The interaction between charged particles and the vacuum fluctuations of the electromagnetic field induces decoherence, and therefore affects the contrast of fringes in an interference experiment. In this article we show that if a double slit experiment is performed near a conducting grating, the fringe visibility is reduced. We find that the reduction of contrast is proportional to the number of grooves in the conducting surface, and that for realistic values of the parameters it could be large enough to be observed. The effect can be computed and understood in terms of the Smith-Purcell radiation produced by the surface currents induced in the conductor.

Superselection rules (SSRs) limit the mechanical and quantum processing resources represented by quantum states. However, SSRs can be violated using reference systems to break the underlying symmetry. We show that there is a duality between the ability of a system to do mechanical work and to act as a reference system. Further, for a bipartite system in a globally symmetric pure state, we find a triality between the system’s ability to do local mechanical work, its ability to do “logical work” due to its accessible entanglement, and its ability to act as a shared reference system.

We analyze shape phase transitions in two-dimensional algebraic models. We apply our analysis to linear-to-bent transitions in molecules and point out what observables are particularly sensitive to the transition (order parameters). We study numerically the scaling behavior of observables and confirm the dependence of the energy gap for phase transitions of $\text{U}\left(n\right)\text{-SO}\left(n+1\right)$ type. We calculate energies of excited states and show their unusual behavior for some values of the Hamiltonian control parameter. This behavior is due to the double-humped nature of the potential and can be associated with the concept of monodromy. Finally, we compute numerically thermodynamic quantities, in particular heat capacities, and show their large variation at and around the critical value of the control parameter.

We describe an experimental test of local realistic and nonlocal realistic theories using polarization-entangled two-photon singlet states created using a fiber-based polarization Sagnac interferometer. We show a violation of Bell’s inequality in the Clauser-Horne-Shimony-Holt form by 15 standard deviations, thus excluding local hidden-variable theories, and a violation of a Leggett-type nonlocal hidden-variable inequality by more than three standard deviations, thus excluding a class of nonlocal hidden-variable theories.

In this article, we study optimal control of dynamics of a linear chain of three spin $1∕2$, weakly coupled with unequal Ising couplings. We address the problem of time-optimal synthesis of multiple spin quantum coherences. We derive time-optimal pulse sequences for creating desired spin order by computing geodesics on a sphere under a special metric. The solution to the geodesic equation is related to the nonlinear oscillator equation and the minimum time to create multiple spin order can be expressed in terms of an elliptic integral. These techniques are used for efficient creation of multiple spin coherences in Ising spin chains with unequal couplings.

Alignment of the $M_3(J=\frac{3}{2})$, $M_4(J=\frac{3}{2})$, and $M_5(J=\frac{5}{2})$ subshell vacancy states produced following photoionization in the $M_i$ $(i=1–5)$ subshells of ${}_{79}\mathrm{Au}$, ${}_{83}\mathrm{Bi}$, ${}_{90}\mathrm{Th}$, and ${}_{92}\mathrm{U}$ have been investigated through angular distribution of the subsequently emitted $M$ x rays. The unpolarized Mn $K$ x rays $(E_{KX}=5.97\mathrm{keV})$ from the ${}^{55}\mathrm{Fe}$ radioisotope were used to ionize the $M_i$ subshells and the emitted $M$ x rays were measured under vacuum using a low energy Ge detector. The $M$ x-ray spectra taken at different emission angles were normalized using the isotropically emitted $K$ shell $(J=\frac{1}{2})$ x rays measured simultaneously from a ${}_{23}\mathrm{V}$ thin target placed adjoining the $M$ x-ray target. The present precision measurements infer that anisotropy in the $M\alpha \beta \gamma$ x-ray emission shows trends and order of magnitude predicted by theoretical calculations, i.e., anisotropy parameter $\left({\beta }_2\right)\sim 0.01$. It clearly rules out strong angular dependence of the $M$ x-ray emission reported earlier. The $M$ x-ray fluorescence integral cross sections have been deduced from the measurements at an emission angle $\left(\psi \right)=126°$, where the second-order Legendre polynomial term, ${\mathrm{P}}_2\left(\cos \psi \right)$, associated with the angular distribution polynomial is zero. The measured fluorescence cross sections are compared with those deduced using theoretical $M_i$ subshell photoionization cross sections, fluorescence yields, and Coster-Kronig transition probabilities.

In a recent publication we presented measurements and distorted-wave Born approximation (DWBA) calculations for spin asymmetries resulting from the scattering of 148 eV spin-polarized electrons from ground-state xenon atoms. While achieving reasonably good agreement between theory and experiment by accounting for many-body exchange effects through a local exchange potential, sizable discrepancies remained in some cases. It was proposed that a more sophisticated treatment of exchange might be the key to obtaining better agreement. Here we present high-precision experimental results at a lower projectile-electron energy of 112 eV. Due to improvements made to our spectrometer, these data are of significantly-improved statistical accuracy over those obtained previously. They are compared to DWBA calculations, within whose framework electron exchange is treated in three different ways. In the first, only exchange between the projectile electron and ejected target electron is considered and that between the continuum- and bound-electrons is ignored. In the second, exchange between the continuum- and bound-electrons is treated through a local approximation. Finally, exchange is included through a Hartree-Fock (HF) calculation of wave functions for both bound- and continuum-electrons. While this more sophisticated treatment of exchange narrows the gap between experiment and theory, the results suggest improvements in other aspects of the problem may be warranted.

We study the response of small- and medium-sized sodium clusters $\left({\text{Na}}_{13–147}\right)$ to dual-pulse laser fields of moderate intensity $I=8\times {10}^{12}\text{W}/{\text{cm}}^2$. The contribution of electron-electron collisions on the cluster dynamics and plasmon-enhanced coupling is analyzed by a systematic comparison of semiclassical Vlasov and Vlasov-Uehling-Uhlenbeck simulations. We find that electron-electron collisions significantly enhance the non-resonant absorption of laser energy and have substantial impact on the time scale on which frequency matching between the laser and the cluster surface-plasmon occurs. The total ionization and energy capture at optimal pulse delays, however, are practically unaffected. This underlines that enhanced absorption through resonant plasmon excitations remains very effective and is only weakly influenced by electron-electron collisions.

We investigate the quantum behavior of a mesoscopic two-boson system produced by number-squeezing ultracold gases of alkali-metal atoms. The quantum Poincaré maps of the wave functions are affected by chaos in those regions of the phase space where the classical dynamics produces features that are comparable to $\hslash$. We also investigate the possibility for quantum control in the dynamics of excitations in these systems. Controlled excitations are mediated by pulsed signals that cause stimulated Raman adiabatic passage (STIRAP) from the ground state to a state of higher energy. The dynamics of this transition is affected by chaos caused by the pulses in certain regions of the phase space. A transition to chaos can thus provide a method of controlling STIRAP.

In this paper, I discuss the selective and efficient vibrational population transfer between electronic states of diatomic molecules by a weak, ultrashort pulse train. The spectrum of each individual pulse in the train is wide enough to simultaneously excite several vibrational levels of the molecule. However, selectivity is achieved by mismatching the frequency comb, associated with the pulse train, to the excited vibrational levels. Efficiency results from the coherent accumulation of population from one pulse in the train to the next.

We derive an effective cross-Kerr Hamiltonian for the four-level atom interacting with three electromagnetic fields in the $N$ configuration. When the atom has relaxed into the ground state, a cross-Kerr nonlinearity arises between two weak probe fields. As a development on earlier work, we derive the form of the atom-field interaction for all detunings and include the spontaneous decay of the upper atomic levels. In general, the atom will also display a linear and self-Kerr response, but if certain resonance conditions are satisfied then only the cross-Kerr interaction will remain. We consider the application of our theory to cold four-level rubidium atoms. The electrical susceptibilities of the probe transitions are explored, and it is shown that a large, pure cross-Kerr nonlinearity can be generated with vanishing absorption of both probe fields.

We analyze the stability of super- and subradiant states in a system of identical two-level atoms in the near-Dicke limit, i.e., when the atoms are very close to each other compared to the wavelength of resonant light. The dynamics of the system are studied using a renormalized master equation, both with multipolar and minimal-coupling interaction schemes. We show that both models lead to the same result and, in contrast to nonrenormalized models, predict that the relative orientation of the (coaligned) dipoles is unimportant in the Dicke limit. Our master equation is of relevance to any system of dipole-coupled two-level atoms, and gives bounds on the strength of the dipole-dipole interaction for closely spaced atoms. Exact calculations for small atom systems in the near-Dicke limit show the increased emission times resulting from the evolution generated by the strong dipole-dipole interaction. However, for large numbers of atoms in the near-Dicke limit, it is shown that as the number of atoms increases, the effect of the dipole-dipole interaction on collective emission is reduced.

The dynamics of an open quantum system can be described by a quantum operation: A linear, complete positive map of operators. Here, I exhibit a compact expression for the time reversal of a quantum operation, which is closely analogous to the time reversal of a classical Markov transition matrix. Since open quantum dynamics are stochastic, and not, in general, deterministic, the time reversal is not, in general, an inversion of the dynamics. Rather, the system relaxes toward equilibrium in both the forward and reverse time directions. The probability of a quantum trajectory and the conjugate, time reversed trajectory are related by the heat exchanged with the environment.

By using the method of density-matrix renormalization-group theory, the ground-state entanglement in the spin-1/2 isotropic antiferromagnetic Heisenberg chain is studied when there are domain walls generated by a boundary magnetic field. It is found that the pairwise entanglement of odd-bond two qubits decreases and even-bond two qubits increases to stable values alternately when the magnetic field increases. The pairwise entanglement of odd bond can be equal to that of even bond for a suitable value of the magnetic field. When the magnetic field increases further, the entanglement of even bond can be larger than that of odd bond. The entanglement entropy increases and then almost saturates as the number of spin sites increases. Due to the domain walls, the entanglement entropy of even bond decreases to a minimum value and then increases as the magnetic field increases.

First, we present a scheme for implementation of$N\to M$ local cloning of coherent states with known phases, based on linear optics, single-mode squeezed sources, and homodyne detection. The amplification of such cloning is accomplished with the help of a strongly single-mode squeezed source. Then we propose a scheme for $N\to M$ nonlocal cloning of such input states by employing an additional two-mode squeezed source. In the case of ideal squeezed sources, the fidelities of the two cloning machines can reach a high level.

Using the positively charged aniline ion $\left({\mathrm{C}}_6{\mathrm{H}}_5{{\mathrm{NH}}_2}^{+}\right)$ as a test molecule, we demonstrate that it is possible to study consecutive photodissociation of complex molecular ions at the single molecule level in an ion trap. When a single ${\mathrm{C}}_6{\mathrm{H}}_5{{\mathrm{NH}}_2}^{+}$ ion is exposed to laser light at $397\mathrm{nm}$ and $294\mathrm{nm}$, direct or consecutive photodissociation leads to the production of a range of smaller polyatomic molecular ions such as ${\mathrm{C}}_5{{\mathrm{H}}_6}^{+}$ and ${\mathrm{C}}_3{{\mathrm{H}}_3}^{+}$. The applied method is very versatile and can, e.g., be used in combination with free electron lasers or synchrotron radiation sources.

We have focused and decelerated benzonitrile $\left({\mathrm{C}}_7{\mathrm{H}}_5\mathrm{N}\right)$ molecules from a molecular beam, using an array of time-varying inhomogeneous electric fields in alternating-gradient configuration. Benzonitrile is prototypical for large asymmetric top molecules that exhibit rich rotational structure and a high density of states. At the rotational temperature of $3.5\mathrm{K}$ in the pulsed molecular beam, many rotational states are populated. Benzonitrile molecules in their absolute ground state are decelerated from $320\text{to}289\mathrm{m}∕\mathrm{s}$, and similar changes in velocity are obtained for excited rotational states. All measurements agree well with the outcome of trajectory calculations. These experiments demonstrate that such large polyatomic molecules are amenable to the powerful method of Stark deceleration.

We rederive uncertainty relations for the angular position and momentum of a particle on a circle by employing the exponential of the angle instead of the angle itself, which leads to circular variance as a natural measure of resolution. Intelligent states minimizing the uncertainty product under the constraint of a given uncertainty in angle or in angular momentum turn out to be given by Mathieu wave functions. We also discuss a number of physically feasible approximations to these optimal states. The theory is applied to the orbital angular momentum of a beam of photons and verified in an experiment that employs computer-controlled spatial light modulators at both the state preparation and analyzing stages.

The notion of fidelity in quantum information science has been recently applied to analyze quantum phase transitions from the viewpoint of the ground-state (GS) overlap for various many-body systems. In this work, we unveil the intrinsic relation between the GS fidelity and the derivatives of GS energy and find that they play an equivalent role in identifying the quantum phase transition. The general connection between the two approaches enables us to understand the different singularity and scaling behaviors of fidelity exhibited in various systems on general grounds. Our general conclusions are illustrated via several quantum spin models which exhibit different kinds of quantum phase transitions.

We show that the wave packet of a biphoton generated via spontaneous parametric down-conversion is strongly anisotropic. Its anisotropic features manifest themselves very clearly in comparison with measurements performed in two different schemes: When the detector scanning plane is perpendicular or parallel to the plane containing the crystal optical axis and the laser axis. The first of these two schemes is traditional whereas the second one gives rise to such unexpected results, such as anomalously strong narrowing of the biphoton wave packet measured in the coincidence scheme and very high degree of entanglement. The results are predicted theoretically and confirmed experimentally.