We present a robust method for quantum process tomography, which yields a set of Lindblad operators that optimally fit the density operators measured at a sequence of time points. The benefits of this method are illustrated using a set of liquid-state nuclear magnetic resonance measurements on a molecule containing two coupled hydrogen nuclei which are sufficient to fully determine its relaxation superoperator. It was found that the complete positivity constraint, which is necessary for the existence of the Lindblad operators, was also essential for obtaining a robust fit to the measurements. The general approach taken here promises to be broadly useful in studying dissipative quantum processes in many of the diverse experimental systems currently being developed for quantum-information processing purposes.

We establish the entangling power of a unitary operator on a general finite-dimensional bipartite quantum system with and without ancillas, and give relations between the entangling power based on the von Neumann entropy and the entangling power based on the linear entropy. Significantly, we demonstrate that the entangling power of a general controlled unitary operator acting on two equal-dimensional qudits is proportional to the corresponding operator entanglement if linear entropy is adopted as the quantity representing the degree of entanglement. We discuss the entangling power and operator entanglement of three representative quantum gates on qudits: the SUM, double SUM, and SWAP gates.

We calculate the cross sections that characterize the frequency shift and broadening of the resonance magnetic hyperfine transition in atomic hydrogen due to collisions with hydrogen atoms in a masing cavity. We consider collision energies that correspond to the temperature range, $10\mathrm{mK}<T<\mathrm{}1000\mathrm{}\mathrm{K},$ and introduce and apply a fully quantal, multichannel collision theory for the entire temperature range under consideration. Our results for the spin-exchange induced cross sections at room temperature, are in harmony with previous calculated values and experimental measurements. For the hyperfine-induced cross sections, our predicted values are in fair agreement with measurements at cryogenic temperatures with the exception of the hyperfine frequency shift at 0.5 K and 323 K for which we obtain a different sign. A study of possible resonance structures shows that they cannot be invoked to resolve the discrepancy and nor can any plausible modification of the $X^1{\Sigma }_g^{+}$ and $b^3{\Sigma }_u^{+}$ interaction potentials.

We used the x-ray extended-range technique to measure the x-ray mass attenuation coefficients of silicon with an accuracy between 0.27% and 0.5% in the $5\mathrm{keV}-20\mathrm{keV}$ energy range. Subtraction of the x-ray scattering contribution enabled us to derive the corresponding x-ray photoelectric absorption coefficients and determine the absolute value of the imaginary part of the atomic form factor of silicon. Discrepancies between the experimental values of the mass attenuation coefficients and theoretically calculated values are discussed. New approaches to the theoretical calculation will be required to match the precision and accuracy of the experimental results.

Energy spectra and angular distributions have been measured of electrons that are emitted upon disassembly of ${\mathrm{Xe}}_n$ clusters $(n=20\mathrm{}000–150000)$ following irradiation by intense ${(10\mathrm{}}^{15}–{10}^{16}{\mathrm{W}\mathrm{}\mathrm{cm}}^{-2})$ laser pulses whose durations are varied over the 100–2200 fs range. The cluster explosion dynamics occur in the hydrodynamic regime. For the smaller clusters in the range that we have studied, a single-electron temperature adequately describes the measured electron energy distribution; in the case of larger clusters, a two-temperature fit becomes necessary. The total electron emission is found to be unexpectedly asymmetric and exhibits a resonance when the laser-pulse duration is $\sim 1\mathrm{ps}.$ These results are rationalized by extending the hydrodynamic model to also take into account the force that the light field exerts on the polarization charge that is induced on the surface of the cluster. We show that the magnitude of this electrostrictive force is comparable to those of the Coulombic and hydrodynamic forces, and it exhibits resonance behavior. Contrary to the findings of the only other earlier report, we find that the low-energy component in the electron energy distribution is connected to the resonance in energy absorption by the cluster. The high-energy component seems to be produced by a mechanism that is not so strongly influenced by the resonance.

The Hamiltonian encoding is a means for revealing the mechanism of controlled quantum dynamics. In this context, the mechanism is defined by the dominant quantum pathways starting from the initial state and proceeding through a set of intermediate states to end at the final state. The nature and interpretation of the mechanism depends on the choice of the states to represent the dynamics. Alternative representations may provide distinct insights into the system mechanism, and representations producing fewer pathways are especially interesting. In addition, a suitable choice of representation may highlight the role of certain couplings in a system that would normally be masked by other, higher magnitude couplings. A simple three-level system is chosen for illustration, where different values for the Rabi frequencies lead to mechanistic analyses that are best described in terms of particular representations. As an examlple, the role of the nonadiabatic terms in stimulated Raman adiabatic passage dynamics is analyzed through the Hamiltonian encoding.

A small atomic sample at a single site of an optical lattice potential is a realization of an anharmonic oscillator with atomic collisions giving rise to the nonlinearity. Due to the atomic collisions, an initial state with a well-defined phase evolves into a complex superposition of states of different phase before returning to the initial state. In this realization, three-body loss (the dominant loss process at high densities) acts as a damping mechanism and decoheres the intermediate superpositions thus reducing the ability of the phase to revive at later times. For high densities the decoherence process is rapid and the system enters the classical regime where revivals do not occur.

We present a scheme for the deterministic generation of N-photon Fock states from N three-level atoms in a high-finesse optical cavity. The method applies an external laser pulse that generates an N-photon output state while adiabatically keeping the atom-cavity system within a subspace of optically dark states. We present analytical estimates of the error due to amplitude leakage from these dark states for general N, and compare it with explicit results of numerical simulations for $N<~5.$ The method is shown to provide a robust source of N-photon states under a variety of experimental conditions and is suitable for experimental implementation using a cloud of cold atoms magnetically trapped in a cavity. The resulting N-photon states have potential applications in fundamental studies of nonclassical states and in quantum information processing.

We show theoretically that measuring an optimum observable of an electromagnetic wave propagating in an atomic medium moving freely in a gravitational field results in a sensitive detection of the acceleration of gravity and its gradient. The best achievable sensitivity of such a measurement is comparable with that of light-pulse atom interferometers based on measuring the atomic internal state. This optical technique is useful for nondestructive detection of ultracold atoms in atomic interferometers.

In this paper, we study the coherent dissociation of a molecular condensate into a multiple-mode atomic condensate during the chemical-potential curve crossing beyond the mean-field approximation. We show that the problem can be reduced to the dissociation of a molecular condensate into a two-mode atomic one. We employ the time-dependent Landau-Zener theory and derive analytical expression for the transition amplitudes. We calculate the number of produced atoms and show that they exist in squeezed state. We also study the formation of multiple-mode atomic condensate by inelastic scatterings of atoms in a single-mode atomic condensate. We show that the problem is also a Landau-Zener-like and exact solution can be found by imposing an additional symmetry.

The predissociation of a diatomic molecule after excitation by an ultrashort laser pulse leads to a succession of partial nuclear wave packets on the dissociative potential curve. Matter wave interferences result from their evolution. We show here theoretically how control pulses inducing Rabi oscillations on the electronic atomic part of the wave function can change totally the shape of the nuclear interference pattern. This is a consequence of the entanglement between the electronic and nuclear parts of the wave function. Application to information storage is finally discussed.

We study the collective Raman cooling of a trapped two-component Fermi gas using quantum master equation in the festina lente regime, where the heating due to photon reabsorption can be neglected. The Monte Carlo simulations show that three-dimensional temperatures of the order of ${0.008T}_F$ can be achieved. We analyze the heating related to background losses, and show that our laser-cooling scheme can maintain the temperature of the gas without significant additional losses.

We develop a numerical method to study the dynamics of a two-component atomic Fermi gas trapped inside a harmonic potential at temperature T well below the Fermi temperature $T_F.$ We examine the transition from the collisionless to the collisional regime down to $T=0.2\mathrm{}{T}_F$ and find a good qualitative agreement with the experiments of B. DeMarco and D.S. Jin [Phys. Rev. Lett. 88, 040405 (2002)]. We demonstrate a twofold role of temperature on the collision rate and on the efficiency of collisions. In particular, we observe a hitherto unreported effect, namely, the transition to hydrodynamic behavior is shifted towards lower collision rates as temperature decreases.

Based on the dynamical diffraction theory, we demonstrate the important role of the multiple scattering effects in an x-ray frequency doubling in crystals, generally leading to a generation of two coupled second-harmonic waves propagating in the forward and scattering direction. The possibility to phase match the process is shown for the Laue and Bragg geometry. Modeling of a frequency-doubling experiment suggests that the harmonic signal should be measurable for x-ray pumping with picosecond pulses of mJ energy.

We demonstrate one- and two-photon diffraction and interference experiments utilizing parametric down-converted photon pairs (biphotons) and a transmission grating. With two-photon detection, the biphoton exhibits a diffraction-interference pattern equivalent to that of an effective single particle that is associated with half the wavelength of the constituent photons. With one-photon detection, however, no diffraction-interference pattern is observed. We show that these phenomena originate from the spatial quantum correlation between the down-converted photons.

We analyze continuous-variable (CV) quantum duality induced by entanglement between the system and the meter in two feasible CV back-action evading measurements and suggest a CV analog of the quantum erasing, previously observed in the atomic interference experiments. The suggested experiments employ only the standard quantum optical devices, such as beam splitters, parametric oscillators, and homodyne detectors.

Echo dynamics and fidelity are often used to discuss stability in quantum-information processing and quantum chaos. Yet fidelity yields no information about entanglement, the characteristic property of quantum mechanics. We study the evolution of entanglement in echo dynamics. We find qualitatively different behavior between integrable and chaotic systems on one hand and between random and coherent initial states for integrable systems on the other. For the latter the evolution of entanglement is given by a classical time scale. Analytic results are illustrated numerically in a Jaynes-Cummings model.

An n-qubit quantum register can in principle be completely controlled by operating on a single qubit that interacts with the register via an appropriate fixed interaction. In this paper, we consider a hypothetical system consisting of n spin-1/2 nuclei that interact magnetically with an electron spin (control and probe particle). We describe control algorithms that implement premeasurements on nontrivial joint observables. We then devote a section to the implementation of unitary gates for the register by acting only on the control spin. Although this model of universal quantum computation is not efficient for large n, it can become efficient if we allow n possible positions for the control particle. This “toy” model of premeasurements shows how specific interactions between the register and the probe particle would support specific types of joint measurements. This means that some joint observables can be measured by performing a simple sequence of operations on the probe particle. In our model, some measurements of joint observables are simpler to perform than for one-qubit observables.

Quantum dense coding is considered to be one of the most remarkable demonstrations of the existence of quantum entanglement. In this paper, we analyze the role of correlations (both classical and quantum) in quantum communication protocols. We study how the capacity of a single-qubit quantum channel depends on correlations between Alice and Bob. In particular, we consider a scenario when Alice and Bob share a pair of qubits in a state such that Alice’s qubit is in the state described by the density operator $\varrho =\frac{1}{2}1+\overrightarrow{n}\cdot \overrightarrow{\sigma }.$ We evaluate a capacity of the noiseless channel for two cases: (1) when Bob performs measurement just on the particle received from Alice, and (2) in the case when he utilizes correlations that have been established prior to the communication. We present a simple classical scenario which might serve as a prototype of quantum dense coding. We generalize our results also for qudits. In addition, we address some aspects of the issue of security in correlation-assisted communication protocols.

The mesic atom deexcitation via an external Auger process on hydrogen molecules (atoms) is considered in a semiclassical approach: relative motion of nuclei is described as a classical process in the potential field, while the motion of the muon and electron is described quantum mechanically. The rates of the hydrogen deexcitation are calculated, beginning from the principal quantum number $n=6.\mathrm{}$ It is shown that as a result of the Auger transition, mesic atoms can either accelerate up to energies $\sim 1\mathrm{eV}$ or form a bound state (molecule) with a target atom. The decay of such a state via predissociation is characterized by a considerable rate $\left(\sim {10}^{12}{\mathrm{s}}^{-1}\right)$ and high-energy release $\left(\sim 100\mathrm{eV}\right).\mathrm{}$