We introduce a different class of thresholdless three-dimensional soliton states that form in higher-order topological insulators based on a two-dimensional Su-Schrieffer-Heeger array of coupled waveguides. The linear spectrum of such structures is characterized by the presence of a topological gap with corner states residing in them. We find that a focusing Kerr nonlinearity allows families of light bullets bifurcating from the linear corner states to exist as stable three-dimensional solitons, which inherit topological protection from their linear corner counterparts and, remarkably, survive even in the presence of considerable disorder. The light bullets exhibit a spatial localization degree that depends strongly on the array dimerization, and may feature large temporal widths in the topological gap near the bifurcation point, thus drastically reducing the otherwise strong instabilities caused by higher-order effects.

An orthodox formulation of quantum mechanics relies on a set of postulates in a Hilbert space supplemented with rules to connect it with classical mechanics such as quantization techniques, correspondence principle, etc. Here, we deduce a qubit and its dynamics straightforwardly from a discrete deterministic dynamics and conservation of the classical collision entropy. No Hilbert space is required, although it can be inferred from this approach if necessary.

Farhi and Gutmann [Phys. Rev. A 57, 2403 (1998)] proved that a continuous-time analog of Grover search (also called spatial search) is optimal on complete graphs. We extend this result by showing that spatial search remains optimal on a complete graph even in the presence of an infinitely long path (or tail). If we view the latter as an external quantum system that has a limited but nontrivial interaction with our finite quantum system, this suggests that spatial search is robust against a coherent infinite one-dimensional probe. Moreover, we show that the search algorithm is oblivious in that it does not need to know whether the tail is present or not, and if so, where it is attached to.

Quantum simulation on near-term quantum hardware is a topic of intense interest. The preparation of novel quantum states of matter provides a quantitative assessment of the capabilities of near-term digital quantum computers to implement circuits with structure of relevance to quantum simulation. Here, we conduct a benchmark study by realizing symmetry-protected topological (SPT) phases of a spin-1/2 Hamiltonian with next-nearest-neighbor hopping on up to 11 qubits on a programmable superconducting quantum processor using adiabatic state preparation. Using recompilation techniques to reduce the gate count to around 50 two-qubit gates, we observe clear signatures of the two distinct SPT phases, such as excitations localized to specific edges and finite string-order parameters. We identify a parasitic phase associated with the two-qubit gate as the dominant imperfection that limits the depth of the circuits, indicating a research topic of interest for future hardware development.

We report experimental studies of electron-impact ionization of nitrogen (${\mathrm{N}}_2$) and oxygen (${\mathrm{O}}_2$) molecules using a cold target recoil ion momentum spectrometer (COLTRIMS, reaction microscope). The recoil ion is detected in coincidence with one outgoing electron such that the momentum vectors and consequently the kinetic energies for these final-state particles are determined. The ionization cross sections for producing the doubly and singly charged parent ions were measured as a function of the incident electron energy ranging from 50 to 600 eV. For molecular dications, e.g., ${\mathrm{O}}_2^{2+}$, the cross sections are obtained by measuring both the ion time of flight and two-dimensional position spectra, which is demonstrated to be an efficient way to suppress the contribution of the dissociation channels with the ${\mathrm{O}}^{+}$ products as both ions have the identical mass-to-charge ratios. The projectile energy-loss spectra correlated to the final-state ions are obtained, which provide direct evidence of the ionization mechanisms of ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ molecules. It is found that the ${\mathrm{O}}_2^{2+}$ dication is produced mainly by Auger process after single ionization in $2{\sigma }_g$ inner-valence shell of ${\mathrm{O}}_2$ molecule, while the ${\mathrm{N}}_2^{2+}$ dication is generated by the direct removal of two electrons from the outermost $3{\sigma }_g$ orbital of ${\mathrm{N}}_2$ molecule.

Electromagnetic waves with a helical-like wavefront are known as optical vortices. One of their main characteristics is a phase structure that has a circulation around a singularity and that they carry orbital angular momentum (OAM). OAM is a fundamental property that governs the interaction of these sources with matter. In the present paper, we study the electron dynamics driven by a so-called spatiotemporal optical vortex (STOV). Contrary to the conventional optical vortices, a STOV carries transverse OAM. By designing a streakinglike technique, we aim to fully characterize the OAM of the STOV. Using both quantum mechanical and semiclassical models, we are able to dissect the spatially resolved photoelectron energy spectra and accurately retrieve the OAM. Our approach paves the way toward a complete understanding of the interaction of complex spatiotemporal light fields with atomic targets.

Relativistic strong-field ionization of hydrogenlike atoms or ions in a constant crossed electromagnetic field is studied. The transition amplitude is formulated within the strong-field approximation in the Göppert-Mayer gauge, with initial and final electron states being described by the corresponding Dirac-Coulomb and Dirac-Volkov wave functions, respectively. Coulomb corrections to the electron motion during tunneling are taken into account by adjusting an established method to the present situation. Total and energy-differential ionization rates are calculated and compared with predictions from other theories in a wide range of atomic numbers and applied field strengths.

Photoionization of rubidium atoms cooled in a magneto-optical trap, characterized by the coexistence of the ground $5S_{1/2}$ and excited $5P_{3/2}$ states, is investigated experimentally and theoretically with the 400-nm femtosecond laser pulses at intensities of $I=(3\times {10}^9)\text{–}(4.5\times {10}^{12})$ $\mathrm{W}/{\mathrm{cm}}^2$. The recoil-ion momentum distribution (RIMD) of ${\mathrm{Rb}}^{+}$ exhibits rich ringlike structures and their energies correspond to one-photon ionization of the $5P_{3/2}$ state and two-photon and three-photon ionizations of the $5S_{1/2}$ state, respectively. With increasing $I$, the dips near zero momentum (NZM) in the experimental RIMDs become shallow dramatically and their peaked ${\mathrm{Rb}}^{+}$ momenta ionized from the $5P_{3/2}$ state move obviously toward zero while the peaks from the $5S_{1/2}$ state do not shift. In addition, the ion-yield ratio of the $5S_{1/2}$ state to the $5P_{3/2}$ state varies from $I$ to $I^{1.5}$ as $I$ increases. These features indicate a transition from perturbative ionization to strongly perturbative ionization for the $5P_{3/2}$ state. Numerical simulations by solving the time-dependent Schrödinger equation (TDSE) can qualitatively explain the measurements of the RIMD, photoion angular distributions, and ion-yield ratio. However, some discrepancies still exist, especially for the NZM dip, which could stem from the electron-electron correlation that is neglected in the present TDSE simulations since we have adopted the single-active-electron approximation.

We report numerical studies of the linear and nonlinear edge dynamics of a non-harmonically-confined macroscopic fractional quantum Hall fluid. In the long-wavelength and weak excitation limit, observable consequences of the fractional transverse conductivity are recovered. The first nonuniversal corrections to the chiral Luttinger liquid theory are then characterized: for a weak excitation in the linear response regime, cubic corrections to the linear wave dispersion and a broadening of the dynamical structure factor of the edge excitations are identified; for stronger excitations, sizable nonlinear effects are found in the dynamics. The numerically observed features are quantitatively captured by a nonlinear chiral Luttinger liquid quantum Hamiltonian that reduces to a driven Korteweg–de Vries equation in the semiclassical limit. Experimental observability of our predictions is finally discussed.

We measure nonlocal third-order spatial correlations in nondegenerate ultracold gases of bosonic (${}^{84}\mathrm{Sr}$) and spin-polarized fermionic (${}^{87}\mathrm{Sr}$) strontium through studies of the formation rates for ultralong-range trimer Rydberg molecules. The trimer production rate is observed to be very sensitive to the effects of quantum statistics with a strong enhancement of up to a factor of 6 (3!), in the case of bosonic ${}^{84}\mathrm{Sr}$ due to bunching and a marked reduction for spin-polarized fermionic ${}^{87}\mathrm{Sr}$ due to antibunching. The experimental results are compared to theoretical predictions and good agreement is observed. The present approach opens the way to in situ studies of higher-order nonlocal spatial correlations in a wide array of ultracold atomic-gas systems.

Non-Hermitian systems with anti-parity-time $\left(\mathrm{a}\mathcal{PT}\right)$ symmetry have revealed rich physics beyond conventional systems. Here, we study optomechanics in an $\mathrm{a}\mathcal{PT}$-symmetric spinning resonator and show that, by tuning the rotating speed to approach the exceptional point (EP) or the non-Hermitian spectral degeneracy, nonreciprocal light transmission with a high isolation ratio can be realized. Accompanying this process, nonreciprocal group delay or advance is also identified in the vicinity of EP. Our work sheds light on manipulating laser propagation with optomechanical EP devices and, in a broader view, can be extended to explore a wide range of $\mathrm{a}\mathcal{PT}$-symmetric effects, such as $\mathrm{a}\mathcal{PT}$-symmetric phonon lasers, $\mathrm{a}\mathcal{PT}$-symmetric topological effects, and $\mathrm{a}\mathcal{PT}$-symmetric force sensing or accelerator.

The generation of mechanical squeezing has attracted a lot of interest for its nonclassical properties, applications in quantum information and high-sensitivity measurements. Here we propose a detuning-switched method that can rapidly generate strong and stationary mechanical squeezing. The pulsed driving can dynamically transpose the optomechanical coupling into a linear optical force and maintain an effective mechanical frequency, which can introduce strong mechanical squeezing in a short time. Moreover, we show the obtained strong mechanical squeezing can be frozen by increasing the pulse intervals, leading to stationary mechanical squeezing with a fixed squeezing angle. Thus, our proposal provides fascinating insights and applications of modulated optomechanical systems.

It is shown that when the linear gain anisotropy is properly accounted for, spin-injected vertical-cavity surface-emitting lasers (spin-VCSELs) offer interesting functionalities. Using the extended spin-flip model (SFM), we predict the existence of exceptional points (EPs) in spin-VCSELs and two interesting phenomena which accompany EPs, namely, (i) the polarization switching manifesting as induced change of field helicity sign and (ii) frequency comb generation without the need of external injection locking. Both effects have tremendous technological potential such as fast on-chip polarization switching, and most importantly, their concepts are not limited only to spin-VCSEL technology. We discuss the concept of anisotropy-engineered non-Hermitian microlasers and we provide a theoretical background to study their polarization dynamics near EPs.

Ptychography, a lensless imaging technology with large field of view (FOV), has been extensively developed in the past decades. However, the demanding coherence of the light source restrains its application in the field of ultrafast imaging, as short light pulses have broad spectral bandwidth and thus poor temporal coherence. Here, we demonstrated the implementation of ultrafast imaging with large FOV by monochromatizing the polychromatic ptychographic diffraction patterns. This scheme relaxes the requirement of bandwidth and successfully realizes the millimeter-scale FOV imaging using ultrashort pulses, while maintaining the good spatial resolution of the order of diffraction limit. Due to the use of a precisely sampled spectrum, the illumination information can be accurately captured allowing more reliable imaging process. Experimental results show that satisfactory spatial resolution can still be achieved even for a relative bandwidth up to 20%. Our work demonstrates the potential of ultrafast diffraction imaging with a large FOV.

The problem of defining a Hermitian quantum phase operator is nearly as old as quantum mechanics itself. Throughout the years, a number of solutions have been proposed, ranging from abstract operator formalisms to phase-space methods. In this work, we make an explicit connection between two of the most prominent approaches by proving that the probability distribution of phase in the Paul formalism follows exactly from the Pegg-Barnett formalism by combining the latter with the quantum-limited amplifier channel. Our findings suggest that the Paul framework may be viewed as a semiclassical limit of the Pegg-Barnett approach.

We present a generalized echo squeezing protocol (GESP) as a generalization of the Schrödinger cat state protocol with the value of the squeezing parameter being an arbitrary number rather than $\pi /2$. We show analytically that over a very broad range of the squeezing parameter the sensitivity of the GESP reaches the Heisenberg limit within a factor of $\sqrt{2}$. For a large number of atoms, $N$, this plateau interval is almost the whole range from zero to $\pi /2$, and the sensitivity is independent of the parity of $N$. Therefore, it is possible to operate a sensor over a wide interval of the squeezing parameter without changing the sensitivity. This is to be contrasted with the conventional echo squeezing protocol (CESP) which only works for a very small interval of the squeezing parameter. We also show that, in contrast to the CESP, the sensitivity of the GESP is close to the quantum Cramér-Rao bound over the whole range of the squeezing parameter, indicating that the phase-shift information contained in the quantum state is near-optimally extracted. We find that the enhancement in sensitivity in the case of the GESP is due to a combination of two parameters: the phase magnification factor (PMF) and the noise amplification factor (NAF). As the value of the squeezing parameter increases, both PMF and NAF increase, while keeping the ratio of the PMF to the NAF essentially constant, yielding a net enhancement of sensitivity at the Heisenberg limit within a factor of $\sqrt{2}$ over the whole plateau interval. An important consequence of this behavior is that the robustness of the GESP against detection noise easily exceeds that of the CESP for a broad range of values of the squeezing parameter. As such, in the context of an experimental study, it should be possible to achieve a net enhancement in sensitivity higher than that for the CESP, under typical conditions where detection noise exceeds the quantum projection noise of an unsqueezed state with the same number of atoms. Finally, we consider the fragility of the GESP against decoherence mechanisms, and show how a balance between the fragility against the decoherence mechanisms and the robustness against detection noise would in practice determine the optimal choice of parameters for the GESP.