A measure of quantum non-Markovianity for an open system dynamics, based on revivals of the distinguishability between system states, has been introduced in the literature using the trace distance as quantifier for distinguishability. Recently it has been suggested to use as measure for the distinguishability of quantum states the trace norm of Helstrom matrices, given by weighted differences of statistical operators. Here we show that this approach, which generalizes the original one, is consistent with the interpretation of information flow between the system and its environment associated to the original definition. To this aim we prove a bound on the growth of the external information, that is information which cannot be accessed by performing measurements on the system only, as quantified by means of the Helstrom matrix. We further demonstrate by means of example that it is of relevance in generalizing schemes for the local detection of initial correlations based on the increase of internal information. Finally we exploit this viewpoint to show the optimality of a previously introduced strategy for the local detection of quantum correlations.

In measurement-based quantum computation (MBQC), a special highly entangled state (called a resource state) allows for universal quantum computation driven by single-qubit measurements and postmeasurement corrections. The large number of qubits necessary to construct the resource state constitutes one of the main down sides to MBQC. However, in some instances it is possible to extend the resource state on the fly, meaning that not every qubit must be realized in the devices simultaneously. We consider the question of the minimal number of physical qubits that must be present in a system to directly implement a given measurement pattern. For measurement patterns which have quantum circuit representation as formalized by the notion of flow, with $n$ inputs, $n$ outputs, and $m$ total qubits, we show that only a minimum of $n+1$ and $m$ qubits are required, while the number of required qubits can be as high as $m-2$ for measurement patterns which implement a unitary but do not have a quantum circuit representation, as formalized by the notion of generalized flow (gflow). We discuss the implications of removing the Clifford part of a measurement pattern, using well-established transformation rules for Pauli measurements, for the presence of flow versus gflow, and hence the effect on the minimum number of physical qubits required to directly realize the measurement pattern.

Quantum illumination takes advantage of quantum entanglement to achieve low error probability for detecting a low reflective object embedded in a noisy thermal bath. The two-mode squeezed state (TMSS), which is a Gaussian state, has been applied to quantum illumination as the detecting states in experiment. The photon-subtracted TMSS has also been proposed to achieve even lower error probability. Here we study quantum illumination with non-Gaussian states generated by photon subtraction and photon addition. Helstrom limit and quantum Chernoff bound are evaluated for comparison between performance of states with the same squeezing strength and with the same signal strength, respectively. Particularly, states generated by asymmetrical coherent superposition of photon subtraction and addition are studied, which are shown by us to have better performance than symmetrical ones. We show that non-Gaussian operations enhance the quantum illumination by introducing both stronger entanglement and signal strength. We then give a strategy on how to choose the optimal states for the best performance in different scenarios.

We establish a lower bound concerning the computational complexity of Grover's algorithms on fractal networks. This bound provides general predictions for the quantum advantage gained for searching unstructured lists. It yields a fundamental criterion, derived from quantum transport properties, for the improvement a quantum search algorithm achieves over the corresponding classical search in a network based solely on its spectral dimension, $d_s$. Our analysis employs recent advances in the interpretation of the venerable real-space renormalization group (RG) as applied to quantum walks. It clarifies the competition between Grover's abstract algorithm, i.e., a rotation in Hilbert space, and quantum transport in an actual geometry. The latter is characterized in terms of the quantum walk dimension $d_w^Q$ and the spatial (fractal) dimension $d_f$ that is summarized simply by the spectral dimension of the network. The analysis simultaneously determines the optimal time for a quantum measurement and the probability for successfully pinpointing a marked element in the network. The RG further encompasses an optimization scheme devised by Tulsi that allows us to tune this probability to certainty, leaving quantum transport as the only limiting process. It considers entire families of problems to be studied, thereby establishing large universality classes for quantum search, which we verify with extensive simulations. The methods we develop could point the way towards systematic studies of universality classes in computational complexity to enable modification and control of search behavior.

We report on total electron-scattering cross sections from pyridine as measured with a magnetically confined electron-beam system for impact energies ranging from 1 to 200 eV, including measurements for energies below 10 eV. Reasonable agreement with previous measurements for energies above 10 eV has been found. Systematic errors arising from elastically and rotationally scattered electrons into the detector acceptance angle have been evaluated. Results are compared with available calculation both for the 10–200 eV and below 10 eV energy ranges. The evaluated data provided in this study will facilitate electron transport modeling in biologically relevant media.

On the basis of the general theoretical approach developed previously by Fofanov et al. [Ya. A. Fofanov et al., Phys. Rev. A 84, 053811 (2011)], we analyze the atomic polarization created by weak monochromatic light in an optically thick, dense, and cold atomic ensemble placed in an electrostatic field. On this basis, we determine complex refractive indexes and the permittivity tensor of the medium. We analyze the dispersion of the permittivity and its dependence on the strength of the field and on the density of the medium. We show that electro-optic effects in dilute and cold gases differ essentially from those in dense ones. In the latter case, electric field modifies the shape, amplitude, and typical linewidth of the atomic resonance. The electric field also modifies the collective Lamb shift. Observed peculiarities are explained as a result of the indirect influence of the field on optical properties of dense gases through modification of polyatomic collective effects caused by interatomic dipole-dipole interactions.

We investigate strong-field ionization of homonuclear diatomic molecules, exemplified with the ${\mathrm{N}}_2$ molecule, by a bichromatic elliptically polarized laser field having corotating components. We assume that both the emitted electron momentum vector and the internuclear vector of the diatomic molecule lay in the laser-field polarization plane. Our analysis of the low-energy electron spectra caused by the direct above-threshold ionization (ATI) and of the high-energy rescattered electron spectra that can form an extended plateau (high-order ATI or HATI) is based on the improved molecular strong-field approximation. The photoelectron spectra obtained by (H)ATI of molecular targets are more complex and have a reacher structure in comparison to the analogous spectra for atomic targets. We explain the observed interference structures by the interference of two electron wave packets emitted from the two centers of the diatomic molecule. Particular attention is devoted to the HATI spectra. For small values of the ellipticity the photoelectron spectra exhibit a plateau whose length can be as high as $17U_{\mathrm{p}}$, with $U_{\mathrm{p}}$ the electron ponderomotive energy. The yield of high-energy electrons emitted nearly antiparallel to the semimajor axis of the laser-field polarization ellipse is one order of magnitude higher for perpendicular than for the parallel molecular orientation.

We study a two-component Bose gas with artificial spin-orbit coupling (SOC) which couples the center-of-mass momentum of an atom to its internal states. We show that in this system resonance can be induced by tuning SOC strength. With a two-dimensional SOC, resonances in two scattering channels can be induced by tuning the aspect ratio of SOC strengths. With a three-dimensional SOC, resonance in all three scattering channels can be induced by tuning the appropriate SOC strength. Our method can also be applied to a Fermi gas where resonance can also be induced with two- or three-dimensional SOC.

We present a technique to diagnose the condensate fraction in a one-dimensional optical lattice of weakly interacting bosons based on the dynamics of the trapped atoms under the influence of a momentum kick. It is shown using the multi-configuration time-dependent Hartree method for bosons that the two extreme cases of the superfluid and Mott insulator states exhibit different behaviors when the lattice is briefly tilted. The current induced by the momentum boost caused by the tilt which depends directly on the amount of phase coherence between the lattice sites is linearly proportional to the condensate fraction. The atom-atom interactions only change the slope of the linear relationship. We discuss the applications of this scheme in magnetic field gradiometery.

We study the equilibrium and dynamical properties of a ferromagnetic spinor atomic Bose-Einstein condensate. In the vicinity of the critical point for a continuous quantum phase transition, universal behaviors are observed both in the equilibrium state and in the dynamics when the quadratic Zeeman shift is swept linearly. Three distinct dynamical regions are identified for different sweeping time scales ($\tau$) when compared to the time scale ${\tau }_{\mathrm{KZ}}\sim N^{(1+\nu z)/\nu d}$ decided by external driving in a system with finite size $N$ ($\nu ,z$ are critical exponents and $d$ the dimensionality). They are manifested by the excitation probability $\mathcal{P}$ and the excess heat density $\mathcal{Q}$. The adiabatic region of $\mathcal{P}\sim \mathcal{Q}\sim {\tau }^{-2}$ follows from the adiabatic perturbation theory when $\tau >{\tau }_{\mathrm{KZ}}$, while the nonadiabatic universal region of $\mathcal{P}\sim \mathcal{Q}\sim {\tau }^{-1}$ in the thermodynamic limit is described by the Kibble-Zurek mechanism when ${\tau }_{\mathrm{KZ}}>\tau >$ the time scale given by initial gap. The Kibble-Zurek scaling hypothesis is augmented with finite-size scaling in the latter region and several experimentally falsifiable features for the finite system we consider are predicted. The region of the fastest sweeping is found to be nonuniversal and far from equilibrium, with $\mathcal{P}$ and $\mathcal{Q}$ essentially being constants independent of $\tau$.

We propose the suppression of dispersive spreading of wave packets governed by the free-space Schrödinger equation with a periodically pulsed nonlinear term. Using asymptotic analysis, we construct stroboscopically dispersionless quantum states that are physically reminiscent of, but mathematically different from, the well-known one-soliton solutions of the nonlinear Schrödinger equation with a constant (time-independent) nonlinearity. Our analytics are strongly supported by full numerical simulations. The predicted dispersionless wave packets can move with arbitrary velocity and can be realized in experiments involving ultracold atomic gases with temporally controlled interactions.

We develop a microscopic theory of the dynamic structure factor to describe the Bogoliubov-Anderson-Goldstone phonon mode and its damping rate in a strongly interacting Fermi gas at finite temperature. It is based on a density functional approach—the so-called superfluid local density approximation. The accuracy of the theory is quantitatively examined by comparing the theoretical predictions with recent experimental measurements for the local dynamic structure factor of a nearly homogeneous unitary Fermi gas at low transferred momentum [S. Hoinka et al., Nat. Phys. 13, 943 (2017)], without any free parameters. We calculate the dynamic structure factor as functions of temperature and transferred momentum, and determine the temperature evolution of the phonon damping rate, by considering the dominant decay process of the phonon mode via scatterings off fermionic quasiparticles. These predictions can be confronted with future Bragg scattering experiments on a unitary Fermi gas near the superfluid transition.

We develop a theoretical description of non-Hermitian time evolution that accounts for the breakdown of the adiabatic theorem. We obtain closed-form expressions for the time-dependent state amplitudes, involving the complex eigenenergies as well as interband Berry connections calculated using basis sets from appropriately chosen Schur decompositions. Using a two-level system as an example, we show that our theory accurately captures the phenomenon of “sudden transitions,” where the system state abruptly jumps from one eigenstate to another.

Autonomous quantum error correction utilizes the engineered coupling of a quantum system to a dissipative ancilla to protect quantum logical states from decoherence. We show that the Knill-Laflamme condition, stating that the environmental error operators should act trivially on a subspace, which then becomes the code subspace, is sufficient for logical qudits to be protected against Markovian noise. It is proven that the error caused by the total Lindbladian evolution in the code subspace can be suppressed up to very long times in the limit of large engineered dissipation, by explicitly deriving how the error scales with both time and engineered dissipation strength. To demonstrate the potential of our approach for applications, we implement our general theory with binomial codes, a class of bosonic error-correcting codes, and outline how they can be implemented in a fully autonomous manner to protect against photon loss in a microwave cavity.

Spectral lines of radiating atoms are broadened by perturbations due to the surrounding plasma environment. In line-broadening calculations, the statistical average of the perturbation is weighted by the density matrix, which, in thermal equilibrium, contains correlations between the radiator and plasma. These correlations, however, have been neglected by all line-broadening theories except for the kinetic theory. The relaxation theory of line broadening is a mathematically exact derivation containing only one physical approximation: neglect of density-matrix correlations. We revisit this derivation and improve it by including the correlations. The line-broadening operator derived with the updated relaxation theory differs from that derived from the kinetic theory, though both derivations are considered to be exact. The kinetic theory derivation predicts that density-matrix correlations result a strong static shift of spectral lines. Our derivation, on the other hand, predicts that the correlations are a frequency-dependent effect that affects the line wings, and there is no shift of the line due to correlations. In addition, we predict that changes in the line shape due to correlations are only noticeable at extremely high densities. To distinguish the more appropriate model, we compare the shifts calculated with the relaxation and kinetic theory with data. The comparison shows support for the relaxation theory and casts into doubt the accuracy of the derivation of the kinetic theory of line broadening.

Hybrid systems of ultracold atoms and trapped ions or Rydberg atoms can be useful for quantum simulation purposes. By tuning the geometric arrangement of the impurities, it is possible to mimic solid-state and molecular systems. Here, we study a single trapped atom interacting with a set of arbitrarily arranged static impurities and show that the problem admits an analytical solution. We analyze in detail the case of two impurities, finding multiple trap-induced resonances which can be used for entanglement generation. Our results serve as a building block for the studies of quantum dynamics of complex systems.

We have performed ultrafast three-dimensional ion momentum imaging spectroscopy on the dissociative single ionization of oxygen molecules using attosecond pulse trains with a broad energy spectrum of 5–30 eV. High-resolution momentum imaging allows clear identification of vibrational structures corresponding to the predissociation of highly excited cationic states. By adding a pump infrared field that is synchronized with and polarized orthogonally to the XUV pulse train, and an additional probe IR field, we demonstrate how the yield of ${\mathrm{O}}^{+}$ ions can be steered between different dissociation channels by coherently controlling the coupling between multiple ${\mathrm{O}}_2^{+*}$ electronic states on an attosecond time scale. Time-dependent calculations in a single active electron approximation allow a qualitative analysis of ion yields for two orientations of the molecular axis.

We investigate tunneling ionization of a model helium atom in a strong circularly polarized short laser pulse using the classical backpropagation method and compare ten different tunneling criteria on the same footing, aiming for a consistent classical picture of the tunneling dynamics. These tunneling criteria are categorized into velocity-based, position-based, and energy-based criteria according to different notions of a tunnel exit. We find that velocity-based criteria give consistent tunneling exit characteristics with nonadiabatic effects fully included. Other criteria are either inconsistent or only able to include nonadiabatic effects partially. Furthermore, we construct a simple tunneling rate formula, identify a term in the rate responsible for the nonadiabatic effects, and demonstrate the importance of this term.

We investigate the time evolution of density, spin, and pairing correlation functions in one-dimensional $t\text{−}J$ models following a quantum quench using the time-dependent density matrix renormalization group. While density and spin correlation functions show the typical light-cone behavior over a wide range of parameters, in pairing correlation functions it is strongly suppressed. This is supported by time-dependent BCS theory, where the light cone in the pairing correlation functions is found to be at least two orders of magnitude weaker than in the density correlator. These findings indicate that in global quantum quenches not all observables are affected equally by the excitations induced by the quench.

We analyze the photon correlations in an optomechanical system containing two nonlinear optical modes and one mechanical mode which are coupled via a three-mode mixing. Under a weak driving condition, we determine the optimal conditions for photon antibunching in the weak-Kerr-nonlinear regime and find that the analytical calculations are consistent with the numerical results. The photon blockade effect is attributed to destructive quantum interference in the two-photon excitation pathways created as a result of the three-mode interaction.