators, and they play a key role in the generation of coherent microresonator frequency combs. In resonators made of amorphous materials, such as fused silica, they can exhibit a spectral redshift due to stimulated Raman scattering. Here we show that this Raman-induced self-frequency-shift imposes a fundamental limit on the duration and bandwidth of temporal CSs. Specifically, we theoretically predict that stimulated Raman scattering introduces a previously unidentified Hopf bifurcation that leads to destabilization of CSs at large pump-cavity detunings, limiting the range of detunings over which they can exist. We have confirmed our theoretical predictions by performing extensive experiments in synchronously driven fiber ring resonators, obtaining results in excellent agreement with numerical simulations. Our results could have significant implications for the future design of Kerr frequency comb systems based on amorphous microresonators.

As everyone knows who has opened a kitchen faucet, pipe flow is laminar at low flow velocities and turbulent at high flow velocities. At intermediate velocities, there is a transition wherein plugs of laminar flow alternate along the pipe with “flashes” of a type of fluctuating, nonlaminar flow that remains poorly understood. In the 19th century, Osborne Reynolds sought to connect these states of flow with quantitative “laws of resistance,” whereby the fluid friction is determined as a function of the Reynolds number. While he succeeded for laminar and turbulent flows, the laws for transitional flows eluded him and remain unknown to this day. By properly distinguishing between laminar plugs and flashes in the transitional regime, we show experimentally and numerically that the law of resistance for laminar plugs corresponds to the laminar law and the law of resistance for flashes is identical to that of turbulence.

A new fluid encapsulation structure, which is characterized by a bubble encapsulating a drop, is reported. It is stably generated from the breakup of a liquid column inside a bubble, which is achieved via the injection of Taylor flow into liquid. A model is constructed to explain the liquid column breakup mechanism. A dimensionless control guidance, which enables the possibility to create different-scale capsules, is provided. The encapsulation stability in external flows is verified, and a method to trigger the release of the encapsulated drop is provided, which supports potential applications with great advantages such as fluid transport.

By adsorbing the same species onto both sides of a suspended, atomically thin membrane, it is possible to couple two distinct surface adsorption systems. This new system, with reflection symmetry about the membrane, is described by a phase diagram with two axes, both representing the chemical potential of the same element, but in distinct half-spaces. For the case of potassium adsorption onto a graphene membrane, the result is a devil’s staircase of fractions for the proportion of adsorbates adhered to one side. Fractions with simpler denominators are favored across wider regions of chemical potential, a pattern reminiscent of other fractional systems across a wide range of physics. Since the system can support multiple devil’s staircases each at a distinct overall adsorbate areal density, points along the boundary between adjacent staircases can come arbitrarily close to violating the Gibbs phase rule. This dual-sided adsorbate geometry provides a means to explore surface science for pairs of weakly coupled surfaces.

We show that topology can protect exponentially localized, zero energy edge modes at critical points between one-dimensional symmetry-protected topological phases. This is possible even without gapped degrees of freedom in the bulk—in contrast to recent work on edge modes in gapless chains. We present an intuitive picture for the existence of these edge modes in the case of noninteracting spinless fermions with time-reversal symmetry (BDI class of the tenfold way). The stability of this phenomenon relies on a topological invariant defined in terms of a complex function, counting its zeros and poles inside the unit circle. This invariant can prevent two models described by the same conformal field theory (CFT) from being smoothly connected. A full classification of critical phases in the noninteracting BDI class is obtained: Each phase is labeled by the central charge of the CFT, $c\in \frac{1}{2}\mathbb{N}$, and the topological invariant, $\omega \in \mathbb{Z}$. Moreover, $c$ is determined by the difference in the number of edge modes between the phases neighboring the transition. Numerical simulations show that the topological edge modes of critical chains can be stable in the presence of interactions and disorder.