When observations must come from incompatible devices and cannot be produced by compatible devices is a property that motivates two integer-valued quantifications of incompatibility, called incompatibility dimension and compatibility dimension. The first one quantifies how many states are minimally needed to detect incompatibility if the test states are chosen carefully, whereas the second one quantifies how many states one may have to use if they are randomly chosen. With concrete examples we show that these quantities have unexpected behavior with respect to noise.

We present an analysis of spatial chirp in subcycle pulsed beams corroborated by spatiotemporal electro-optic sampling measurements of terahertz radiation. Modeling the subcycle pulsed beam as a superposition of monochromatic Gaussian beams, free-space propagation is shown to directly lead to lateral spatial chirp with the spectrum on-axis substantially bluer than the overall energy spectrum. The two-transverse $+$ one-temporal dimensional profile of terahertz subcycle pulsed beams from organic crystals DSTMS and OH1 are measured and compared with observed spatiospectral correlations consistent with the model.

Typically in many-body systems the correlation energy, which is defined as the difference between the exact ground-state energy and the mean-field solution, has been a measure of the system's total correlations. However, under the quantum information context, it is possible to define some quantities in terms of the system's constituents that measure the classical and quantum correlations, such as the entanglement entropy, mutual information, quantum discord, one-body entropy, etc. In this work we apply concepts of quantum information in fermionic systems in order to study traditional correlation measures (the relative correlation energy). Concretely, we analyze the two- and three-level Lipkin models, which are exactly solvable (but nontrivial) models commonly used in the context of the many-body problem. We conclude that the correlation energy is not a good estimator of the total correlation of the system, and information theoretic measures should be considered.

Entangled states can help in quantum-state discrimination by local operations and classical communication (LOCC). For example, a Bell state is necessary (and sufficient) to perfectly discriminate a set of either three or four Bell states by LOCC. In this paper, we consider the task of LOCC discrimination of the states of noisy Bell ensembles, where a given ensemble consists of the states obtained by mixing the Bell states with an arbitrary two-qubit state with nonzero probabilities. It is proved that a Bell state is required for optimal discrimination by LOCC, even though the ensembles do not contain, in general, any maximally entangled state and, in specific instances, any entangled state.

We investigate the high-order harmonic generation in graphene irradiated by a linearly polarized intense laser, addressing the ellipticity or polarization properties of the harmonics. We exploit time-dependent density functional theory to calculate the harmonic spectra for a laser wavelength of 4770 nm and an intensity of 1.7 $\mathrm{TW}/{\mathrm{cm}}^2$, and our numerical results can qualitatively reproduce recent experimental data. Our simulations also reveal that the harmonic ellipticity depends on both the harmonic order and the orientation angle between the graphene symmetric axis and the laser polarization direction. It can reach 0.68 for the ninth-order harmonic at the orientation angle of ${20}^{\circ }$. To understand the mechanism of the high ellipticity, we develop a two-band model based on the tight-binding approximation. We may explain the ellipticity of high-order harmonic generation by investigating the transition dipole moments in the two-band model. Our theory further predicts a sensitive dependence of the harmonic ellipticity on the laser intensity for various laser wavelengths.

The mechanism of atomic collisions in excited bands plays an important role in both the atomic dynamics in high bands of optical lattices and the simulation of condensed matter physics. Atoms distributed in an excited band of an optical lattice can collide and decay to other bands through different scattering channels. In the excited bands of a one-dimensional lattice there is no significant difference between the cross sections to different scattering channels, due to the sameness of all of the geometrical couplings. Here, we investigate the collisional scattering channels for atoms in the excited bands of a triangular optical lattice and demonstrate a dominant scattering channel in the experiment. A shortcut method is utilized to load Bose-Einstein condensate of ${}^{87}\mathrm{Rb}$ atoms into the $\mathrm{\Gamma }$ point of the first $D$ band with zero quasimomentum in the triangular optical lattice. After some evolution time, the number of atoms scattering into the $S$ band induced by two-body collisions is around four times the number that scatter into the second most populated band. Our numerical calculation shows that the $ss$ scattering channel is dominant, which is roughly consistent with the experimental measurement. The appearance of dominant scattering channels in a triangular optical lattice is owing to nonorthogonal lattice vectors. This work is helpful for the research on many-body systems and directional enhancement in optical lattices.

Although heralding signals in quantum optics experiments are typically based on the detection of exactly one photon, it has recently been theoretically shown that heralding based on the detection of zero photons can be useful in a number of quantum information applications. Here we experimentally investigate a technique for “heralding on zero photons” using conventional single-photon detectors. We illustrate how detector efficiency and dark count rates play a counterintuitive role in the ability to accurately detect zero photons, and we use a variant of the Hong-Ou-Mandel interferometer to study the deleterious effects of limited detector efficiency when heralding on zero.

Ordered ensembles of atoms, such as atomic arrays, exhibit distinctive features from their disordered counterpart. In particular, while collective modes in disordered ensembles show a linear optical response, collective subradiant excitations of subwavelength arrays are endowed with an intrinsic nonlinearity. Such nonlinearity has both a coherent and a dissipative component: two excitations propagating in the array scatter off each other leading to formation of correlations and to emission into free-space modes. We show how to take advantage of such nonlinearity to coherently prepare a single excitation in a subradiant (dark) collective state of a one-dimensional array as well as to perform an entangling operation on dark states of parallel arrays. We discuss the main source of errors represented by disorder introduced by atomic center-of-mass fluctuations, and we propose a practical way to mitigate its effects.

We shift the paradigm of feedback control from the control of quantum states to the control of phase transitions in quantum systems. We show that feedback allows tuning the universality class of phase transitions via modifying its critical exponent. We expand our previous treatment [Ivanov et al., Phys. Rev. Lett. 124, 010603 (2020)] of the Dicke model and go beyond the approximation of the adiabatically eliminated light field. Both linearized and nonlinear models of spin ensembles are considered. The tunability of quantum fluctuations near the critical point by the feedbacks of nontrivial shapes is explained by considering the fluctuation spectra and the system behavior at single quantum trajectories.

The evolution of a driven quantum system is said to be adiabatic whenever the state of the system stays close to an instantaneous eigenstate of its time-dependent Hamiltonian. The celebrated quantum adiabatic theorem ensures that such pure state adiabaticity can be maintained with arbitrary accuracy, provided one chooses a small enough driving rate. Here, we extend the notion of quantum adiabaticity to closed quantum systems initially prepared at finite temperature. In this case adiabaticity implies that the (mixed) state of the system stays close to a quasi-Gibbs state diagonal in the basis of the instantaneous eigenstates of the Hamiltonian. We prove a sufficient condition for the finite temperature adiabaticity. Remarkably, it turns out that the finite temperature adiabaticity can be more robust than the pure state adiabaticity with respect to increasing the system size. This can be the case for one-body systems with large Hilbert spaces, such as a particle in a large box, as well as for certain many-body systems. In particular, we present an example of a driven many-body system where, in the thermodynamic limit, the finite temperature adiabaticity is maintained, while the pure state adiabaticity breaks down. On the other hand, for generic many-body systems the scaling of the finite temperature adiabatic condition with the system size is exponential, analogously to pure state adiabatic conditions.

We propose a robust and efficient way to store and transport quantum information via one-dimensional discrete time quantum walks. We show how to attain an effective dispersionless wave packet evolution using only two types of local unitary operators (quantum coins or gates), properly engineered to act at predetermined times and at specific lattice sites during the system's time evolution. In particular, we show that a qubit initially localized about a Gaussian distribution can be almost perfectly confined during long times or sent hundreds lattice sites away from its original location and later almost perfectly reconstructed using only Hadamard and ${\sigma }_x$ gates.

A beautiful idea about the incompatibility of physical context (IPC) was introduced in Phys. Rev. A 102, 050201(R) (2020). Here, a context is defined as a set of a quantum state and two sharp rank-one measurements, and the incompatibility of physical context is defined as the leakage of information while implementing those two measurements successively in that quantum state. In this work, we show the limitations in their approach. The three primary limitations are that (i) their approach is not generalized for positive operator-valued measurements; (ii) they restrict information theoretic agents Alice, Eve, and Bob to specific quantum operations and do not consider most general quantum operations, i.e., quantum instruments; and (iii) their measure of IPC can take negative values in specific cases in a more general scenario, which implies the limitation of their information measure. Thereby, we have introduced a generalization and modification to their approach in a more general and convenient way, such that this idea is well defined for generic measurements, without these limitations. We also present a comparison of the measure of the IPC through their and our methods. Lastly, we show, how the IPC reduces in the presence of memory using our modification, which further validates our approach.

The numerical solution of partial differential equations by discretization techniques is ubiquitous in computational physics. In this work we investigate this approach in the quantum realm by solving the heat equation for a square plate subject to fixed temperatures at the edges and random heat sources and sinks within the domain. The hybrid classical-quantum approach consists of the solution on a quantum computer of the coupled linear system of equations that results from the discretization step. Owing to the limitations on the number of qubits and their connectivity, we use the Gauss-Seidel method to divide the full system of linear equations into subsystems, which are solved iteratively in block fashion. Each of the linear subsystems was solved using 2000Q and Advantage quantum computers developed by D-Wave Systems Inc. By comparing classical numerical and quantum solutions, we observe that the errors and chain break fraction are, on average, greater on the 2000Q system. Unlike in the classical Gauss-Seidel method, the errors of the quantum solutions level off after a few iterations of our algorithm. This is partly a result of the span of the real-number line available from the mapping of the chosen size of the set of qubit states. We verified this by using techniques to progressively shrink the range mapped by the set of qubit states at each iteration (increasing floating-point accuracy). As a result, no leveling off is observed. However, an increase in qubits does not translate to an overall lower error. This is believed to be indicative of the increasing length of chains required for the mapping to real numbers and the ensuing limitations of hardware.

Entangled states are undoubtedly an integral part of various quantum information processing tasks. On the other hand, absolutely separable states which cannot be made entangled under any global unitary operations are useless from the resource theoretic perspective, and hence, identifying non–absolutely separable states can be an important issue for designing quantum technologies. Here we report that nonlinear witness operators provide significant improvements in detecting non–absolutely separable states over their linear analogs, by invoking examples of states in various dimensions. We also address the problem of closing the detection loophole and we find the critical efficiency of detectors above which no fake detection of non–absolutely separable (non–absolutely positive partial transposed) states is possible.

The strong-field approximation for high-harmonic generation in near-infrared and infrared laser pulses is formulated in the accelerated Kramers-Henneberger frame. The accompanying physical picture is discussed and the nature of the leading-order term is contrasted with that of the three-step model following the strong-field-approximation formulation in the length or velocity gauges. The theory is illustrated by high-harmonic generation spectra for atomic hydrogen.

We study cross correlation of a single-cycle circularly polarized laser pulse with its delayed and/or attenuated replica in the attoclock field configuration. In this configuration, tunneling ionization of the target atom is observed in the polarization plane of the driving laser pulse. The two-pulse attoclock displays a robust interference pattern in the photoelectron energy spectrum and the real time correlation between the photon absorption rates from the two pulses. This interference and correlation can be used for characterization of single-cycle laser pulses and monitoring the target population dynamics during the ionization process.

We apply a hyperspherical formulation to a trapped Bose-Einstein condensate with dipolar and contact interactions. Central to this approach is a general correspondence between $K$-harmonic hyperspherical methods and a suitable Gaussian ansatz to the Gross-Pitaevskii equation, regardless of the form of the interparticle potential. This correspondence allows one to obtain hyperspherical potential energies for a wide variety of physical problems. In the case of the dipolar Bose-Einstein condensate, this motivates the inclusion of a beyond-mean-field term within the hyperspherical picture, which allows us to describe the energies and wave functions of excitations of self-bound dipolar droplets outside the mean-field limit.

The paper studies the modulational instability (MI), both theoretically and numerically, of the helicoidal spin-orbit coupled Bose-Bose mixture. An expression of the MI growth rate is found through the linear stability analysis of continuous wave, followed by a comprehensive parametric study of the MI regions, emphasizing the effects of the spin-orbit coupling, the helicoidal gauge potential, and interatomic interactions. Direct numerical simulations concur with the analytical predictions. Under suitable balance between nonlinear and dispersive effects, trains of solitonlike objects are obtained, and their behaviors are very sensitive to parameter variations. Attention is particularly paid to the impact of the left- and right-handed helicoidal spin-orbit couplings on the appearance of matter waves that have the form soliton molecules in the Bose-Bose mixture. Additionally, for qualitative support of the obtained structures, the formation of a bright solitons train is also reported numerically using two neighboring solitons subjected to a fixed phase difference. Their behavior under the action of the helicoidal spin-orbit coupling is also debated, especially when left- and right-handed helicoidal couplings are interchanged.

Delay differential equation model of a nonlinear optical–nonlinear amplifying loop mirror mode-locked laser is developed that takes into account the finite relaxation rate of the gain medium and asymmetric beam splitting at the entrance of the nonlinear mirror loop. Asymptotic linear stability analysis of the continuous wave solutions performed in the limit of large delay indicates that in a class-B laser flip instability is preceded by the modulational instability and therefore cannot give rise to stable square wave patterns. Numerically it is shown that the model can demonstrate large windows of regular fundamental and harmonic mode-locked regimes with single and multiple pulses per cavity round trip time separated by domains of irregular pulsing.

We study the system of strongly interacting spinor bosons in a square lattice subject to the isotropic Rashba spin-orbit coupling $\alpha =\beta$. It supports a collinear spin-bond correlated magnetic Y-x phase, a gapped coplanar in-commensurate (IC) XY-y phase, and a noncoplanar commensurate (C) $3\times 3$ Skyrmion crystal phase. The state at the Abelian point $\alpha =\beta =\pi /2$ is just a ferromagnetic state in a rotated basis. Slightly away from the point, we identify a spurious $\text{U}\left(1\right)$ symmetry, develop a nonperturbative method to calculate not only the gap but also the excitation spectrum due to the order from quantum disorder (OFQD) mechanism. We construct a symmetry-based effective action to investigate the quantum Lifshitz transition from the Y-x state to the IC XY-y state and establish the connection between the phenomenological parameters in the effective action and those evaluated by the microscopic nonperturbative OFQD analysis in the large $S$ limits. Experimental implications on cold atoms and some $4d$ or $5d$ Kitaev materials are discussed.