We predict that junctions between an antiferromagnetic insulator and a superconductor provide a robust platform to create a one-dimensional topological superconducting state. Its emergence does not require the presence of intrinsic spin-orbit coupling nor noncollinear magnetism, but arises solely from repulsive electronic interactions on interfacial solitonic states. We demonstrate that a topological superconducting state is generated by repulsive interactions at arbitrarily small coupling strength, and that the size of the topological gap rapidly saturates to one of the parent trivial superconductor. Our results put forward antiferromagnetic insulators as a new platform for interaction-driven topological superconductivity.

Connections between continuous and discrete worlds tend to be elusive. One example is curvature. Even though there exist numerous nonequivalent definitions of graph curvature, none is known to converge in any limit to any traditional definition of curvature of a Riemannian manifold. Here we show that Ollivier curvature of random geometric graphs in any Riemannian manifold converges in the continuum limit to Ricci curvature of the underlying manifold, but only if the definition of Ollivier graph curvature is properly generalized to apply to mesoscopic graph neighborhoods. This result establishes a rigorous link between a definition of curvature applicable to networks and a traditional definition of curvature of smooth spaces.

We study the crossover between equilibrium and off-equilibrium dynamical universality classes in the Vicsek model near its ordering transition. Starting from the incompressible hydrodynamic theory of Chen et al. [Critical phenomenon of the order-disorder transition in incompressible active fluids, New J. Phys. 17, 042002 (2015)], we show that increasing the activity leads to a renormalization group (RG) crossover between the equilibrium ferromagnetic fixed point, with dynamical critical exponent $z=2$, and the off-equilibrium active fixed point, with $z=1.7$ (in $d=3$). We run simulations of the classic Vicsek model in the near-ordering regime and find that critical slowing down indeed changes with activity, displaying two exponents that are in remarkable agreement with the RG prediction. The equilibrium to off-equilibrium crossover is ruled by a characteristic length scale, beyond which active dynamics takes over. The larger the activity is, the smaller is such a length scale, suggesting the existence of a general trade-off between activity and the system's size in determining the dynamical universality class of active matter.

We study the transport and spectral properties of a non-Hermitian one-dimensional disordered lattice, the diagonal matrix elements of which are random complex variables taking both positive (loss) and negative (gain) imaginary values: Their distribution is either the usual rectangular one or a binary pair-correlated one possessing, in its Hermitian version, delocalized states and unusual transport properties. Contrary to the Hermitian case, all states in our non-Hermitian system are localized. In addition, the eigenvalue spectrum, for the binary pair-correlated case, exhibits an unexpected intricate fractallike structure on the complex plane and with increasing non-Hermitian disorder, the eigenvalues tend to coalesce in particular small areas of the complex plane, a feature termed “eigenvalue condensation.” Despite the strong Anderson localization of all eigenstates, the system appears to exhibit transport not by diffusion but by a new mechanism through sudden jumps between states located even at distant sites. This seems to be a general feature of open non-Hermitian random systems. The relation of our findings to recent experimental results is also discussed.

We study the nonequilibrium steady states of a one-dimensional fluid in a finite-space region of length $L$. Particles interact among themselves by multi-particle collisions and are in contact with two thermal-wall heat reservoirs, located at the boundaries of the region. After an initial ballistic regime, we find a crossover from a normal (kinetic) transport regime to an anomalous (hydrodynamic) one, above a characteristic size $L_{*}$. We argue that $L_{*}$ is proportional to the cube of the collision time among particles. Motivated by the physics of emissive divertors in fusion plasma, we consider the more general case of thermal walls injecting particles with given average (nonthermal) velocity. For fast and relatively cold particles, short systems fail to establish local equilibrium and display non-Maxwellian distributions of velocities.

The Marchenko integral, key to inverse scattering problems across many disciplines, is a long-standing equation that relates single-sided reflection data and Green's functions for virtual source locations inside of an inaccessible, one-dimensional volume. The concept was later expanded to two and three dimensions, yielding important advances in imaging complex media, particularly in the context of geophysics. However, this expansion is based on a set of coupled Marchenko equations which requires up and down decomposition of the wave fields at both the level of the measurement surface and the level of the virtual source of the desired Green's function. The underlying theory implies that the recently developed Marchenko relations, while enabling novel applications, carry intrinsic limitations. For example, this scheme cannot incorporate evanescent or refracted waves, and in turn practical implementations must discard data to meet such requirements. We present a derivation that circumvents these limitations, thereby yielding a Marchenko integral akin to those in recent advances, but that is more general than previously assumed. We set up a wave equation based framework to describe the physical concept of focusing functions by introducing homogeneous Green's functions of the second kind. Based on this, we derive integral representations for both closed and open boundary volumes. Owing to our perspective on the integral formalism, we present an inverse scattering approach for retrieving Green's functions from single-sided reflection data—with the same practical applicability of recent methods but without any limitations due to one-way decomposition. Finally, we illustrate the capability of the Marchenko method to obtain the full wave field, including evanescent and refracted waves, within an unknown scattering medium by means of a numerical example.

Axion-like particles (ALPs) are predicted to mediate exotic interactions between spin and mass. We propose an ALP-searching experiment based on the levitated micromechanical oscillator, which is one of the most sensitive sensors for spin-mass forces at a short distance. The proposed experiment tests the spin-mass resonant interaction between the polarized electron spins and a diamagnetically levitated microsphere. By periodically flipping the electron spins, the contamination from nonresonant background forces can be eliminated. The levitated microoscillator can prospectively enhance the sensitivity by nearly ${10}^3$ times over current experiments for ALPs with mass in the range 4 meV to 0.4 eV.

Hardware-efficient transpilation of quantum circuits to a quantum device native gate set is essential for the execution of quantum algorithms on noisy quantum computers. Typical quantum devices utilize a gate set with a single two-qubit Clifford entangling gate per pair of coupled qubits; however, in some applications access to a non-Clifford two-qubit gate can result in more optimal circuit decompositions and also allows more flexibility in optimizing over noise. We demonstrate calibration of a low-error non-Clifford controlled-$\frac{\pi }{2}$ phase (cs) gate on a cloud-based IBM Quantum system using the Qiskit Pulse framework. To measure the gate error of the calibrated cs gate we perform non-Clifford cnot-dihedral interleaved randomized benchmarking. We are able to obtain a gate error of $5.9\left(7\right)\times {10}^{-3}$ at a gate length 263 ns, which is close to the coherence limit of the associated qubits, and lower error than the back-end standard calibrated cnot gate.

Outgassing remains a pertinent issue as it typically is the first stage of possible plasma formation, and can lead to effects such as breakdown, surface flashover, and pulse shortening in high power systems. Here two pertinent aspects are probed: (i) a model-based assessment of outgassing and associated temperature-dependent rates from a copper electrode based on molecular dynamics simulations, and (ii) calculations for the sticking coefficients of hydrogen gas atoms as a function of incident energy and angle. Our results of temperature dependent diffusion coefficients for hydrogen in copper, agree well with experimental reports over a wide range from 300 to 1350 K, and show reduction in the presence of vacancies. Results also show low reflection coefficients at both high and low energies, with a maxima at around 6.5 eV. A curve fit to the data is predicted to roughly hold for a range of incident angles. Adsorption is predicted to occur for incident energies below 10 eV, with absorption dominating above 10 eV.

The dynamics of nonequilibrium spatially extended systems are often dominated by fluctuations, e.g., due to deterministic chaos or intrinsic stochasticity. This reflects into generic scale invariant or kinetic roughening behavior that can be classified into universality classes defined by critical exponent values and by the probability distribution function (PDF) of field fluctuations. Suitable geometrical constraints are known to change secondary features of the PDF while keeping the values of the exponents unchanged, inducing universality subclasses. Working on the Kuramoto-Sivashinsky equation as a paradigm of spatiotemporal chaos, we show that the physical nature of the prevailing fluctuations (chaotic or stochastic) can also change the universality class while respecting the exponent values, as the PDF is substantially altered. This transition takes place at a nonzero value of the stochastic noise amplitude and may be suitable for experimental verification.

Emission of photons by atoms can occur into modes which extend into a region causally disconnected with the emitter. For example, a uniformly accelerated ground-state atom emits a photon into the Unruh-Minkowski mode which is exponentially larger in the causally disconnected region. This makes an impression that photon emission is acausal. Here we show that conventional quantum optical analysis yields that a detector atom will not detect the emitted photon in the region noncausally connected with the emitter. However, joint excitation probability of atoms in the causally disconnected regions can be correlated due to entanglement of Minkowski vacuum and be much larger than the product of independent excitation probabilities. Moreover, atoms uniformly accelerated in the same Rindler wedge cannot become simultaneously excited without changing the state of the field, that is, the Unruh-Minkowski photon emitted by one atom cannot be absorbed by the other atom. We discuss examples demonstrating interesting features of Minkowski vacuum entanglement.

We report on the fate of the quantum Hall effect in graphene under intense laser illumination. By using Floquet theory combined with both a low energy description and full tight-binding models, we clarify the selection rules, the quasienergy band structure, as well as their connection with the two-terminal and multiterminal conductance in a device setup as relevant for experiments. We show that the well-known dynamical gaps that appear in the Floquet spectrum at $\pm \hslash \mathrm{\Omega }/2$ lead to a switch-off of the quantum Hall edge transport for different edge terminations except for the armchair one, where two terms cancel out exactly. More interestingly, we show that near the Dirac point changing the laser polarization (circular right or circular left) controls the Hall conductance, by allowing to switch it on or off, or even by flipping its sign, thereby reversing the chirality of the edge states. This might lead to new avenues to fully control topologically protected transport.

Non-Gaussian receivers for optical communication with coherent states can achieve measurement sensitivities beyond the limits of conventional detection, given by the quantum-noise limit (QNL). However, the amount of information that can be reliably transmitted substantially degrades if there is noise in the communication channel, unless the receiver is able to efficiently compensate for such noise. Here, we investigate the use of a deep neural network as a computationally efficient estimator of phase and amplitude channel noise to enable a reliable method for noise tracking for non-Gaussian receivers. The neural network uses the data collected by the non-Gaussian receiver to estimate and correct for dynamic channel noise in real time. Using numerical simulations, we find that this noise tracking method allows the non-Gaussian receiver to maintain its benefit over the QNL across a broad range of strengths and bandwidths of phase and intensity noise. The noise tracking method based on neural networks can further include other types of noise to ensure sub-QNL performance in channels with many sources of noise.

Squeezed light has evolved into a powerful tool for quantum technology, ranging from quantum-enhanced sensing and quantum-state engineering based on partial postselection techniques. The pulsed generation of squeezed light is of particular interest, as it can provide accurate time stamps and a physically defined temporal mode, which are highly preferred in complex communication networks and large-scale information processing. However, the multimode feature of pulsed squeezing limits the purity of the output state, negatively impacting its application in quantum technology. Previous demonstrations and analysis of pulsed squeezing focus on single-pass configurations and synchronously pumped free-space cavities. In this paper, we propose a different approach to generate pulsed squeezing with high temporal purity, where a parametric down-conversion process in integrated photonic cavities is pumped by a single-pass pulse. We show that the effective mode number of the output pulsed squeezing approaches unity. Such a high-purity squeezed light can be realized with broad parameters and a low pump power, providing a robust approach to generating large-scale quantum resources.

We theoretically investigate the crystalline anisotropy of topological phase transitions in phase-controlled planar Josephson junctions (JJs) subject to spin-orbit coupling and in-plane magnetic fields. It is shown how topological superconductivity (TS) is affected by the interplay between the magnetic field and the orientation of the junction with respect to its crystallographic axes. This interplay can be used to electrically tune between different symmetry classes in a controlled fashion and thereby optimize the stability and localization of Majorana bound states in planar Josephson junctions. Our findings can be used as a guide for achieving the most favorable conditions when engineering TS in planar JJs and can be particularly relevant for setups containing noncollinear junctions which have been proposed for performing braiding operations on multiple Majorana pairs.

The variational quantum eigensolver (VQE) is a promising algorithm to compute eigenstates and eigenenergies of a given quantum system that can be performed on a near-term quantum computer. Obtaining eigenstates and eigenenergies in a specific symmetry sector of the system is often necessary for practical applications of the VQE in various fields ranging from high-energy physics to quantum chemistry. It is common to add a penalty term in the cost function of the VQE to calculate such a symmetry-resolving energy spectrum; however, systematic analysis of the effect of the penalty term has been lacking, and the use of the penalty term in the VQE has not been justified rigorously. In this paper, we investigate two major types of penalty terms for the VQE that were proposed in previous studies. We show that a penalty term of one of the two types works properly in that eigenstates obtained by the VQE with the penalty term reside in the desired symmetry sector. We further give a convenient formula to determine the magnitude of the penalty term, which may lead to faster convergence of the VQE. Meanwhile, we prove that the other type of penalty term does not work for obtaining the target state with the desired symmetry in a rigorous sense and even gives completely wrong results in some cases. We finally provide numerical simulations to validate our analysis. Our results apply to general quantum systems and lay the theoretical foundation for the use of the VQE with the penalty terms to obtain the symmetry-resolving energy spectrum of the system, which fuels the application of a near-term quantum computer.

We study the evolution of a cold single ${\mathrm{BaRb}}^{+}$ molecule while it continuously collides with ultracold Rb atoms. The initially weakly bound molecule can undergo a sequence of elastic, inelastic, reactive, and radiative processes. We investigate these processes by developing methods for discriminating between different ion species, electronic states, and kinetic ion energy ranges. By analyzing the experimental data while taking into account theoretical insights, we obtain a consistent description of the typical trajectory through the manifold of available atomic and molecular states. Monte Carlo simulations describe the measured dynamics well. As a further result, we determine rates for collisional and radiative relaxation as well as photodissociation, spin-flip collisions, and chemical reactions.

Realistic attosecond wave packets have complex profiles that, in dispersive conditions, rapidly broaden or split in multiple components. Such behaviors are encoded in sharp features of the wave packet spectral phase. Here we exploit the quantum beating between one- and two-photon transitions in an attosecond photoionization experiment to measure the photoelectron spectral phase continuously across a broad energy range. Supported by numerical simulations, we demonstrate that this experimental technique is able to reconstruct sharp fine-scale features of the spectral phase, continuously as a function of energy and across the full spectral range of an attosecond pulse train. In a proof-of-principle experiment, we observe the periodic modulations of the spectral phase of an attosecond pulse train due to the individual chirp of each harmonic.

Flopping mode qubits in double quantum dots (DQDs) allow for coherent spin-photon hybridization and fast qubit gates when coupled to either an alternating external or a quantized cavity electric field. To achieve this, however, electronic systems rely on synthetic spin-orbit interaction (SOI) by means of a magnetic field gradient as a coupling mechanism. Here we theoretically show that this challenging experimental setup can be avoided in heavy-hole (HH) systems in germanium (Ge) by utilizing the sizable cubic Rashba SOI. We argue that the resulting natural flopping mode qubit possesses highly tunable spin coupling strengths that allow for qubit gate times in the nanosecond range when the system is designed to function in an optimal operation mode which we quantify.

We perform direct numerical simulations to compute the net circular polarization of gravitational waves from helical (chiral) turbulent sources in the early Universe for a variety of initial conditions, including driven (stationary) and decaying turbulence. We investigate the resulting gravitational wave signal assuming different turbulent geneses such as magnetically or kinetically driven cases. Under realistic physical conditions in the early Universe we compute numerically the wave number-dependent polarization degree of the gravitational waves. We find that the spectral polarization degree strongly depends on the initial conditions. The peak of the spectral polarization degree occurs at twice the typical wave number of the source, as expected, and for fully helical decaying turbulence, it reaches its maximum of nearly 100% only at the peak. We determine the temporal evolution of the turbulent sources as well as the resulting gravitational waves, showing that the dominant contribution to their spectral energy density happens shortly after the activation of the source. Only through an artificially prolonged decay of the turbulence can further increase of the gravitational wave amplitude be achieved. We estimate the detection prospects for the net polarization, arguing that its detection contains clean information (including the generation mechanisms, time, and strength) about the sources of possible parity violations in the early Universe.