We present experimental and theoretical differential cross sections for elastic electron scattering from benzene. The present experimental results are obtained at incident electron energies ranging from 1 to 50 eV and for scattering angles from ${10}^{\circ }$ to ${130}^{\circ }$. The experimental measurements are compared to available results from 1 to 10 eV and to theoretical results from 10 to 50 eV, both based on the Schwinger multichannel (SMC) method. Different electron scattering calculations are carried out by employing varied basis sets and multichannel coupling schemes. This allows us to (i) obtain relatively converged cross sections with respect to the inclusion of Rydberg excited states, (ii) observe significant variations in the forward scattering as a function of the multichannel coupling scheme, and (iii) explore possible effects stemming from states lying above the ionization threshold. Overall, the agreement between experiments and models is found to be very good to excellent. The remaining discrepancies point out areas in which the SMC method should be improved.

The non-Hermitian skin effect (NHSE) describes the exponential localization of all eigenstates toward boundaries in non-Hermitian systems, and has attracted intense research interest of late. Here we theoretically propose a scheme in which the NHSE significantly impacts the external motion of a single trapped ion through complex spin-motion dynamics. On the one hand, we show the competition between the NHSE and the coherent Bloch dynamics. On the other hand, since the NHSE manifests as a nonreciprocal flow in occupied phonon modes, we demonstrate that such dynamics can have potential applications in cooling and sensing. Our proposal can be readily implemented using existing experimental techniques and offers a scalable (in terms of the available ions and phonon modes) simulation platform for relevant non-Hermitian physics.

Gases of doubly dipolar particles, with both magnetic and electric dipole moments, offer intriguing novel possibilities. We show that the interplay between doubly dipolar interactions, quantum stabilization, and external confinement results in a rich ground-state physics of supersolids and incoherent droplet arrays in doubly dipolar condensates. Our study reveals novel possibilities for engineering quantum droplets and droplet supersolids, including supersolid-supersolid transitions and the realization of supersolid arrays of pancake droplets.

In this paper, we theoretically and numerically investigate the resonant modes and reflectance of an optical grating consisting of a wavy dielectric slab by applying the spectral element method. The presence of the wavy shape transforms the waveguide modes into leaky resonant modes. A few resonant modes with specific longitudinal wave numbers have infinitely large-$Q$ factors, whereas the other resonant modes have finite-$Q$ factors. For the leaky resonant mode with a zero longitudinal wave number, the $Q$ factor is inversely proportional to the amplitude of the wavy shape. An array of multiple low-$Q$ wavy gratings has a high reflectance in a large bandwidth. A double-layer wavy grating forms a Fabry-Pérot cavity, which hosts Fabry-Pérot bound states in the continuum (BICs) at the resonant frequency. The $Q$ factor of the Fabry-Pérot cavity can be tuned by adjusting the distance between the two wavy slabs. The wavy shape could be generated by a vibrational wave in a flat dielectric slab so that the BICs and wideband reflectance could be controlled on demand.

We apply a recently proposed modified Pearson correlation coefficient to investigate the statistical properties of intensity fluctuations in the second harmonic as well as fundamental mode of a multimode $Q$-switched Nd:yttrium aluminum garnet laser. Above threshold the system displays a photonic ferromagneticlike spontaneous mode-locking phase with replica symmetry breaking (RSB). Differently from the photonic glassy phase in random lasers in which modes do not oscillate synchronously, in the present case a relevant fraction of modes oscillate coherently, but the synchronous oscillation of some modes is frustrated with the RSB arising from the fact that a given subset of modes is randomly activated at each excitation pulse with dominating coherent oscillation, frustrating the emergence of other modes. From a single set of measurements, the modified Pearson coefficient allows to identify that the onset of the RSB phenomenon between distinct spectra as the lasing threshold is crossed coincides with the emergence of correlations between different wavelengths in the same spectrum. As different subsets of longitudinal modes are nondeterministically activated after each shot, the RSB property indicates that distinct spectra can be either correlated or anticorrelated around both the fundamental mode and the second harmonic.

Solid-state high-order harmonic generation (HHG) continues to attract a lot of interest. From the theory and simulation standpoint, two issues are still open; The first is the so-called transition-dipole phase problem. It has been recognized that the dipoles must be treated as complex-valued quantities and that their corresponding Berry connections must be included to ensure phase-gauge invariance. However, while this has been successfully implemented for lower-dimensional systems, fully vectorial and three-dimensional simulations remain challenging. The second issue concerns the symmetry of the high-order harmonic response, when simulations sometimes fail to honor the symmetry of the crystalline material. This work addresses both of these problems with the help of a HHG-simulation approach which (a) is manifestly free of the transition-dipole phase problem, (b) does not require calculation of dipole moments, (c) can account for the contributions from the entire Brillouin zone, and (d) faithfully preserves the symmetry of the simulated crystalline material. We use the method to show that high-order harmonic sources are distributed throughout the Brillouin zone with various phase shifts giving rise to significant cancellations. As a consequence, for the simulated response to correctly capture the material symmetry, contributions from the entire Brillouin zone must be included. Our results have important implications for a number of HHG applications, including all-optical band and dipole reconstruction.

Over the last three decades, plasmonics using metallic nanostructures has become central to nanophotonics research. Recently, its targets have been extended to nonlinear optical phenomena. In a nonlinear regime, quantum mechanical effects, as exemplified by electron spill-out on the surface of nanostructures, significantly influence the optical response. Quantum hydrodynamic theory (QHT) is a promising basis for the analysis of nonlinear optical responses involving such quantum mechanical effects. Herein, QHT was applied to calculate the third-order optical nonlinearity of a spherical metallic nanostructure. The results demonstrate how electron spill-out strongly affects plasmon resonance and third-order optical nonlinearity.

We study, experimentally and numerically, in-plane light scattering (side scattering) and near- and far-field collective excitations in plasmonic metasurfaces consisting of periodic arrays of Au $\mathsf{V}$-shaped nanoantennas. Each unit cell in these arrays includes a pair of such nanoantennas facing each other ($<>$). We show that, depending on the polarization of the incident light, such metasurfaces can support two types of supercells ($<+>$ and $>+<$). $<+>$ is a braket supercell wherein two nanoantennas of the same unit cells are coupled to each other via their plasmonic fields. When the polarization of the incident light is rotated by ${90}^{\circ }$, a plasmonic conjugate supercell is formed ($>+<$). In this case the plasmon fields couple two $\mathsf{V}$-shaped nanoantennas of the neighboring cells, supporting a geometrical and near-field coupling process different from $<+>$. We show that braket and conjugate supercells support sharp infrared resonances at two different wavelengths, offering a high extinction polarization optical switching process associated with the transition between $<+>$ and $>+<$. Our results show that while $<+>$ supercells tend to be optically noninteracting with each other, $>+<$ supercells can get coupled to the lattice modes, spatially extending coherent properties across the arrays. We investigate the in-plane scattering of these arrays, demonstrating how variations of the charge configuration and phase by the incident light polarization can be used to coherently control the in-plane scattering of $\mathsf{V}$-shaped nanoantennas arrays.

Resonance fluorescence from atomic systems consists of a single spectral peak that evolves into a Mollow triplet for a strong excitation field. Photons from different peaks of the triplet show distinct timing correlations that make the fluorescence a useful light source for quantum information purposes. We characterize the fluorescence of a single optically trapped ${}^{87}\mathrm{Rb}$ atom that is excited resonantly at different power levels. Second-order correlation measurements reveal the single photon nature of the fluorescence concurrently with Rabi oscillations of a strongly excited atom. The asymmetry in correlations between photons from two sidebands of the fluorescence spectrum when the atom is exposed to an off-resonant field indicates that there is a preferred time ordering of the emitted photons from different sidebands.

The control and maintenance of the quantum properties of an open quantum system along its evolution are essential for both fundamental and technological aims. We assess the capability of a frequency-modulated qubit embedded in a leaky cavity to exhibit enhancement of its dynamical quantum features. The qubit transition frequency is sinusoidally modulated by an external driving field. We show that a properly optimized quantum witness effectively identifies quantum coherence protection due to frequency modulation, while a standard quantum witness fails. We also find an evolution speedup of the qubit through proper manipulation of the modulation parameters of the driving field. Importantly, by introducing a figure of merit ${\mathfrak{R}}_{\mathfrak{g}}$, we discover that the relation between quantum speed limit time and non-Markovianity depends on the system initial state, which generalizes previous connections between these two dynamical features. The frequency-modulated qubit model thus manifests insightful dynamical properties with potential utilization against decoherence.

The construction of nonlocal sets of quantum states has attracted much attention in recent years. We first introduce two lemmas related to the triviality of orthogonality-preserving local measurements. Then, we propose a general construction of nonlocal set of $n(d-1)+1$ orthogonal product states in ${\left({\mathbb{C}}^d\right)}^{\otimes n}$. The sets of nonlocal orthogonal product states are also put forward for multipartite quantum systems with arbitrary dimensions. Our construction gives rise to nonlocal sets of orthogonal product states with much less members and thus reveals the phenomenon of nonlocality without entanglement more efficiently.

The efficient decomposition of multicontrolled gates is a significant factor in quantum compiling, both in circuit depth and $T$-gate count. Recent work has demonstrated that qudits have the potential to reduce resource requirements from linear to logarithmic depth and to avoid fractional phase rotations. Here, we argue, based on the scaling of decoherence in high-index states, that circuit depth is not the primary factor, and that both the choice of entangling gate and interaction network topology act together to determine the spread of errors and the ultimate failure rate in a circuit. We further show that for certain linear-depth circuits, additional error mitigation is possible via selective application of resources.

Three-dimensional (3D) excitonic complexes influence the optoelectronic properties of bulk semiconductors. More generally, correlated few-particle molecules and ions, held together by pairwise Coulomb potentials, play a fundamental role in a variety of fields in physics and chemistry. Based on statistically exact diffusion quantum Monte Carlo calculations, we have studied excitonic three- and four-body complexes (trions and biexcitons) in bulk 3D semiconductors, as well as a range of small molecules and ions in which the nuclei are treated as quantum particles on an equal footing with the electrons. We present interpolation formulas that predict the binding energies of these complexes, either in bulk semiconductors or in free space. By evaluating pair distribution functions within quantum Monte Carlo simulations, we examine the importance of harmonic and anharmonic vibrational effects in small molecules.

We present microwave cavity resonance spectroscopy as a technique to determine excited-state photoionization cross sections of laser-cooled atoms. We demonstrate this technique by measuring the photoionization cross section of the $5{}^{2}P_{3/2}(F^{\prime }=4)$ excited state of ${}^{85}\mathrm{Rb}$ to continuum by creating an ultracold plasma inside a 5-GHz resonant microwave cavity. We find a photoionization cross section of $\sim 5\times {10}^{-22}{\mathrm{m}}^2$ for photon excess energies of 50, 100, 200, and 500 K above the ionization threshold. The measurements yield a cross section which is approximately 2 to 3 times smaller than the value provided by existing theory for the $5p$ excited state of ${}^{85}\mathrm{Rb}$ and two earlier measurements performed with other techniques for similar excess energies.

We report on the implementation of degenerate Raman sideband cooling of ${}^{40}\mathrm{K}$ atoms. The scheme incorporates a three-dimensional (3D) optical lattice, which confines the atoms and drives the Raman transitions. The optical cooling cycle is closed by two optical pumping beams. The wavelength of the laser beams forming the lattice is close to the ${\mathrm{D}}_2$ atomic transition, while the optical pumping is operated near the ${\mathrm{D}}_1$ transition. With this cooling method, we achieve a temperature of $\sim 1\mu \mathrm{K}$ of a cloud with $\sim {10}^7$ atoms. This corresponds to a phase space density of $\ge {10}^{-3}$. Moreover, the fermionic ensemble is spin polarized to conditions which are favorable for subsequent evaporative cooling. We study the dependence of the cooling scheme on several parameters, including the applied magnetic field and the detuning, duration, and intensity profile of the optical pumping beams. Adding this optical cooling stage to current Fermi gas experiments can improve the final conditions and increase the data rate.

An electron wave function is characterized by the phase and amplitude distributions over position or momentum space. Recent attosecond technologies allow us to obtain the phase information of photoelectrons using interference between several optical transition pathways. We demonstrate that by employing a two-path photoionization interference process, the complex wave function of the photoelectron is fully mapped in two-dimensional momentum space. We ionize neon gas by an extreme ultraviolet (XUV) attosecond pulse train consisting of both odd and even harmonics in the presence of an infrared (IR) laser field. By controlling the generation process of the attosecond pulse train, we isolate two ionization pathways for interfering with the photoelectrons: one is the two-photon ionization process due to the odd harmonic excitation with one IR photon absorption, and the other is the one-photon ionization by an even harmonic. We record the photoelectron momentum distributions via velocity map imaging as a function of the XUV and IR delay. Using three different experimental conditions, we show that the detailed structure of the amplitude and phase distributions of photoelectrons can be resolved in the two-dimensional momentum space within the bandwidth determined by the attosecond XUV pulse. We separate the measured photoelectron wave function into those produced by each ionization pathway. Our method will be applicable to more complex molecules.

In this paper we propose an efficient scheme to continuously generate two-mode two-photon bundles by using a three-level atomic system, in which two correlated channels are created for the bundle emission in different modes. It is shown that the antibunching effect not only happens between the same modes but also between two different modes. While the former is due to the dominance of cavity decay over atomic decay, the latter is based on the atomic coherence between the two final states of bundle emission. The antibunched two-mode two-photon bundles are achievable in a wide range of parameters and robust against environmental noise. This scheme provides a kind of photon sources for two-signal-based applications such as quantum information processing, high-precision metrology, and ultrasensitive biosensing.

It is known that quantum indeterminacy sets a lower bound on the power spectrum of voltage fluctuations in any conducting medium. The low-frequency asymptotic of this bound is $\sim 1/f$ in the case of freely propagating charge carriers. It is found that on account of the charge-carrier–phonon interaction, the asymptotic becomes $\sim 1/f^{\gamma },\gamma \ne 1.$ Its general expression is derived in the case of charge carriers subject to the piezoelectric interaction with acoustic phonons. The sign of $(\gamma -1)$ depends on the state filling for charge carriers, so that $\gamma$ may exceed unity despite the absence of a low-frequency cutoff. It is shown that under stationary physical conditions the voltage variance grows with time as $t^{\gamma -1},$ in agreement with observations. It is proved that despite this growth the power spectrum is well defined and finite for $\gamma <2.\mathrm{A}$ practical attainability of the quantum bound is discussed. A comparison with the experimental data on $1/f$ noise in InGaAs quantum wells and high-temperature superconductors is made. It is demonstrated that the account of $\gamma \ne 1$ brings the calculated noise levels uniformly within an order of magnitude of the measured. This holds true whether the voltage fluctuations are measured along or across the electric current, though the observed noise levels in the two cases are significantly different.

In a breakthrough work, Bravyi, Gosset, and König (BGK) [Science 362, 308 (2018)] unconditionally proved that constant-depth quantum circuits are more powerful than their classical counterparts. Their result is equivalent to saying that a particular family of constant-depth quantum circuits takes classical circuits at least $\mathrm{\Omega }(logn)$ depth to “simulate,” in a certain sense. In our paper, we formalize their sense of simulation, which we call “possibilistic simulation” or “$p$-simulation,” and construct explicit classical circuits that can $p$-simulate any depth-$d$ quantum circuit with Clifford and $tT$-gates in depth $O(d+t)$. Our classical circuits use $\{\mathrm{NOT},\mathrm{AND},\mathrm{OR}\}$ gates of fan-in $\le 2$.