We study the gravity-mediated scattering of scalar fields based on a parameterization of the Lorentzian quantum effective action. We demonstrate that the interplay of infinite towers of spin zero and spin two poles at imaginary squared momentum leads to scattering amplitudes that are compatible with unitarity bounds, causal, and scale-free at trans-Planckian energy. Our construction avoids introducing nonlocalities or the massive higher-spin particles that are characteristic in string theory.

A nontrivial $\mathcal{S}$ matrix generally implies a production of entanglement: starting with an incoming pure state, the scattering generally returns an outgoing state with nonvanishing entanglement entropy. It is then interesting to ask if there exists a nontrivial $\mathcal{S}$ matrix that generates no entanglement. In this Letter, we argue that the answer is the $\mathcal{S}$-matrix for the scattering of classical black holes. We study the spin entanglement in the scattering of arbitrary spinning particles. Augmenting the $\mathcal{S}$-matrix with Thomas–Wigner rotation factors, we derive the entanglement entropy from the gravitational induced $2\to 2$ amplitude. In the Eikonal limit, we find that the relative entanglement entropy, defined here as the difference between the entanglement entropy of the in and out states, is nearly zero for minimal coupling irrespective of the in state and increases significantly for any nonvanishing spin multipole moments. This finding suggests that minimal couplings of spinning particles, whose classical limit corresponds to a Kerr black hole, have the unique feature of generating near zero entanglement.

The cascading 3.21 and 4.44 MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65 MeV excitation energy in ${}^{12}\mathrm{C}$, excited by the ${}^{12}\mathrm{C}(p,p^{\prime })$ reaction at 10.7 MeV proton energy. From the proton-$\gamma \text{−}\gamma$ triple coincidence data, a value of ${\mathrm{\Gamma }}_{\mathrm{rad}}/\mathrm{\Gamma }=6.2(6)\times {10}^{-4}$ was obtained for the radiative branching ratio. Using our results, together with ${\mathrm{\Gamma }}_{\pi }^{E0}/\mathrm{\Gamma }$ from Eriksen et al. [Phys. Rev. C 102, 024320 (2020)] and the currently adopted ${\mathrm{\Gamma }}_{\pi }(E0)$ values, the radiative width of the Hoyle state is determined as ${\mathrm{\Gamma }}_{\mathrm{rad}}=5.1(6)\times {10}^{-3}\mathrm{eV}$. This value is about 34% higher than the currently adopted value and will impact models of stellar evolution and nucleosynthesis.

The conventional characterization of periodically driven systems usually necessitates the time-domain information beyond Floquet bands, hence lacking universal and direct schemes of measuring Floquet topological invariants. Here we propose a unified theory, based on quantum quenches, to characterize generic $d$-dimensional Floquet topological phases in which the topological invariants are constructed with only minimal information of the static Floquet bands. For a $d$-dimensional phase that is initially static and trivial, we introduce the quench dynamics by suddenly turning on the periodic driving. We show that the quench dynamics exhibits emergent topological patterns in ($d-1$)-dimensional momentum subspaces where Floquet bands cross, from which the Floquet topological invariants are directly obtained. This result provides a simple and unified characterization in which one can extract the number of conventional and anomalous Floquet boundary modes and identify the topologically protected singularities in the phase bands. These applications are illustrated with one- and two-dimensional models that are readily accessible in cold-atom experiments. Our study opens a new framework for the characterization of Floquet topological phases.

We develop a rigorous theoretical framework for interaction-induced phenomena in the waveguide quantum electrodynamics (QED) driven by mechanical oscillations of the qubits. Specifically, we predict that the simplest setup of two qubits, harmonically trapped over an optical waveguide, enables the ultrastrong coupling regime of the quantum optomechanical interaction. Moreover, the combination of the inherent open nature of the system and the strong optomechanical coupling leads to emerging parity-time ($\mathcal{P}\mathcal{T}$) symmetry, quite unexpected for a purely quantum system without artificially engineered gain and loss. The $\mathcal{P}\mathcal{T}$ phase transition drives long-living subradiant states, observable in the state-of-the-art waveguide QED setups.

The impact of a droplet on an undercooled surface is a complex phenomenon as it simultaneously instigates several physical processes that cover a broad spectrum of transport phenomena and phase transition. Here, we report and explain an unexpected but highly relevant phenomenon of fingered growth of the solid phase. It emerges during the impact of a binary droplet that freezes from the outside prior to the impact on the undercooled surface. We establish that the presence of presolidified material at the advancing contact line fundamentally changes the resulting dynamics, namely, by modifying the local flow mobility that leads to an instability analogous to viscous fingering. Moreover, we delineate the interplay between the interfacial deformations of the impacting droplet and patterned growth of the solid phase as disconnected patterns emerge at faster impacts.

The mobility of a fakir state droplet on a structured surface is fundamentally determined by the effective length of a microscopic contact line. However, it is largely unknown how the surface topography determines the effective contact line length. Based on the direct measurement of droplet adhesion force and the visualization of contact line, this work shows that effective contact line length is topography dependent as opposed to prior notion. On pored surfaces, contact line is not distorted, and the effective length approaches the droplet apparent perimeter regardless of pore dimensions. On pillared surfaces, the distortion of contact line is significantly dependent on the packing density of the pillar structures so that the effective length is as small as a pillar diameter on densely packed pillars and as large as a pillar perimeter on sparsely-packed pillars, while changing linearly between the two extremes.

The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide ($2\mathrm{H}\text{−}{\mathrm{TaS}}_2$) at low temperature is boosted by applying hydrostatic pressures to study both vibrational and magnetic transport properties. Around $P_c$, we observe a superconducting dome with a maximum superconducting transition temperature $T_c=9.1\mathrm{K}$. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods. While a $d$ wave and single-gap BCS prediction cannot describe the lower critical field $H_{c1}$ data, the temperature dependence of the $H_{c1}$ can be well described by a single-gap anisotropic $s$-wave order parameter.

We study the transverse dynamical susceptibility of an antiferromagnetic spin-$1/2$ chain in the presence of a longitudinal Zeeman field. In the low magnetization regime in the gapless phase, we show that the marginally irrelevant backscattering interaction between the spinons creates a nonzero gap between two branches of excitations at small momentum. We further demonstrate how this gap varies upon introducing a second neighbor antiferromagnetic interaction, vanishing in the limit of a noninteracting “spinon gas.” In the high magnetization regime, as the Zeeman field approaches the saturation value, we uncover the appearance of two-magnon bound states in the transverse susceptibility. This bound state feature generalizes the one arising from string states in the Bethe ansatz solution of the integrable case. Our results are based on numerically accurate, unbiased matrix-product-state techniques as well as analytic approximations.

We report the observation of an intriguing behavior in the transport properties of nanodevices operating in a regime between the Fabry-Pérot and the Kondo limits. Using ultrahigh quality nanotube devices, we study how the conductance oscillates when sweeping the gate voltage. Surprisingly, we observe a fourfold enhancement of the oscillation period upon decreasing temperature, signaling a crossover from single-electron tunneling to Fabry-Pérot interference. These results suggest that the Fabry-Pérot interference occurs in a regime where electrons are correlated. The link between the measured correlated Fabry-Pérot oscillations and the SU(4) Kondo effect is discussed.

We study the microscopic origin of nonlocality in dense granular media. Discrete element simulations reveal that macroscopic shear results from a balance between microscopic elementary rearrangements occurring in opposite directions. The effective macroscopic fluidity of the material is controlled by these velocity fluctuations, which are responsible for nonlocal effects in quasistatic regions. We define a new micromechanically based unified constitutive law describing both quasistatic and inertial regimes, valid for different system configurations.

Shock waves are examples of the far-from-equilibrium behavior of matter; they are ubiquitous in nature, yet the underlying microscopic mechanisms behind their formation are not well understood. Here, we study the dynamics of dispersive quantum shock waves in a one-dimensional Bose gas, and show that the oscillatory train forming from a local density bump expanding into a uniform background is a result of quantum mechanical self-interference. The amplitude of oscillations, i.e., the interference contrast, decreases with the increase of both the temperature of the gas and the interaction strength due to the reduced phase coherence length. Furthermore, we show that vacuum and thermal fluctuations can significantly wash out the interference contrast, seen in the mean-field approaches, due to shot-to-shot fluctuations in the position of interference fringes around the mean.

Adiabatic quantum computing enables the preparation of many-body ground states. Realization poses major experimental challenges: Direct analog implementation requires complex Hamiltonian engineering, while the digitized version needs deep quantum gate circuits. To bypass these obstacles, we suggest an adiabatic variational hybrid algorithm, which employs short quantum circuits and provides a systematic quantum adiabatic optimization of the circuit parameters. The quantum adiabatic theorem promises not only the ground state but also that the excited eigenstates can be found. We report the first experimental demonstration that many-body eigenstates can be efficiently prepared by an adiabatic variational algorithm assisted with a multiqubit superconducting coprocessor. We track the real-time evolution of the ground and excited states of transverse-field Ising spins with a fidelity that can reach about 99%.

Resonance fluorescence has played a major role in quantum optics with predictions and later experimental confirmation of nonclassical features of its emitted light such as antibunching or squeezing. In the Rayleigh regime where most of the light originates from the scattering of photons with subnatural linewidth, antibunching would appear to coexist with sharp spectral lines. Here, we demonstrate that this simultaneous observation of subnatural linewidth and antibunching is not possible with simple resonant excitation. Using an epitaxial quantum dot for the two-level system, we independently confirm the single-photon character and subnatural linewidth by demonstrating antibunching in a Hanbury Brown and Twiss type setup and using high-resolution spectroscopy, respectively. However, when filtering the coherently scattered photons with filter bandwidths on the order of the homogeneous linewidth of the excited state of the two-level system, the antibunching dip vanishes in the correlation measurement. Our observation is explained by antibunching originating from photon-interferences between the coherent scattering and a weak incoherent signal in a skewed squeezed state. This prefigures schemes to achieve simultaneous subnatural linewidth and antibunched emission.

All known examples suggesting an exponential separation between classical simulation algorithms and stoquastic adiabatic quantum computing (StoqAQC) exploit symmetries that constrain adiabatic dynamics to effective, symmetric subspaces. The symmetries produce large effective eigenvalue gaps, which in turn make adiabatic computation efficient. We present a classical algorithm to subexponentially sample from an effective subspace of any $k$-local stoquastic Hamiltonian $H$, without a priori knowledge of its symmetries (or near symmetries). Our algorithm maps any $k$-local Hamiltonian to a graph $G=(V,E)$ with $|V|=O\mathbf{(}\mathrm{poly}(n)\mathbf{)}$, where $n$ is the number of qubits. Given the well-known result of Babai [Graph isomorphism in quasipolynomial time, in Proceedings of the Forty-Eighth Annual ACM Symposium on Theory of Computing (2016), pp. 684–697], we exploit graph isomorphism to study the automorphisms of $G$ and arrive at an algorithm quasipolynomial in $|V|$ for producing samples from effective subspace eigenstates of $H$. Our results rule out exponential separations between StoqAQC and classical computation that arise from hidden symmetries in $k$-local Hamiltonians. Our graph representation of $H$ is not limited to stoquastic Hamiltonians and may rule out corresponding obstructions in nonstoquastic cases, or be useful in studying additional properties of $k$-local Hamiltonians.

We present an algorithm for studying quantum systems at finite temperature using continuous matrix product operator representation. The approach handles both short-range and long-range interactions in the thermodynamic limit without incurring any time discretization error. Moreover, the approach provides direct access to physical observables including the specific heat, local susceptibility, and local spectral functions. After verifying the method using the prototypical quantum XXZ chains, we apply it to quantum Ising models with power-law decaying interactions and on the infinite cylinder, respectively. The approach offers predictions that are relevant to experiments in quantum simulators and the nuclear magnetic resonance spin-lattice relaxation rate.

The ${}^{80}\mathrm{Ge}$ structure was investigated in a high-statistics $\beta$-decay experiment of ${}^{80}\mathrm{Ga}$ using the GRIFFIN spectrometer at TRIUMF-ISAC through $\gamma$, $\beta \text{−}e$, $e\text{−}\gamma$, and $\gamma \text{−}\gamma$ spectroscopy. No evidence was found for the recently reported $0_2^{+}$ 639-keV level suggested as evidence for low-energy shape coexistence in ${}^{80}\mathrm{Ge}$. Large-scale shell model calculations performed in ${}^{78,80,82}\mathrm{Ge}$ place the $0_2^{+}$ level in ${}^{80}\mathrm{Ge}$ at 2 MeV. The new experimental evidence combined with shell model predictions indicate that low-energy shape coexistence is not present in ${}^{80}\mathrm{Ge}$.