A broad range of phenomena in correlated electrons traveling in moiré lattices has emerged in both scenarios, experiments and theory. In this paper we report the observation of a dynamic stability that arises in an analogous system to that of electrons, a weakly interacting spinor Bose gas of ultracold ${}^{23}\mathrm{Na}$ atoms lying in a single layer having a moiré pattern with square and hexagonal symmetries. Our paper is based on the dynamical description of two magnetic domains represented by two hyperfine spin components of a Bose condensate initially localized in the left and right halves of a moiré lattice defined by a specific angle $\theta$ plus a harmonic confinement. To demonstrate the persistence of such an initial condition under the competence of the moiré pattern and harmonic confinements we studied both single noninteracting and double-domain interacting cases. We solve the time dependent Gross-Pitaevskii equations, and track the time evolution of several observables on each half as a function of the twisting angle. In the case of square moiré lattices we found a dynamic stability for angles larger than a special one ${\theta }_s$, except for the Pythagorean angles. The value of such an angle depends on the existence of a harmonic trap when interactions are absent, while this dependence is negligible for the interacting case. Hexagonal moiré lattices exhibit the dynamic stability starting from a certain angle that also depends on the harmonic confinement for the noninteracting case.

We present an experimental and theoretical study of circular dichroism observed in a symmetric nanohole array in a metal film—a nonchiral planar structure exhibiting extraordinary optical transmission. The effect is attributed to an extrinsic chirality dynamically induced in the excitation of surface plasmon polariton waves via spin-momentum locking.

We propose a photonics-based, continuous-variable (CV) form of remote entanglement utilizing strictly second-order nonlinear optical interactions that does not require the implementation of a state-projective measurement (i.e., remote entanglement without conditioning). This scheme makes use of two separate down-converters, wherein the corresponding nonlinear crystals are driven by strong classical fields as prescribed by the parametric approximation, as well as a fully quantum-mechanical model of nondegenerate second-harmonic generation (SHG) whose evolution is described by the trilinear Hamiltonian of the form ${\widehat{H}}_{\text{shg}}=i\hslash \kappa (\widehat{a}\widehat{b}{\widehat{c}}^{†}-{\widehat{a}}^{†}{\widehat{b}}^{†}\widehat{c})$. By driving the SHG process with the signal modes of the two down-converters, we show entanglement formation between the generated second-harmonic mode (SH mode) and the noninteracting joint-idler subsystem without the need for any state-reductive measurements on the interacting modes.

We propose an evanescent-field interferometer in the Mach-Zehnder configuration, which can provide an interferometric measure of the phase of evanescent fields and can be used to quantify the Hartman effect. The phase difference for such tunneled fields saturates to $\pi /2$, thus confirming the classic Hartman effect. An analytical model of the proposed evanescent-field interferometer is presented and validated by numerical simulations based on the finite-difference time-domain technique. In the numerical treatment, the requisite elements of the interferometer, such as beam splitters and mirrors, are realized through carefully designed cavities in a two-dimensional photonic crystal system. The underlying cavity modes are further engineered using a weak perturbation to spatially maneuver the evanescent field along preferred directions. The evanescent-field interferometer offers multiple avenues of introducing phase delay and has higher sensitivity that is spatially delocalized in comparison to the conventional propagating-field interferometers.

We study photon scattering errors in stimulated Raman driven quantum logic gates. For certain parameter regimes, we find that previous simplified models of the process significantly overestimate the gate error rate due to photon scattering. This overestimate is shown to be due to previous models neglecting the detuning dependence of the scattered photon frequency and Lamb-Dicke parameter, a second scattering process, interference effects on scattering rates to metastable manifolds, and the counterrotating contribution to the Raman transition rate. The resulting improved model shows that there is no fundamental limit on gate error due to photon scattering for electronic ground-state qubits in commonly used trapped-ion species when the Raman laser beams are red detuned from the main optical transition. Additionally, photon scattering errors are studied for qubits encoded in the metastable $D_{5/2}$ manifold, showing that gate errors below ${10}^{-4}$ are achievable for all commonly used trapped ions.

We propose a class of randomized quantum Krylov diagonalization (rQKD) algorithms capable of solving the eigenstate estimation problem with modest quantum resource requirements. Compared to previous real-time evolution quantum Krylov subspace methods, our approach expresses the time evolution operator $e^{-i\widehat{H}\tau }$ as a linear combination of unitaries and subsequently uses a stochastic sampling procedure to reduce circuit depth requirements. While our methodology applies to any Hamiltonian with fast-forwardable subcomponents, we focus on its application to the explicitly double-factorized electronic-structure Hamiltonian. To demonstrate the potential of the proposed rQKD algorithm on near-term quantum devices, we provide numerical benchmarks for a variety of molecular systems with circuit-based state-vector simulators including the effects of sampling noise, achieving ground-state energy errors of less than $1 \mathrm{kcal} {\mathrm{mol}}^{-1}$ with circuit depths orders of magnitude shallower than those required for low-rank deterministic Trotter-Suzuki decompositions.

We perform theoretical studies of the interplay between disorder, spin-orbit coupling (SOC), and Rabi fields, and show that both SOC and Rabi fields can be used to dramatically control the degree of Anderson localization of a Bose-Einstein condensate in bichromatic lattices. We obtain ground-state phase diagrams in the SOC and Rabi field plane for different values of disorder strength and use realistic experimental parameters compatible with ${}^{39}\mathrm{K}$. We find cases of fixed disorder and SOC (Rabi field), where the Rabi field (SOC) reduces the threshold for localization and controls the localization length. We also show regimes of fixed disorder and Rabi field, where the extent of the ground-state wave function is periodic in the SOC, leading to alternating regions of stronger and weaker localization as SOC changes. Lastly, we describe examples of fixed disorder and SOC, where tuning the Rabi field leads to a strong localization peak.

We report on the achievement of quantum degeneracy in both components of a Bose-Fermi mixture of metastable helium atoms, ${}^4\mathrm{He}^{*}$ and ${}^3\mathrm{He}^{*}$. Degeneracy is achieved via Doppler cooling and forced evaporation for ${}^4\mathrm{He}^{*}$, and sympathetically cooling ${}^3\mathrm{He}^{*}$ with ${}^4\mathrm{He}^{*}$. We discuss our modified implementation, along with the high versatility of our system. This technique is able to produce a degenerate Fermi gas with a minimum reduced temperature of $T/T_F=0.14\left(1\right)$, consisting of $2.5\times {10}^4$ ${}^3\mathrm{He}^{*}$ atoms. Due to the high internal energy of both isotopes single atom detection is possible, opening the possibility of a large number of experiments into Bose-Fermi mixtures.

Binary Bose-Einstein condensates of two distinct atomic species are studied, with interesting effects being verified due to the imbalance of atomic masses. We study the formation of vortices and associated turbulence flows created by stirring two mass-imbalanced coupled Bose-Einstein condensates in the miscible regime. We consider the mixture in the laboratory frame, confined in a pancake-like harmonic trap slightly perturbed elliptically by a time-dependent periodic potential, which introduces turbulent dynamics with final stable patterns of vortices, induced by the periodicity of the perturbation. Derived rotational frequencies are shown to be associated with the observed vortex patterns in the asymptotic regime, with a correspondence between the stirring procedure and a direct time-independent approach being indicated. It was also confirmed that a larger pattern of vortices is supported by the higher mass condensed species at the same frequency. For the experimentally accessible mixtures ${}^{85}\mathrm{Rb}\text{−}{}^{133}\mathrm{Cs}$ and ${}^{85}\mathrm{Rb}\text{−}{}^{87}\mathrm{Rb}$, we numerically solve the equations of motion and analyze the system dynamics using a range of measures including energy spectra. In the transient turbulent regime, spectral analysis shows evidence for the characteristic $k^{-5/3}$ Kolmogorov classical law associated with turbulence, modified by the universal $k^{-3}$ vortex core scaling in the ultraviolet regime.

Dispersive shock waves (DSWs) are fascinating wave phenomena occurring in media when nonlinearity overwhelms dispersion (or diffraction). Creating DSWs with low generation power and realizing their active controls is desirable but remains a longstanding challenge. Here, we propose a scheme to generate weak-light DSWs and realize their manipulations in an atomic gas involving strongly interacting Rydberg states under the condition of electromagnetically induced transparency. We show that for a two-dimensional Rydberg gas, weak nonlocality of optical Kerr nonlinearity can significantly change and induce a singular behavior of the edge speed and hence an instability of the DSWs, which, however, can be suppressed by increasing the degree of the Kerr nonlocality. We also show that in a three-dimensional Rydberg gas, DSWs can be created and propagate stably when the system works in the intermediate nonlocality regime. Different from the DSWs reported before, the DSWs found here have extremely low generation power. Moreover, such DSWs can be actively controlled; in particular, they can be stored and retrieved with high efficiency and fidelity through switching off and on a control laser field. The results reported here are useful not only for unveiling intriguing physics of DSWs but also for finding promising applications of nonlinear and nonlocal Rydberg media.

Robustness of quantum operations or controls is important to build reliable quantum devices. The robustness-infidelity measure (${\text{RIM}}_p$) is introduced to statistically quantify in a single measure the robustness and fidelity of a controller as the $p\mathrm{th}$ order Wasserstein distance between the fidelity distribution of the controller under any uncertainty and an ideal fidelity distribution. The ${\text{RIM}}_p$ is the $p\mathrm{th}$ root of the $p\mathrm{th}$ raw moment of the infidelity distribution. Using a metrization argument, we justify why ${\text{RIM}}_1$ (the average infidelity) is a good practical robustness measure. Based on the ${\text{RIM}}_p$, an algorithmic robustness-infidelity measure (ARIM) is developed to quantify the expected robustness and fidelity of controllers found by a control algorithm. The utility of the RIM and ARIM is demonstrated on energy landscape controllers of spin-$\frac{1}{2}$ networks subject to Hamiltonian uncertainty. The robustness and fidelity of individual controllers as well as the expected robustness and fidelity of controllers found by different popular quantum control algorithms are characterized. For algorithm comparisons, stochastic and nonstochastic optimization objectives are considered. Although high fidelity and robustness are often conflicting objectives, some high-fidelity, robust controllers can usually be found, irrespective of the choice of the quantum control algorithm. However, for noisy or stochastic optimization objectives, adaptive sequential decision-making approaches, such as reinforcement learning, have a cost advantage compared to standard control algorithms and, in contrast, the high infidelities obtained are more consistent with high RIM values for low noise levels.

We study super-resolution imaging theoretically using a distant $n$-mode interferometer in the microwave regime for passive remote sensing, used, e.g., for satellites like the “Soil Moisture and Ocean Salinity” (SMOS) mission to observe the surface of the Earth. We give a complete quantum-mechanical analysis of multiparameter estimation of the temperatures on the source plane. We find the optimal detection modes by combining incoming modes with an optimized unitary that enables the most informative measurement based on photon counting in the detection modes and saturates the quantum Cramér-Rao bound from the symmetric logarithmic derivative for the parameter set of temperatures. In our numerical analysis, we achieved a quantum-enhanced super-resolution by reconstructing an image using the maximum likelihood estimator with a pixel size of 3 km, which is ten times smaller than the spatial resolution of SMOS with comparable parameters. Further, we find the optimized unitary for uniform temperature distribution on the source plane, with the temperatures corresponding to the average temperatures of the image. Even though the corresponding unitary was not optimized for the specific image, it still gives a super-resolution compared to local measurement scenarios for the theoretically possible maximum number of measurements.

A method for the analysis of internal dynamics of nonlinear weakly bound polymolecular systems based on the effective-mode approach is proposed. The method enables one to estimate the number of the governing collective degrees of freedom of the system of interest at a preset accuracy under particular conditions and analyze the character of the modes depending on the activation energy of the system and the duration of its dynamic propagation, which provides qualitative and quantitative information about the coupling of diverse motions and the respective energy redistribution. The method is applied to the analysis of the dynamics of small water clusters stabilized by hydrogen bonds, which are unique spectacular examples of the systems with a pronounced coupling between the intramolecular and substantially delocalized intermolecular oscillations. The dynamic trajectories were generated in the adiabatic approximation at the Born-Oppenheimer level with the use of the quantum chemical description of selected clusters at the $\mathrm{MP}2/6-311++G\left(d,p\right)$ level. The initial conditions corresponded to different variants of the excitation of low-frequency normal modes, and the dynamic runs were carried out at a time step of 0.5 fs and the whole duration of 50 to 100 ps. Different prevailing characters of the cluster dynamics were identified depending on the molecular size, the total activation energy, and the mean potential-energy-increment to kinetic-energy-increment ratio, from an efficient accumulation of the excess kinetic energy on the effective modes of the cluster to the dissociation of the cluster into constituting fragments. The signs of the corresponding processes in the overlap matrices of the effective-mode vectors, kinetic-energy distribution over the modes, and the correlation between the number of the modes and the mean kinetic energy of the cluster are distinguished.

Electron-impact double ionization (DI) including direct and indirect processes of B-like ${\mathrm{C}}^{+}$ to ${\mathrm{Ne}}^{5+}$ ions is investigated using a two-step approach. The direct process is treated as a primary single ionization (SI) and subsequent knockout mechanism. The indirect process is considered as a primary SI of an inner-shell $1s$ electron followed by a single autoionization. Based on the two-step approach, the DI cross sections are calculated using flexible atomic code with distorted-wave approximation. The potential of the intermediate ion is approximately equivalent to the mean potential constructed by three ions involved for calculating the cross section of the direct process, while using the potential of the ionizing ion in each step of the present two-step approach overestimates results. The present cross sections agree with available theoretical and experimental data.

We investigate the application of deep learning to the retrieval of the internuclear distance in the two-dimensional ${\mathrm{H}}_2^{+}$ molecule from the momentum distribution of photoelectrons produced by strong-field ionization. We study the effect of the carrier-envelope phase on the prediction of the internuclear distance with a convolutional neural network and investigate the possibility of reconstruction of the internuclear distance from one-dimensional momentum distributions. We apply the transfer learning technique to make our convolutional neural network applicable to distributions obtained for parameters outside the ranges of the training data. The convolutional neural network is compared with alternative approaches to this problem, including the direct comparison of momentum distributions, support-vector machines, and decision trees. These alternative methods are found to possess very limited transferability. Finally, we use the occlusion-sensitivity technique to extract the features that allow a neural network to make its decisions.

In this work, we generalize the concept of the roton-softening mechanism of the spatial crystalline transition to time crystals in open quantum systems. We study a dissipative Dicke model as a prototypical example, which exhibits both continuous time crystal and discrete time crystal phases. We find that, on approaching the time crystalline transition, the response function diverges at a finite frequency, which determines the period of the upcoming time crystal. This divergence can be understood as softening of the relaxation rate of the corresponding collective excitation, which can be clearly seen by the poles of the response function on the complex plane. Using this mode-softening analysis, we predict a time quasicrystal phase in our model, in which the self-organized period and the driving period are incommensurate.

We numerically investigate the quantum transport in coupled kicked rotors with $\mathcal{PT}$-symmetric potential. We find that spontaneous $\mathcal{PT}$-symmetry breaking of wave functions emerges when the amplitude of the imaginary part of the complex potential is beyond a threshold value, which can be modulated by the coupling strength effectively. In the regime of $\mathcal{PT}$-symmetry breaking, the particles driven by the periodical kicks move unidirectionally in momentum space, indicating the emergence of a directed current. Meanwhile, with increasing the coupling strength, we find a transition from the ballistic energy diffusion to a kind of modified ballistic energy diffusion where the width of the wave packet also increases with time in a power law. Our findings suggest that the decoherence effect induced by the interplay between the interparticle coupling and the non-Hermitian driving potential is responsible for these particular transport behaviors.

We prove that the classical Dirac equation in the presence of an external (nondynamical) electromagnetic field is a relativistically causal theory. As a corollary, we show that it is impossible to use quantum tunneling to transmit particles or information faster than light. When an electron tunnels through a barrier, it is bound to remain within its future light cone. In conclusion, the relativistic quantum tunneling (if modeled using the Dirac equation) is an entirely subluminal process, and it is not instantaneous.

In the present work, we propose a generalization of tunneling time in parity and time $\left(\mathcal{P}\mathcal{T}\right)$-symmetric systems. The properties of tunneling time in $\mathcal{P}\mathcal{T}$-symmetric systems are studied with a simple contact interaction periodic finite-size diatomic $\mathcal{P}\mathcal{T}$-symmetric model. The physical meaning of negative tunneling time in $\mathcal{P}\mathcal{T}$-symmetric systems and its relation to spectral singularities is discussed.