Simulating quantum nonlocality and steering requires augmenting preshared randomness with nonvanishing communication cost. This prompts the question of how one may provide such an operational characterization for the quantumness of correlations due to even unentangled states. Here we show that for a certain class of states, such quantumness can be pointed out by superlocality, the requirement for a larger dimension of the preshared randomness to simulate the correlations than that of the quantum state that generates them. This provides an approach to define the nonclassicality of local multipartite correlations in convex operational theories.

In a previous article [Phys. Rev. Lett. 117, 213201 (2016)] we have determined the angular resolved and the total energy spectrum of a positron produced via nonlinear Breit-Wheeler pair production by a high-energy photon counterpropagating with respect to a tightly focused laser beam. Here, we first generalize the results in that article by including the possibility that the incoming photon is not exactly counterpropagating with respect to the laser field. As main focus of the present paper, we determine the photon angular resolved and total energy spectrum for the related process of nonlinear Compton scattering by an electron impinging into a tightly focused laser beam. Analytical integral expressions are obtained under the realistic assumption that the energy of the incoming electron is the largest dynamical energy of the problem and that the electron is initially almost counterpropagating with respect to the laser field. The crossing symmetry relation between the two processes in a tightly focused laser beam is also elucidated.

Testing quantum theory on macroscopic scales is a longstanding challenge whose solution could have a significant impact on physics. For example, laboratory tests (such as those anticipated in nanomechanical or biological systems) may look to rule out macroscopic realism: the idea that the properties of macroscopic objects exist objectively and can be noninvasively measured. Such investigations are likely to suffer from (i) stringent experimental requirements, (ii) marginal statistical significance, and (iii) logical loopholes. We address all of these problems by refining two tests of macroscopic realism, or “quantum witnesses”, and implementing them in a microscopic test on a photonic qubit and qutrit. The first witness heralds the invasiveness of a blind measurement; its maximum violation has been shown to grow with the dimensionality of the system under study. The second witness heralds the invasiveness of a generic quantum channel and can achieve its maximum violation in any dimension; it therefore allows for the highest quantum signal-to-noise ratio and most significant refutation of the classical point of view.

Many scenarios in the sciences and engineering require simultaneous optimization of multiple objective functions, which are usually conflicting or competing. In such problems the Pareto front, where none of the individual objectives can be further improved without degrading some others, shows the tradeoff relations between the competing objectives. This paper analyzes the Pareto-front shape for the problem of quantum multiobservable control, i.e., optimizing the expectation values of multiple observables in the same quantum system. Analytic and numerical results demonstrate that with two commuting observables the Pareto front is a convex polygon consisting of flat segments only, while with noncommuting observables the Pareto front includes convexly curved segments. We also assess the capability of a weighted-sum method to continuously capture the points along the Pareto front. Illustrative examples with realistic physical conditions are presented, including NMR control experiments on a ${}^1\mathrm{H}-{}^{13}\mathrm{C}$ two-spin system with two commuting or noncommuting observables.

Orbital angular momentum of photons is an intriguing system for the storage and transmission of quantum information, but it is rapidly degraded by atmospheric turbulence. Understanding the noise processes that affect photons is essential if we desire to protect them. In this paper we use the infinitesimal propagation equation of Roux to derive a discrete Lindblad equation, and numerically study the form of the most relevant Lindblad operators. We find that the dominant Lindblad operators are those that shift the angular momentum by one unit. We explore possible schemes to protect quantum information across multiple photons by concatenating a standard quantum error-correcting code with an error-detecting code for orbital angular momentum.

Isotope shifts and total transition frequencies are calculated for the $2{}^{2}S-3{}^{2}S$ transition of the lithium isotopes ${}^6\mathrm{Li}, {}^7\mathrm{Li}, {}^8\mathrm{Li}, {}^9\mathrm{Li}$, and the halo nucleus ${}^{11}\mathrm{Li}$. The accuracy is improved for previously calculated relativistic and quantum electrodynamic corrections, and in particular a disagreement for the Bethe logarithm is resolved for the ground ${}^{2}S$ state. Our previous result is confirmed for the $2{}^{2}P$ state. We use the pseudostate expansion method to perform the sum over virtual intermediate states. Results for the second-order relativistic recoil term of order ${\alpha }^2{(\mu /M)}^2$ Ry are shown to make a significant contribution relative to the theoretical uncertainty, but because of accidental cancellations the final result for the isotope shift is nearly unchanged. However, the spin-orbit term makes an unexpectedly large contribution to the splitting isotope shift (SIS) for the $2{}^{2}P_{1/2}-2{}^{2}P_{3/2}$ fine structure, increasing the theoretical value for the ${}^6\mathrm{Li}-{}^7\mathrm{Li}$ isotopes to $0.55631\left(7\right)\pm 0.001$ MHz. A comparison is made with high-precision measurements and other calculations for the SIS and for the total $2{}^{2}S-3{}^{2}S$ transition frequency.

Photoionization cross sections and photoelectron angular distribution asymmetry parameters are calculated for the $4d^{10},5s^2,5p^6$, and $6s^2$ subshells of atomic barium as a test of the relativistic multiconfiguration Tamm-Dancoff (RMCTD) method. The shape resonance present in the near-threshold region of the $4d$ subshell is studied in detail in the $4d$ photoionization along with the $5s, 5p$, and $6s$ subshells in the region of the $4d$ thresholds, as the $4d$ shape resonance strongly influences these subshells in its vicinity. The results are compared with available experiment and other many-body theoretical results in an effort to assess the capabilities of the RMCTD methodology. The electron correlations addressed in the RMCTD method give relatively good agreement with the experimental data, indicating that the important many-body correlations are included correctly.

We analyze the mechanism of THz generation in a gas medium with intense two-color infrared lasers pulses. The dependence of the amplitude of THz emission on the relative phase between the fundamental color (800 nm) and its second harmonic (400 nm) is shown to be identical to the residual current as well as to the asymmetry of the above-threshold-ionization (ATI) photoelectrons along the left versus the right side of the linear polarization axis of the laser, thus confirming the validity of the semiclassical photocurrent model for the THz emission. We further analyze the even vs odd angular momentum distributions of the ATI electrons. The degree of overlap between the even-parity dominant electrons and the odd-parity dominant electrons within each ATI peak determines the strength of the THz emission, thus favoring the model that THz is generated through free-free transitions in the laser field. A model is also provided to obtain the same phase dependence as the four-wave mixing model.

Vacuum consists of a bath of balanced and symmetric positive- and negative-frequency fluctuations. Media in relative motion or accelerated observers can break this symmetry and preferentially amplify negative-frequency modes as in quantum Cherenkov radiation and Unruh radiation. Here, we show the existence of a universal negative-frequency-momentum mirror symmetry in the relativistic Lorentzian transformation for electromagnetic waves. We show the connection of our discovered symmetry to parity-time ($\mathcal{PT}$) symmetry in moving media and the resulting spectral singularity in vacuum fluctuation-related effects. We prove that this spectral singularity can occur in the case of two metallic plates in relative motion interacting through positive- and negative-frequency plasmonic fluctuations (negative-frequency resonance). Our work paves the way for understanding the role of $\mathcal{PT}$-symmetric spectral singularities in amplifying fluctuations and motivates the search for $\mathcal{PT}$ symmetry in novel photonic systems.

Based on a formalism that describes atom-light interactions in terms of the classical electromagnetic Green's function, we study the optical response of atoms and other quantum emitters coupled to one-dimensional photonic structures, such as cavities, waveguides, and photonic crystals. We demonstrate a clear mapping between the transmission spectra and the local Green's function, identifying signatures of dispersive and dissipative interactions between atoms. We also demonstrate the applicability of our analysis to problems involving three-level atoms, such as electromagnetically induced transparency. Finally we examine recent experiments, and anticipate future observations of atom-atom interactions in photonic band gaps.

We present an extension of the Gerchberg-Saxton algorithm to allow for phase retrieval from interferometric data recorded at multiple sample-detector distances. A system of coupled waves is introduced where the information exchange can be controlled by use of relaxation parameters. Optimal parameters are investigated by numerical simulation and demonstrated to work with experimental data. We demonstrate that systematic errors such as pointing instabilities of the interfering waves involved and position uncertainties of the detector can be overcome by dimensional extension of the search space. Further it is shown that the proposed approach offers superior reconstruction quality as compared to conventional Gerchberg-Saxton type algorithms. We expect that the method described here will open up numerous possibilities for the correction of systematic errors in optical phase retrieval and lensless microscopy.

A unified view of the phenomenon of monogamy exhibited by Bell inequalities and noncontextuality inequalities arising from the no-signaling and no-disturbance principles is presented using the graph-theoretic method introduced by Ramanathan et al. [Phys. Rev. Lett. 109, 050404 (2012)]. We propose a hitherto unexplored tradeoff relation, namely, Bell inequalities that do not exhibit monogamy features of their own can be activated to be monogamous by the addition of a local contextuality term. This is illustrated by means of the well-known ${\mathcal{I}}_{3322}$ inequality and reveals a resource trade-off between bipartite correlations and the local purity of a single system. In the derivation of no-signaling monogamies, we uncover a unique feature, namely, that two-party Bell expressions that are trivially classically saturated can become nontrivial upon the addition of an expression involving a third party with a single measurement input.

We experimentally demonstrate the strictly nonclassical behavior in a many-atom system using a recently derived criterion [E. Kot et al., Phys. Rev. Lett. 108, 233601 (2012)] that explicitly does not make use of quantum mechanics. We thereby show that the magnetic-moment distribution measured by McConnell et al. [Nature (London) 519, 439 (2015)] in a system with a total mass of $2.6\times {10}^5$ atomic mass units is inconsistent with classical physics. Notably, the strictly nonclassical behavior affects an area in phase space ${10}^3$ times larger than the Planck quantum $\hslash$.

The dynamics of systems subjected to noise is called Markovian in the absence of memory effects, i.e., when its immediate future depends only on its present. Time correlations in the noise source may generate non-Markovian effects that, sometimes, can be erased by appropriately coarse graining the time evolution of the system. In general, the coarse-graining time $t_{\text{CG}}$ is taken to be much larger than the correlation time $\tau$ but no direct relation between them is established. Here we analytically obtain a relation between $t_{\text{CG}}$ and $\tau$ for the dynamics of a qubit subjected to a time-correlated environment. Our results can be applied in principle to any distribution of the environmental correlations and can be tested through a collisional model where the qubit sequentially interacts with correlated qutrits.

Use of time delays in high-order-harmonic generation (HHG) driven by intense two-color, few-cycle pulses is investigated in order to determine means of optimizing HHG intensities and plateau cutoff energies. Based upon numerical solutions of the time-dependent Schrödinger equation for the H atom as well as analytical analyses, we show that introducing a time delay between the two-color, few-cycle pulses can result in an enhancement of the intensity of the HHG spectrum by an order of magnitude (or more) at the cost of a reduction in the HHG plateau cutoff energy. Results for both positive and negative time delays as well as various pulse carrier-envelope phases are investigated and discussed.

The ab initio description of the nonlinear response of many-electron systems to strong-laser fields remains a major challenge. In order to address larger systems, alternative methods need to be developed that bypass the exponential scaling with particle number inherent to conventional wave-function-based approaches. In this paper we present a fully three-dimensional implementation of the time-dependent two-particle reduced-density-matrix (TD-2RDM) method for many-electron atoms. We benchmark this approach by a comparison with multiconfigurational time-dependent Hartree-Fock results for the harmonic spectra of beryllium and neon. We show that the TD-2RDM is very well suited to describe the nonlinear atomic response and to reveal the influence of electron-correlation effects.

High-order-harmonic generation (HHG) results from the interaction of ultrashort laser pulses with matter. It configures an invaluable tool to produce attosecond pulses, moreover, to extract electron structural and dynamical information of the target, i.e., atoms, molecules, and solids. In this contribution, we introduce an analytical description of atomic and molecular HHG, that extends the well-established theoretical strong-field approximation (SFA). Our approach involves two innovative aspects: (i) First, the bound-continuum and rescattering matrix elements can be analytically computed for both atomic and multicenter molecular systems, using a nonlocal short range model, but separable, potential. When compared with the standard models, these analytical derivations make possible to directly examine how the HHG spectra depend on the driven media and laser-pulse features. Furthermore, we can turn on and off contributions having distinct physical origins or corresponding to different mechanisms. This allows us to quantify their importance in the various regions of the HHG spectra. (ii) Second, as reported recently [N. Suárez et al., Phys. Rev. A 94, 043423 (2016)], the multicenter matrix elements in our theory are free from nonphysical gauge- and coordinate-system-dependent terms; this is accomplished by adapting the coordinate system to the center from which the corresponding time-dependent wave function originates. Our SFA results are contrasted, when possible, with the direct numerical integration of the time-dependent Schrödinger equation in reduced and full dimensionality. Very good agreement is found for single and multielectronic atomic systems, modeled under the single active electron approximation, and for simple diatomic molecular systems. Interference features, ubiquitously present in every strong-field phenomenon involving a multicenter target, are also captured by our model.

A scheme for achieving high temporal resolution in gas electron diffraction is proposed, in which time-dependent electron diffraction patterns can be obtained from energy-resolved angular distributions of electrons scattered by molecules in dynamical processes under the presence of a single-cycle THz-wave pulse. Derived formulae of the differential cross section and numerical simulations of electron signals scattered by Ar atoms and $\mathrm{C}{\mathrm{l}}_2$ molecules show that the temporal resolution of the proposed method can be $<10\mathrm{fs}$ in the pump-probe measurement without scanning the time delay.

We develop an exact approach to study the quench dynamics of hard-core bosons initially in thermal equilibrium in one-dimensional lattices. This approach is used to study the sudden expansion of thermal states after confining potentials are switched off. We find that a dynamical fermionization of the momentum distribution occurs at all temperatures. This phenomenon is studied for low initial site occupations, for which the expansion of the cloud is self-similar. In this regime, the occupation of the natural orbitals allows one to distinguish hard-core bosons from noninteracting fermions. We also study the free expansion of initial Mott insulating domains at finite temperature and show that the emergence of off-diagonal one-body correlations is suppressed gradually with increasing temperature. Surprisingly, the melting of the Mott domain is accompanied by an effective cooling of the system. We explain this phenomenon analytically using an equilibrium description based on an emergent local Hamiltonian.

In this work we discuss the Landau criterion for anisotropic superfluidity. To this end we consider a pointlike impurity moving in a uniform Bose-Einstein condensate with either interparticle dipole-dipole interaction or Raman-induced spin-orbit coupling. In both cases we find that the Landau critical velocity $v_{\mathrm{c}}$ is generally smaller than the sound velocity in the moving direction. Beyond $v_{\mathrm{c}}$, the energy dissipation rate is explicitly calculated via a perturbation approach. In the plane-wave phase of a spin-orbit-coupled Bose gas, the dissipationless motion is suppressed by the Raman coupling even in the direction orthogonal to the recoil momentum. Our predictions can be tested in the experiments with ultracold atoms.