We present a nondemolition quantum information processing task of probing the information of a class of quantum state. In this task, the information is extracted by some unitary evolution with the introduced probing qubit assisted, but the probed quantum state (density matrix) is undisturbed at any time and independent of the choice of the initial probing state. We give a sufficient and necessary condition on the Hamiltonian that can lead to the successful realization of such a task. We prove that, for any feasible scheme, the probed plus probing system will always stay at a disentangled state with one side's quantum discord absent and the other side's inevitably produced in the probing process. An explicit example is given for the demonstration, which shows that the ratio of quantum discord to the total correlation will have to reduce to zero for the maximal accessible information. In this sense, we say that quantum discord plays a dual role in this case.

We discuss how to generate entangled coherent states of four microwave resonators (a.k.a. cavities) coupled by a three-level superconducting device (qutrit). We also show that a Greenberger-Horne-Zeilinger (GHZ) state of four superconducting qubits embedded in four different resonators can be created with this scheme. In principle, the proposed method can be extended to create an entangled coherent state of $n$ resonators and to prepare a (GHZ) state of $n$ qubits distributed over $n$ cavities in a quantum network. In addition, it is noted that four resonators coupled by a coupler qutrit may be used as a basic circuit block to build a two-dimensional quantum network, which is useful for scalable quantum information processing.

The ultimate accuracy as regards controlling a qubit with a coherent field is studied in terms of degradation of the fidelity by employing a fully quantum mechanical treatment. While the fidelity error accompanied by $\pi /2$ pulse control is shown to be inversely proportional to the average photon number in a way similar to that revealed by J. Gea-Banacloche's [Phys. Rev. A 65, 022308 (2002)] results, our results show that the error depends strongly on the initial state of the qubit. When the initial state of the qubit is the ground state, the error is about 20 times smaller than that of the control started from the excited state, no matter how large $N$ is. This dependency is explained in the context of an exact quantum mechanical description of the pulse area theorem. Using the result, the error accumulation tendency of successive pulse controls is found to be both nonlinear and initial state dependent.

We propose approximate separable solutions for the Schrödinger equation corresponding to the four-body Coulomb problem. By exploring several asymptotic limits, we find configurations where the nondiagonal terms of the Hamiltonian can be introduced in the two-body Coulomb-like distortion factors leading to modified momenta. The present effective momenta model is used to study the double ionization of He by ion impact at the fully differential level. Possible differences arising from proton and antiproton impact are explored at impact energies in the range 700 keV/amu–6 MeV/amu. The results are represented in terms of contour plots as a function of the electrons’ emission angles, and the four-body dynamics for the double-ionization process is analyzed. Finally, we compare diverse representations for the fully differential cross section. A detailed interpretation of the features of the process is done by overlapping the recoil level lines over the contour plots. Ternary plots are tested as an alternative tool to explore the different physical mechanisms involved in the double emission process at the fully differential level.

A charged particle emits bremsstrahlung while traversing matter. We calculate the radiation cross section for bare heavy ions penetrating amorphous materials and single crystals at highly relativistic energies. The main component originates in scattering of the virtual photons of screened target nuclei on the projectile. It appears at, approximately, $2\gamma$ times the energy of the giant dipole resonance of the projectile, approximately 25$\gamma$ MeV for a lead nucleus ($\gamma \equiv E/M{c}^2$, where $E$ and $M$ denote the projectile energy and mass). The emission pertains to relatively close impacts, with impact parameters ranging to, at maximum, the screening radius of the target atoms. As a result, the main bremsstrahlung component shows channeling dips, that is, dips in yield upon variation of the incidence angle to major crystallographic directions of a single crystal. The minimum yield increases with $\gamma$ but saturates at a very low value. Incoherent interaction with single target electrons gives rise to two additional bremsstrahlung components, a modest component due to scattering of virtual photons of the electrons on the projectile and a strong low-energy component due to scattering of the virtual photons of the projectile on the electrons. The difference in radiation levels can be traced to the mass of the scatterer. Since target electrons are more widely distributed than nuclei in a crystal channel the variation of the electron component of the bremsstrahlung with incidence angle to a major crystallographic direction is less abrupt than the variation of the nuclear component. In consequence, the shape of the total bremsstrahlung spectrum changes when the crystal is tilted and the individual components may be singled out. Pair creation is also sensitive to the orientation of a crystalline material, resulting in a pronounced directional dependence of the energy loss of bare heavy ions at extreme relativistic energies.

The 38-GHz microwave field required to effect 50$\%$ ionization of Li atoms decreases from 60 V/cm at $n=37$, where the microwave frequency is one-third the Kepler frequency, to 45 V/cm at $n=56$, where the microwave frequency is equal to the Kepler frequency. For $n>56$ the 50$\%$ ionization field is constant at $E\approx 2.4{\omega }^{5/3}$ until $\sim 80$ GHz below the ionization limit, where it begins to decline. In spite of the fact that the pulse is 8000 cycles long, we detect approximately 5$\%$ of the initial population in very-high-lying ($n>215$) states subsequent to the microwave pulse. We suggest that these atoms are trapped in metastable atom-field states during the microwave pulse and relax to the high-lying states when the field is turned off. The existence of such states is predicted by the simple man's model of above-threshold ionization.

We propose and illustrate numerically the possibility of imaging rotational wave packets in angular space and time by using different pump-probe spectroscopic techniques. A general theoretical framework to perform such rotational mapping is derived and three specific spectroscopies, namely, birefringence, high harmonic generation, and angle-resolved photoelectron spectroscopy, are numerically explored. All three approaches are shown to provide direct mapping of the rotational coherences of molecules but they are not equivalent; comparison of their results yields interesting insights into their relative merits. Finally, we illustrate the role played by the symmetry of the molecular orbitals in determining the quality of the images generated by high harmonic and photoelectron signals. The potential of rotational imaging as a route to both intramolecular coupling mechanisms and the interaction of molecules with different environments is discussed.

We present an experimental concept and setup for laser-microwave double-resonance spectroscopy of highly charged ions in a Penning trap. Such spectroscopy allows a highly precise measurement of the Zeeman splittings of fine- and hyperfine-structure levels due the magnetic field of the trap. We have performed detailed calculations of the Zeeman effect in the framework of quantum electrodynamics of bound states as present in such highly charged ions. We find that apart from the linear Zeeman effect, second- and third-order Zeeman effects also contribute to the splittings on a level of ${10}^{-4}$ and ${10}^{-8}$, respectively, and hence are accessible to a determination within the achievable spectroscopic resolution of the ARTEMIS experiment currently in preparation.

The Zitterbewegung effect in spin-orbit-coupled spin-1 cold atoms is investigated in the presence of the Zeeman field and a harmonic trap. It is shown that the Zeeman field and the harmonic trap have significant effect on the Zitterbewegung oscillatory behaviors. The external Zeeman field could suppress or enhance the Zitterbewegung amplitude and change the frequencies of oscillation. A much slowly damping Zitterbewegung oscillation can be achieved by adjusting both the linear and the quadratic Zeeman fields. Multifrequency Zitterbewegung oscillation can be induced by the applied Zeeman field. In the presence of the harmonic trap, the subpackets corresponding to different eigenenergies would always keep coherent, resulting in the persistent Zitterbewegung oscillations. The Zitterbewegung oscillation would display very complicated and irregular oscillation characteristics due to the coexistence of different frequencies of the Zitterbewegung oscillation. Furthermore, numerical results show that the Zitterbewegung effect is robust even in the presence of interaction between atoms.

We investigate bound states in the one-dimensional two-particle Bose-Hubbard model with an attractive ($V>0$) impurity potential. This is a one-dimensional, discrete analogy of the hydrogen negative ion (H${}^{-}$) problem. There are several different types of bound states in this system, each of which appears in a specific region. For given $V$, there exists a (positive) critical value $U_{c1}$ of $U$ (the on-site atom-atom interaction), below which the ground state is a bound state. Interestingly, close to the critical value ($U\lesssim U_{c1}$), the ground state can be described by the Chandrasekhar-type variational wave function, which was initially proposed for H${}^{-}$. For $U>U_{c1}$, the ground state is no longer a bound state. However, there exists a second (larger) critical value $U_{c2}$ of $U$, above which a molecule-type bound state is established and stabilized by the repulsion. We have also tried to solve for the eigenstates of the model using the Bethe ansatz. The model possesses a global ${\mathbb{Z}}_2$ symmetry (parity) which allows classification of all eigenstates into even and odd states. It is found that all states with odd parity have the Bethe form, but none of the states in the even-parity sector. This allows us to identify analytically two odd-parity bound states, which appear in the parameter regions $-2V<U<-V$ and $-V<U<0$, respectively. Remarkably, the latter one can be embedded in the continuum spectrum with appropriate parameters. Moreover, in part of these regions, there exists an even-parity bound state accompanying the corresponding odd-parity bound state with almost the same energy.

The difference between boson and fermion dynamics in quasi-one-dimensional lattices is studied by calculating the persistent current in small quantum rings and by exact simulations of the time evolution of the many-particle state in two cases: expansion of a localized cloud and collisions in a Newton's cradle. We consider three different lattices which in the tight-binding model exhibit flat bands. The physical realization is considered to be an optical lattice with bosonic or fermionic atoms. The atoms are assumed to interact with a repulsive short-range interaction. The different statistics of bosons and fermions lead to different dynamics. Spinless fermions are easily trapped in the flat-band states due to the Pauli exclusion principle, which prevents them from interacting, while bosons are able to push each other out from the flat-band states.

We perform lattice Monte Carlo simulations for up to 66 unitary fermions in a finite box using a highly improved lattice action for nonrelativistic spin-$\frac{1}{2}$ fermions. We obtain a value of $0.{366}_{-0.011}^{+0.016}$ for the Bertsch parameter, defined as the energy of the unitary Fermi gas measured in units of the free gas energy in the thermodynamic limit. In addition, for up to four unitary fermions, we compute the spectrum of the lattice theory by exact diagonalization of the transfer matrix projected onto irreducible representations of the octahedral group for small to moderate size lattices, providing an independent check of our few-body simulation results. We compare our exact numerical and simulation results for the spectrum to benchmark studies of other research groups as well as perform an extended analysis of our lattice action improvement scheme, including an analysis of the errors associated with higher partial waves and finite temporal discretization.

The generation of Faraday waves in superfluid Fermi-Bose mixtures in elongated traps is investigated. The generation of waves is achieved by periodically changing a parameter of the system in time. Two types of modulations of parameters are considered: a variation of the fermion-boson scattering length and the boson-boson scattering length. We predict the properties of the generated Faraday patterns and study the parameter regions where they can be excited.

We develop a cavity model including transverse field variations and gain saturation to elucidate the operating characteristics of a femtosecond amplification cavity (fsAC) with an incident infrared frequency comb. After validating the model against experimental data, we explore how the fsAC operating characteristics, such as average output power and beam quality, vary with factors such as the pump beam and the output coupler reflectivity. We also examine the locking range of the fsAC and how the nonlinear Kerr effect places a limitation on the pulse duration that can be successfully amplified by the fsAC.

We study the relation between semi- and fully-device-independent protocols. As a tool, we use the correspondence between Bell inequalities and dimension witnesses. We present a method for converting the former into the latter, and vice versa. This relation provides us with interesting results for both scenarios. First, we find random-number-generation protocols with higher bit rates for both the semi- and fully-device-independent cases. As a byproduct, we obtain classes of Bell inequalities and dimension witnesses. Then, we show how optimization methods used in studies on Bell inequalities can be adopted for dimension witnesses.

In this paper, we propose a scheme for generating steady-state entanglement of remote micromechanical oscillators in unidirectionally coupled cavities. For the system of two mechanical oscillators, we show that when two cavity modes in each cavity are driven at red- and blue-detuned sidebands, respectively, a stationary two-mode squeezed vacuum state of the two mechanical oscillators can be generated by the cascaded cavity coupling. The degree of squeezing is controllable by adjusting the relative strength of the pump lasers. Our calculations also show that the achieved mechanical entanglement is robust against thermal fluctuations. For the case of multiple mechanical oscillators, we find that steady-state genuine multipartite entanglement can also be built up among the remote mechanical oscillators. The present scheme does not require nonclassical light input or conditional quantum measurements, and it can be realized with current experimental technology.

Many-photon interference in linear-optics setups can be exploited to generate and to detect multipartite entanglement. Without recurring to any interparticle interaction, many entangled states have been created experimentally, and a panoply of theoretical schemes for the generation of various classes of entangled states is available. Here, we present a unifying framework that accommodates the present experiments and theoretical protocols for the creation of multiparticle entanglement via interference. A general representation of the states that can be created is provided for bosons and fermions for any particle number and for any dimensionality of the entangled degree of freedom. Using this framework, we derive an upper bound on the generalized Schmidt number of the states that can be generated, and we establish bounds on the dimensionality of the manifold of these states. We show that—at the expense of a smaller success probability—more states can be created with bosons than with fermions and give an intuitive interpretation of the state representation and of the established bounds in terms of superimposed many-particle paths.

An alkene-based antirelaxation coating for alkali-metal vapor cells exhibiting Zeeman relaxation times up to 77 s was recently identified by Balabas et al. The long relaxation times, two orders of magnitude longer than in paraffin- (alkane-) coated cells, motivate revisiting the question of what the mechanism is underlying wall-collision-induced relaxation and renew interest in applications of alkali-metal vapor cells to secondary frequency standards. We measure the width and frequency shift of the ground-state hyperfine $m{}_F=0\to m{}_F^{\prime }=0$ transition (clock resonance) in vapor cells with ${}^{85}$Rb and ${}^{87}$Rb atoms, with an alkene antirelaxation coating. We find that the frequency shift is slightly larger than for paraffin-coated cells and that the Zeeman linewidth scales linearly with the hyperfine frequency shift.

We report the experimental study of field-free molecular alignment in CO${}_2$ gas mixtures induced by intense femtosecond laser pulses in the presence of collisional processes. We demonstrate that the alignment signals exhibit specific features due to nontrivial collisional propensity rules that tend to preserve the orientation of the rotational angular momentum of the molecules. The analysis is performed with a quantum approach based on the modeling of rotational $J$- and $M$-dependent state-to-state transfer rates. The present work paves the way for strong-field spectroscopy of collisional dynamics.

The influence of the power variation on the evolution of an arbitrary beam in strongly nonlocal nonlinear media is investigated on the basis of the mode-decomposition method. The variation of the power changes the longitudinal positions of the beam patterns and induces the longitudinal scaling. Although there is a one-to-one correspondence between the patterns before and after the power variation, the pattern sizes in the two cases differ from each other; therefore the transverse scaling occurs. The effect of the three-dimensional nonuniform scaling effect can be represented with a simple formula, with which the analytical solution for the beam after the power variation can be readily obtained from its counterpart before the power variation.