The potential of diamond films for future semiconductor applications is partly limited by current growth techniques. This limitation can be addressed by achieving an atomic-level understanding of the growth processes. Using atomic force microscopy with atomic resolution, we examined diamond surfaces and observed specific structures, where odd numbers of dimers form ribbonlike configurations. Formed in the nonequilibrium environment of plasma, these structures were evaluated as the most stable configurations through density-functional-theory calculations. Our findings provide a crucial foundation for optimizing the film growth process.

Fast entangling gate operations are a fundamental prerequisite for quantum simulation and computation. We propose an entangling scheme for arbitrary pairs of ions in a linear crystal, harnessing the high electric polarizability of highly excited Rydberg states. An all-to-all quantum gate connectivity is based on an initialization of a pair of ions to a superposition of ground and Rydberg states by laser excitation, followed by the entangling gate operation, which relies on a state-dependent frequency shift of collective vibrational modes of the crystal. This gate operation requires applying an electric waveform to trap electrodes. Employing transverse collective modes of oscillation, we reveal operation times on the order of microseconds within any of the qubit pairs in a small crystal. In our calculation, we take into account realistic experimental conditions and feasible electric field ramps. The proposed gate operation is ready to be combined with a scalable processor architecture to reconfigure the qubit register, either by shuttling ions or by dynamically controlling optical tweezer potentials.

Attosecond photoelectron interferometry based on the combination of an attosecond pulse train and a synchronized infrared field is a fundamental technique for the temporal characterization of attosecond waveforms and for the investigation of electron dynamics in the photoionization process. In this approach, the comb of extreme ultraviolet harmonics typically lies above the ionization threshold of the target under investigation, thus releasing a photoelectron by single-photon absorption. The interaction of the outgoing photoelectron with the infrared pulse results in the absorption or emission of infrared photons, thereby creating additional peaks in the photoelectron spectrum, referred to as sidebands. While, in the absence of resonances in the first ionization step, the phases imparted on the photoionization process evolve smoothly with the photon energy, the presence of intermediate resonances imprints a large additional phase on the outgoing photoelectron wave packet. In this paper, using a comb of harmonics below and above the ionization threshold of neon, we investigate the effect of intermediate bound excited states on attosecond photoelectron interferometry. We show that the phase of the oscillations of the sidebands and their angular distributions are strongly affected by such resonances. By slightly tuning the photon energies of the extreme ultraviolet harmonics, we show how the contributions of selected resonances can be enhanced or suppressed.

The studies of this paper on the spin-$1/2\mathrm{XXZ}$ chain at finite temperatures $T>0$ have two complementary goals. The first is to identify the spin carriers of all its $S_q>0$ energy eigenstates and to show that their spin elementary currents fully control the spin-transport quantities. Here, $S_q$ is the $q$-spin of the continuous $S{U}_q\left(2\right)$ symmetry of the model for anisotropy $\mathrm{\Delta }>1$. To achieve this goal, our studies rely on a suitable exact physical-spin representation valid in throughout the Hilbert space and associated with normal spin operators, rather than fractionalized particles such as spinons. Both the spin stiffness and the zero-field spin-diffusion constant are expressed in terms of thermal expectation values of the square of the elementary currents carried by the spin carriers. Our second goal is to confirm that the zero-field and finite-temperature spin transport is normal diffusive for $\mathrm{\Delta }>1$. We use two complementary methods that rely on an inequality for the $T>0$ spin stiffness and the above thermal expectation values, respectively, to show that the contributions to ballistic spin transport vanish. Complementarily, for $T>0$ and $\mathrm{\Delta }>1$, the spin-diffusion constant is found to be finite and enhanced upon lowering $T$, reaching its largest yet finite values at low temperatures. Evidence suggests that it diverges in the $\mathrm{\Delta }\to 1$ limit for $T>0$, consistent with $T>0$ anomalous superdiffusive spin transport at $\mathrm{\Delta }=1$ and zero field.

Complex quantum many-body dynamics spread initially localized quantum information across the entire system. Information scrambling refers to such a process whose simulation is one of the promising applications of quantum computing. We demonstrate the Hayden-Preskill recovery protocol and the interferometric protocol for calculating out-of-time-ordered correlators to study the scrambling property of a one-dimensional kicked-Ising model on 20-qubit trapped-ion quantum processors. The simulated quantum circuits have a geometrically local structure that exhibits the ballistic growth of entanglement, resulting in the circuit depth being linear in the number of qubits for the entire state to be scrambled. We experimentally confirm the growth of signals in the Hayden-Preskill recovery protocol and the decay of out-of-time-ordered correlators at late times. As an application of the created scrambling circuits, we also experimentally demonstrate the calculation of the microcanonical expectation values of local operators adopting the idea of thermal pure quantum states. Our experiments are made possible by extensively utilizing one of the highest-fidelity quantum processors currently available and, thus, should be considered as a benchmark for the current status of the most advanced quantum computers.

In strongly correlated systems, interactions give rise to critical fluctuations surrounding the quantum critical point (QCP) of a quantum phase transition. Quasicrystals allow the study of quantum critical phenomena in aperiodic systems with frustrated magnetic interactions. Here, we study the magnetic field and temperature scaling of the low-temperature specific heat for the quantum critical Yb-Au-Al quasicrystal. We devise a scaling function that encapsulates the limiting behaviors as well as the area where the system goes from a temperature-limited to a field-limited quantum critical region, where the magnetic field acts as a cutoff for critical fluctuations. The zero-field electronic specific heat is described by a power-law divergence, $C_{\text{el}}/T\propto T^{-0.54}$, aligning with previously observed ac-susceptibility and specific-heat measurements. The field dependence of the electronic specific heat at high magnetic fields shows a similar power law $C_{\text{el}}/T\propto B^{-0.50}$. In the zero-field and low-field region, we observe two small but distinct anomalies in the specific heat, located at 0.7 and 2.1 K.

The excited bands in optical lattices manifest an important tool for studying quantum simulation and many-body physics, making it crucial to measure high-band scattering dynamics under strong interactions. This work investigates both experimentally and theoretically the collisional scattering of ${}^6\mathrm{Li}_2$ molecular Bose-Einstein condensate in the $D$ band of a one-dimensional optical lattice, with interaction strength directly tunable via magnetic Feshbach resonance. We find a clear dependence of the $D$-band lifetimes on the interaction strength within the strongly interacting regime, which arises from the fact that the scattering cross section is proportional to the square of the scattering length. The maximum lifetime versus lattice depth is measured to reveal the effects of interactions. We also investigate the scattering channels of $D$-band molecules under different interaction levels and develop a reliable two-body scattering rate equation. This work provides insight into the interplay between interaction and the collisional scattering of high-band bosons in optical lattices, paving the way for research into strong correlation effects in high-band lattice systems.

Recent experiments show that metachronal waves (MCWs) can emerge from a chain of symmetrically beating nematodes aligned at the edge of sessile droplets. Our study, employing a coupled elastohydrodynamic model of active filaments, elucidates that a misalignment caused by a tilt against the bounding wall disrupts the synchronization and generates a constant time lag between adjacent filaments, giving rise to MCWs. The MCWs, enhancing the fluid circulation, achieve their maximum thermodynamic efficiency over the same range of tilt angles observed in the nematode experiments.

Krylov complexity has recently emerged as a new paradigm to characterize quantum chaos in many-body systems. However, which features of Krylov complexity are a prerogative of quantum chaotic systems and how they relate to more standard probes, such as spectral statistics or out-of-time-order correlators (OTOCs), remain open questions. Recent insights have revealed that in quantum chaotic systems Krylov state complexity exhibits a distinct peak during time evolution before settling into a well-understood late-time plateau. In this work we propose that this Krylov complexity peak (KCP) is a hallmark of quantum chaotic systems and suggest that its height could serve as an order parameter for quantum chaos. We demonstrate that the KCP effectively identifies chaotic-integrable transitions in two representative quantum-mechanical models at both infinite and finite temperature: the mass-deformed Sachdev-Ye-Kitaev model and the sparse Sachdev-Ye-Kitaev model. Our findings align with established results from spectral statistics and OTOCs while introducing an operator-independent diagnostic for quantum chaos, offering more universal insights and a deeper understanding of the general properties of quantum chaotic systems.

The two-dimensional photon-excitation photon-emission (PhexPhem) map of the archetypal ${\mathrm{H}}_2$ molecule, constructed from dispersed-fluorescence measurements after energy-scanning excitation by small-bandwidth photons, displays complete systems of its Condon diffraction bands. They originate from spontaneous radiative dissociation of individual bound electronically excited rovibronic levels, one of the most relevant processes for the destruction of ${\mathrm{H}}_2$ in space. For the $B\text{−}X$ system of electronic states, we show that specific measured spectral characteristics of individual bands are extremely sensitive to the difference $\mathrm{\Delta }R$ of the internuclear distances, where the two potential energy curves each have their minimum. Using data from recorded ${\mathrm{H}}_2$-PhexPhem maps, it is possible to experimentally validate the calculations of $\mathrm{\Delta }R$ down to an accuracy of at least $50\mathrm{f}\mathrm{m}$. This accuracy may be used as a sensitive experimental test to calculational accuracy. It is well feasible to improve the currently achieved experimental accuracy, for an even more accurate benchmark of advanced potential energy curve calculations.

We propose a measurement of laser-induced vacuum birefringence through the use of pulsed lasers coupled to femtosecond optical enhancement cavities. This measurement technique features cavity-enhanced pump and probe pulses, as well as an independent control pulse. The control pulse allows for a differential measurement where the final signal is obtained using high-frequency lock-in detection, greatly mitigating time-dependent cavity birefringence as an important and possibly prohibitive systematic effect. In addition, the method features the economical use of laser power and results in a relatively simple experimental setup.

We realize a magneto-optical trap (MOT) of titanium (Ti) atoms, performing laser cooling on the 498 nm transition between the long-lived $3{\mathrm{d}}^3\left({}^4\mathrm{F}\right)4\mathrm{s}{\mathrm{a}}^5{\mathrm{F}}_5$ metastable state and the $3{\mathrm{d}}^3\left({}^4\mathrm{F}\right)4\mathrm{p}{\mathrm{y}}^5{\mathrm{G}}_6^{\mathrm{o}}$ excited state. Without the addition of any repumping light, we observe MOTs of the three stable, $I=0$ bosonic isotopes, ${}^{46}\mathrm{Ti}, {}^{48}\mathrm{Ti}$, and ${}^{50}\mathrm{Ti}$. Up to $8.30\left(26\right)\times {10}^5{}^{48}\mathrm{Ti}$ atoms are trapped at a maximum density of $1.3\left(4\right)\times {10}^{11}{\mathrm{cm}}^{-3}$ and at a temperature of $90\left(15\right)\mu \mathrm{K}$. By measuring the decay of the MOT, we constrain the leakage branching ratio of the cooling transition ($\le 2.5\times {10}^{-6}$) and the two-body loss coefficient ($\le 2\times {10}^{-10}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$). Our approach to laser cooling Ti can be applied to other transition metals, enabling a significant expansion of the elements that can be laser cooled.

Understanding the nature of surface states and their exchange gaps in magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ (MBT) thin films is crucial for achieving robust topological Chern and axion insulating phases where the quantum anomalous Hall effect and topological magneto-electric effect can be realized. Here we focus on some rather unexplored features of possible surface reconstructions of interstitial-2H and peripheral-2H types, which are likely to occur in experiments. Using first-principles calculations together with molecular dynamics simulations accelerated by a machine learning force field, we demonstrate that interstitial-2H and peripheral-2H type atomic reconstructions play a crucial role in modifying the exchange gap and surface characteristics of MBT thin films, alongside previously reported factors such as surface magnetism, stacking configurations, and native defects. Moreover, these surface reconstructions have important implications for the topological indices and the nature of quasi-one-dimensional side-wall edge states dominating quantum transport. Specifically, the calculation of the energy landscape and barriers for the proposed surface reconstructions indicates that the interstitial-2H reconstruction is thermodynamically more stable than the peripheral-2H reconstruction. The latter case is also hypothesized as providing a plausible explanation for the Rashba surface states observed in angle-resolved photoemission spectroscopy measurements. Our analysis provides a theoretical framework to elucidate the nature and effect of reconstructions in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ thin films, with predictions for the experimental realization of different topological phases.

Topological vortex textures in magnetic disks have garnered great attention due to their interesting physics and diverse applications. However, prior studies have primarily focused on microscale ferromagnetic disks with oscillation frequencies limited to the GHz range. To address this critical bottleneck, we propose a novel mechanism to generate vortex states in antiferromagnetic nanodots surrounded by heavy metals, which enables ultrasmall sizes—hundreds to thousands of times smaller in area—and ultrafast frequencies—hundreds to thousands of times higher. We demonstrate that, interestingly, the interfacial Dzyaloshinskii-Moriya interaction (iDMI) induced by the heavy metal at the dot boundary acts as an effective chemical potential for vortices. Analogous to the formation of superfluid vortices through rotation, we show that the iDMI can stabilize a magnetic vortex state. Moreover, applying an electric current induces vortex oscillations via spin-transfer torque, reaching THz frequencies that can be further tuned with external magnetic fields. These theoretical predictions are validated via micromagnetic simulations. Furthermore, we show that coherent coupling between vortices can be achieved via an antiferromagnetic link, which depends on the vortex polarity and topological charge and is also tunable via the vortex resonance frequency. Our proposal overcomes the critical bottleneck and opens a new vortex-based path toward next-generation information technologies and high-density data storage.

Superconductor–semiconductor hybrid systems play a crucial role in realizing nanoscale quantum devices, including hybrid qubits, Majorana bound states, and Kitaev chains. For such hybrid devices, subgap states play a prominent role in their operation. In this paper, we study these subgap states via Coulomb and tunneling spectroscopy through a superconducting island defined in a semiconductor nanowire fully coated by a superconductor. We systematically explore regimes ranging from an almost decoupled island to the open configuration. In the weak-coupling regime, the experimental observations are very similar in the absence of a magnetic field and when one flux quantum pierces the superconducting shell. Conversely, in the strong-coupling regime, significant distinctions emerge between the two cases. We attribute this distinct behavior to the existence of subgap states at one flux quantum, which become observable only for sufficiently strong coupling to the leads. We support our interpretation using a simple model to describe transport through the island. Our study highlights the importance of studying a broad range of tunnel couplings for understanding the rich physics of hybrid devices.