We provide an approach for characterization of quantum Hamiltonian systems via utilizing a single measurement device. Specifically, we demonstrate how external quantum correlations can be used for Hamiltonian identification tasks. We explicitly introduce experimental procedures for direct estimation of single- and two-qubit Hamiltonian parameters, and also for simultaneous estimation of transverse and longitudinal relaxation rates, using a single Bell-state analyzer. An advantage of our method over the earlier approaches is that it has a built-in feature which makes it suitable for partial characterization of Hamiltonian parameters.

In this paper we investigate electron and positron total and electron vibrational excitation scattering cross sections for propene and cyclopropane molecules. The electron and positron total cross sections were measured over the energy range 0.2–1000 eV using a retarding-potential time-of-flight method while the electron impact vibrational excitation cross sections were measured using a crossed-beam method. For both molecules, bending and stretching vibrational modes are studied at loss energies 0.12 and 0.37 eV, respectively, for propene, and 0.13 and 0.37 eV, respectively, for cyclopropane, at the scattering angle of 90° and impact energy range 1–16 eV.

We study the effect of a magnetic field on the spin-depolarization cross sections of $\text{N}_2{}^{+}\left({}^2\Sigma \right)$ in collisions with ${}^3\text{H}\text{e}$ and ${}^4\text{H}\text{e}$ at very low collision energy. The fundamental states of the two nuclear spin isomers of $\text{N}_2{}^{+}$ are shown to respond quite differently to the application of the field. When the applied magnetic filed increases, the spin-depolarization cross sections are found to monotonously decrease for the fundamental paralevel $N=0$ of $\text{N}_2{}^{+}$ whereas they monotonously increase for the fundamental ortholevel $N=1$ of $\text{N}_2{}^{+}$. This effect is found to be amplified around the resonances and an explanation based on the different mechanism of spin flipping acting for these two levels is proposed. We discuss the sensitivity of the results on the change of relative mass when ${}^3\text{H}\text{e}$ is replaced by ${}^4\text{H}\text{e}$. We also explore the tuning of the inelastic cross sections by examining the variation of the scattering length as a function of the magnetic field and locate several narrow Feshbach resonances.

Quantal systems are predicted to show a changeover from exponential to a slower decay rate at very long times. Asymptotically most models predict power-law decay with integer exponents. However, the postexponential decay of a trapped particle from a potential can occur with a continuous range of power-law exponents. We show that this happens when the outer part of the potential is repulsive and decreases with the inverse square of the distance. We demonstrate a simple relation between the strength of the long-range tail and the power-law exponent. We also give explicit forms for the postexponential decay laws when the outer part of the potential is weakly attractive. These systems are amenable to experimental scrutiny.

An efficient and selective laser-induced molecular stabilization with respect to dissociation is obtained by an appropriate shaping of an intense ultraviolet-visible laser pulse which ensures the adiabatic transport of the system onto an infinitely long-lived, zero-width Floquet resonance. Such a resonance is obtained for a frequency-dependent critical field intensity specific to each field-free initial vibrational state. This mechanism, illustrated here for the photodissociation of ${\text{H}}_2^{+}$ under a $\lambda =400\text{nm}$ intense laser pulse, opens the way to a control scenario of a selective preparation of a given vibrational level that could be used in the context of molecular cooling.

The energy levels involving the ${}^{1}0^{+}$, ${}^1(-1)^{+}$, and ${}^1(-2)^{+}$ states of the helium atom in strong magnetic fields up to $2.35\times {10}^7\mathrm{T}$ are investigated. Our computational approach is a configuration-interaction (CI) method based on a set of anisotropic Hylleraas-Gaussian orbitals nonlinearly optimized for different field strength. A detailed comparison between our method and the CI method with the Gaussian-type basis functions in cylindrical coordinates is made. The advantages of the Hylleraas-Gaussian basis over the Gaussian basis are discussed.

Hamiltonian Encoding (HE) has been proposed as a technique for analyzing the mechanism of controlled quantum dynamics, where mechanism is understood in terms of the set of amplitudes of the dominant pathways connecting the initial and final states of the system. The choice of representation for the system wave function is often motivated by seeking simplicity for the structure of the Hamiltonian and not necessarily for the generated dynamics. However, the mechanism revealed by HE is strongly dependent on the basis in which the wave function is represented. The degree of mechanistic complexity is indicated by the relevant orders of the Dyson series contributing to the dynamics. An appropriate choice of representation can yield a simpler view of the dynamical mechanism by shifting some of the complexity into the representation itself. In this work the choice of representation is set up as the solution to a variational optimization problem. For unconstrained basis transformations, the optimization of the representation is formally equivalent to solving the time-dependent Schrödinger equation; different constrained basis transformations provide distinct dynamical perspectives. Specific constrained variational Ansätze are compared and analyzed by performing HE on several simple Hamiltonians with an observation of the extent to which the mechanism assessment varies with representation. The general variational formulation for determining representation can flexibly admit other Ansätze with the ultimate aim of balancing the ease of determining and understanding the representation with the reduction in mechanistic complexity.

Lateral effects are analyzed in the antibunching of a beam of free noninteracting fermions. The emission of particles from a source is dynamically described in a three-dimensional full quantum-field-theoretical framework. The size of the source and the detectors, as well as the temperature of the source are taken into account and the behavior of the visibility is scrutinized as a function of these parameters.

The superfluid-insulator transition of fermionic atoms in optical lattices is investigated by two-site dynamical mean-field theory. It is shown that the Mott transition occurs as a result of multiband effects. The quasiparticle weight in the superfluid state decreases significantly as the system approaches the Mott transition point. By changing the interaction and the orbital splitting, we obtain the phase diagram at half filling. The numerical results are discussed in comparison with the effective boson model.

We study spatiotemporal light localization in truncated one-dimensional arrays of optical waveguides in the presence of gain and loss. We demonstrate the existence of classes of continuous-discrete spatiotemporal Ginzburg-Landau solitons, which include the so-called dissipative surface light bullets localized near the edges of the truncated waveguide arrays. We find the domains of existence of such solitons in the relevant parameter space, for both on-site and intersite spatiotemporal dissipative solitons and for the states localized at the different distances from the surface. We show that the on-site solitons are stable in the whole domain of their existence, whereas all intersite solitons are unstable. We describe the complex instability-induced scenarios of the dynamics of dissipative surface solitons.

The complementarity principle of quantum mechanics relates qualitatively the visibility of quantum interference with path indistinguishability. A quantitative study was recently presented [Z. Y. Ou, Phys. Rev. A 74, 063808 (2006)]. Following the formalism of this study, we investigate another scheme for characterizing quantitatively the degree of temporal distinguishability of an $N$-photon state, based on constructive quantum interference between an $N$-photon and a single-photon state. This scheme is related to a generalized photon bunching effect in the form of a “bump,” in contrast to the “dip” for the destructive interference effect in the previous study. Generalization to other more complicated cases is straightforward and is much simpler than for the scheme of destructive interference in the previous study. A degree of (in)distinguishability is defined and can be determined experimentally by the measurement of the size of the constructive multiphoton interference effect.

Using a full density matrix formalism we show that for a lifetime broadened four-level scheme with a signal wave gain medium a large nonlinear phase shift can be induced without signal wave slowdown and attenuation. In this system the signal wave acquires a large nonlinear phase shift and travels with superluminal propagation characteristics. This raises the possibility of rapidly responding nonlinear phase shifting and possibly phase gating devices for information science.

Decoherence is believed to deteriorate the ability of a purification scheme that is based on the idea of driving a system to a pure state by repeatedly measuring another system in interaction with the former and hinder for a pure state to be extracted asymptotically. Nevertheless, we find a way out of this difficulty by deriving an analytic expression of the reduced density matrix for a two-qubit system immersed in a bath. It is shown that we can still extract a pure state if the environment brings about only dephasing effects. In addition, for a dissipative environment, there is a possibility of obtaining a dominant pure state when we perform a finite number of measurements.

We investigate above-threshold ionization and high-order harmonic generation processes by mid-infrared $\left(3.3\mu \text{m}\right)$ and far-infrared $\left(10\mu \text{m}\right)$ laser pulses using the numerical solution of the time-dependent Schrödinger equation for the potassium atom. When the laser intensity is reasonably high, we find two plateaus in the above-threshold ionization spectra. With the use of two different potential models and a classical analysis, we clarify that the first plateau mainly originates from the contribution of the excited bound states while the second one is associated with the well-known backscattering process. Corresponding results for the harmonic spectra verify our interpretation.

We show theoretically that a short specific elliptically polarized laser pulse driving an asymmetric top molecule can induce postpulse revivals of three-dimensional (3D) alignment. By choosing the field ellipticity resulting in the best compromise between the alignment of two molecular axes, we demonstrate that efficient 3D alignment can be achieved at low temperature. In the experiment, the field-free alignment of moderately cool ethylene molecules is probed by using a technique based on the optical Kerr effect. Control of 3D field-free alignment opens the door to a large range of applications in chemistry as well as in molecular optics.

Manipulation of the quantum state by the Majorana transition in a spinor Bose-Einstein condensed system has been realized by altering the rotation frequency of the magnetic field direction. This kind of manipulation method has no limitation on the transition speed in principle, and the system is well closed. When this method is used on a pulsed atom laser, a multicomponent spinor atom laser is generated. We demonstrate that the experimental results agree with the theoretical prediction.

This paper addresses the minimal disturbance quantum discrimination among a set of $N$ pure states which are symmetrically distributed on a Bloch sphere equator. The optimal trade-off between the discrimination probability and the state disturbance is exactly derived. The minimal disturbance discrimination which saturates the trade-off turns out to be a convex combination of the minimum error discrimination and the identity maps.

Energy absorption of xenon clusters embedded in helium nanodroplets from strong femtosecond laser pulses is studied theoretically. Compared to pure clusters we find earlier and more efficient energy absorption in agreement with experiments. This effect is due to resonant absorption of the helium nanoplasma whose formation is catalyzed by the xenon core. For very short double pulses with variable delay, two plasma resonances, due to the helium shell and the xenon core, are identified and the experimental conditions are given which should allow for a simultaneous observation of both of them.

Maps that are not completely positive (CP) are often useful to describe the dynamics of open systems. An apparent violation of complete positivity can occur because there are prior correlations of the principal system with the environment, or if the applied transformation is correlated with the state of the system. We provide a physically motivated definition of accessible non-CP maps and derive two necessary conditions for a map to be accessible. We also show that entanglement between the system and the environment is not necessary to generate a non-CP dynamics. We describe two simple approximations that may be sufficient for some problems in process tomography, and then outline what these methods may be able to tell us in other situations where non-CP dynamics naturally arise.

Two noninteracting qubits, initially prepared in an entangled state, are coupled to their own independent environments and evolve under their influence. The reduced non-Markovian dynamics of two qubits is exact for arbitrary model parameters. Necessary and sufficient conditions for nonvanishing entanglement are formulated for both zero and nonzero temperatures and arbitrary time. It is shown that (i) entanglement dynamics can effectively be controlled by a finite quantum system coupled to one of the qubits and (ii) dynamical symmetry of the controlling quantum system can influence significantly entanglement of the qubits and results in its nonmonotonic decay.