We demonstrate high-rate randomized data-encryption through optical fibers using the inherent quantum-measurement noise of coherent states of light. Specifically, we demonstrate $650\mathrm{Mbit}∕\mathrm{s}$ data encryption through a $10\mathrm{Gbit}∕\mathrm{s}$ data-bearing, in-line amplified 200-km-long line. In our protocol, legitimate users (who share a short secret key) communicate using an $M$-ry signal set while an attacker (who does not share the secret key) is forced to contend with the fundamental and irreducible quantum-measurement noise of coherent states. Implementations of our protocol using both polarization-encoded signal sets as well as polarization-insensitive phase-keyed signal sets are experimentally and theoretically evaluated. Different from the performance criteria for the cryptographic objective of key generation (quantum key-generation), one possible set of performance criteria for the cryptographic objective of data encryption is established and carefully considered.

We study a possible physical realization of a quantum controlled-NOT gate with the use of two weakly coupled asymmetric quantum dots. Solving the time-dependent Schrödinger equation for the model two-electron system, we simulate the infrared-radiation-induced quantum transitions that correspond to basic gate operations. We require the transition probabilities to be close to 1 and optimize the parameters of the nanostructure in order to make the gate operation time as short as possible. In the simulations, we have taken into account the entire energy spectrum, which can be populated by the absorption or emission of the infrared radiation. We discuss the consequences of the existence of many bound two-electron states on the probability of radiative transitions.

Variationally calculating the ground state of a many-electron quantum system using only the two-electron reduced-density-matrix (2-RDM) requires $N$-representability conditions that constrain the 2-RDM to correspond to an $N$-electron wave function. A systematic hierarchy of $N$-representability conditions, known as $p$-positivity conditions, has been developed [D. A. Mazziotti and R. M. Erdahl, Phys. Rev. A 63, 042113 (2001)], and many-electron atoms and molecules in nonminimal basis sets have been solved with useful accuracy by a variational 2-RDM method with 2-positivity conditions [D. A. Mazziotti, Phys. Rev. Lett. 93, 213001 (2004)]. This paper considers two forms of partial 3-positivity conditions, the lifting conditions and the $T_1∕T_2$ conditions, to further enhance the accuracy of the 2-RDM methods without the computational cost of full 3-positivity conditions. Variational 2-RDM methods with different $N$-representability constraints including 2-positivity conditions, the two types of partial 3-positivity conditions, as well as the complete 3-positivity conditions are applied to compute the ground state of the Lipkin spin model. The energies and 2-RDMs are compared to the results from full and truncated configuration interaction, many-body perturbation theory, and couple cluster theory with single and double excitations. Implications of using partial 3-positivity for variational 2-RDM calculations of many-electron atoms and molecules will be discussed.

Charge-transfer processes due to collisions of ${\mathrm{N}}^{+}$ with atomic hydrogen and ${\mathrm{H}}^{+}$ with atomic nitrogen are investigated using the quantum-mechanical molecular-orbital close-coupling (MOCC) method. The MOCC calculations utilize ab initio adiabatic potentials and nonadiabatic radial and rotational couplings obtained with the multireference single- and double-excitation configuration interaction approach. Total and state-selective cross sections for the energy range $0.1\mathrm{meV}∕\mathrm{u}\text{–}1\mathrm{keV}∕\mathrm{u}$ are presented and compared with existing experimental and theoretical data. A large number of low-energy resonances are obtained for exoergic channels and near thresholds of endoergic channels. Rate coefficients are also obtained and comparison to previous calculations suggests nonadiabatic effects dominate for temperatures greater than 20 000 K, but that the spin-orbit interaction plays a major role for lower temperatures.

Standard classical and quantum-mechanical methods are used to characterize the momentum-transfer cross section needed in energy-loss calculations and simulations for heavy, swift charges moving in an electron gas. By applying a well-known, finite-range screened Coulombic potential energy to model the two-body collision, the quantitative applicability range of the classical cross section is investigated as a function of charge $\left(Z\right)$, screening length $\left(R\right)$, and scattering relative velocity $\left(v\right)$. The a posteriori condition $(Z∕R)∕v^2<1$, as an upper bound for heavy charges, is deduced for this applicability range from the comparative study performed.

An analytic description of adiabatic evolution of a two-state system in the presence of dephasing processes is presented. The adiabatic solution of the Bloch equation is derived in the most general case of a mixed initial state. Examples of the general analytic solution are given in the particular cases of energy-level crossing, no crossing, and half crossing.

We use quantum Monte Carlo simulations to show the presence and study the properties of solitons in the one-dimensional soft-core bosonic Hubbard model with near-neighbor interaction in traps. We show that when the half-filled charge density wave (CDW) phase is doped, solitons are produced and quasi-long-range order established. We discuss the implications of these results for the presence and robustness of this solitonic phase in Bose-Einstein condensates on one-dimensional optical lattices in traps and study the associated excitation spectrum. The density profile exhibits the coexistence of Mott insulator, CDW, and superfluid regions.

We present an ab initio nonpertubative investigation of the mechanisms responsible for the production of very-high-order harmonic generation (HHG) from Ar atoms and ${\mathrm{Ar}}^{+}$ ions by means of the self-interaction-free time-dependent density-functional theory recently developed. Further, by introducing an effective charge concept, we can study at which laser intensity the contribution to the high-energy HHG from ${\mathrm{Ar}}^{+}$ ions precede over the Ar atoms. Comparing the HHG behavior from Ar atoms and ${\mathrm{Ar}}^{+}$ ions in the superintense laser field, we conclude that the high-energy HHG observed in the recent experiment originated from the ionized Ar atoms.

We exploit the analogy between the transfer of a pulse across a scattering medium and Aharonov’s weak measurements to resolve the long standing paradox between the impossibility to exceed the speed of light and the seemingly “superluminal” behavior of a tunneling particle in the barrier or a photon in a “fast-light” medium. We demonstrate that superluminality occurs when the value of the duration $\tau$ spent in the barrier is uncertain, whereas when $\tau$ is known accurately, no superluminal behavior is observed. In all cases only subluminal durations contribute to the transmission which precludes faster-than-light information transfer, as observed in a recent experiment.

We demonstrate that a very recently obtained formula for the force on a slab in a material planar cavity based on the calculation of the vacuum Lorentz force [C. Raabe and D.-G. Welsch, Phys. Rev. A 71, 013814 (2005)] describes a (medium) modified Casimir force and, in addition to it, a medium-assisted force. The latter force also describes the force on the cavity medium. For dilute media, it implies the atom-mirror interaction of the Casimir-Polder type at large and of the Coulomb type at small atom-mirror distances of which the sign is insensitive to the polarizability type (electric or magnetic) of the atom.

An optical-field-ionization soft x-ray laser using a clustered gas jet was demonstrated. Pd-like xenon lasing at 41.8 nm with 95-nJ pulse energy and 5.2-mrad divergence was achieved, indicating near-saturation amplification. By using a prepulse to control the expansion of ionized clusters, it was found that the microscopic uniformity of the plasma is essential for efficient lasing. In addition, the optimal atom density for maximum lasing reported previously is verified to result from the tradeoff between large gain coefficient and short gain length due to ionization-induced refraction.

We study the relation between local unitary (LU) equivalence and local Clifford (LC) equivalence of stabilizer states. We introduce a large subclass of stabilizer states, such that every two LU equivalent states in this class are necessarily LC equivalent. Together with earlier results, this shows that LC, LU, and stochastic local operation with classical communication equivalence are the same notions for this class of stabilizer states. Moreover, recognizing whether two given stabilizer states in the present subclass are locally equivalent only requires a polynomial number of operations in the number of qubits.

We study the relation between entanglement and quantum chaos in one- and two-dimensional spin-$1∕2$ lattice models, which exhibit mixing of the noninteracting eigenfunctions and transition from integrability to quantum chaos. Contrary to what occurs in a quantum phase transition, the onset of quantum chaos is not a property of the ground state but takes place for any typical many-spin quantum state. We study bipartite and pairwise entanglement measures—namely, the reduced von Neumann entropy and the concurrence—and discuss quantum entanglement sharing. Our results suggest that the behavior of the entanglement is related to the mixing of the eigenfunctions rather than to the transition to chaos.

We construct interesting entanglement distillation protocols by interpolating between the recurrence and hashing protocols. This leads to asymptotic two-way distillation protocols, resulting in an improvement of the distillation rate for all mixed Bell diagonal entangled states, even for the ones with very high fidelity. We also present a method for how entanglement-assisted distillation protocol can be converted into nonentanglement-assisted protocols with the same yield.

We report analyses and comparison of three high quality ab initio KRb $a{}^3\Sigma ^{+}$ state potentials [Rousseau et al., J. Mol. Spectrosc. 203, 235 (2000); Park et al., Chem. Phys. 257, 135 (2000); Kotochigova et al., Phys. Rev. A 68, 011501 (2003)]. Eigenvalues for all vibrational levels and precision binding energies for high-lying levels (and corresponding triplet scattering lengths) are presented. By adjusting the inner wall of a potential, we show that we can reproduce measured scattering lengths of two isotopomers and predict scattering lengths of all isotopomers.

Using the time-dependent close-coupling method, we have investigated the decay dynamics of autoionization states of He, induced by resonant photon excitations. The resulting autoionization decay rates agree very well with those obtained both from experiments and other theoretical methods. Utilizing the advantage of the time-dependent method, we have also studied the dynamical behaviors of the two electrons “trapped” in autoionization states of He. The calculated interelectron distances in triplet and singlet $P$ states explain the large difference of their decay rates. Moreover, we also explored the possibility of modifying autoionization decay by additional few-cycle pulses.

A method is described that yields a series of $(D+1)$-element wave-vector sets giving rise to ($D=2$ or 3)-dimensional coherent sparse lattices of any desired Bravais symmetry and primitive cell shape, but of increasing period relative to the excitation wavelength. By applying lattice symmetry operations to any of these sets, composite lattices of $N>D+1$ waves are constructed, having increased spatial frequency content but unchanged crystal group symmetry and periodicity. Optical lattices of widely spaced excitation maxima of diffraction-limited confinement and controllable polarization can thereby be created, possibly useful for quantum optics, lithography, or multifocal microscopy.

Fermionic atoms in an optical superlattice can realize a very peculiar Anderson lattice model in which impurities interact with each other through a discretized set of delocalized levels. We show that under these finite-size features strongly correlated phases appear. By tuning the parameters of the superlattice two different phases could be observed: a Kondo-singlet phase in which each impurity forms a singlet with the Fermi level of its neighboring conducting islands, and a magnetic phase in which long range magnetic order is established between the impurities mediated by the intermediate islands. The interplay between these two phenomena depends on the parity of the number of particles per discretized set. We show how Kondo-induced resonances of measurable size can be observed through the atomic interference pattern.

The entanglement of quantum $XY$ spin chains of arbitrary length is investigated via a recently developed global measure suitable for generic quantum many-body systems. This entanglement is determined over the phase diagram and found to exhibit rich structure. In particular, the field derivative of the entanglement density becomes singular along the critical line. The form of this singularity is dictated by the universality class controlling the quantum phase transition.

High-efficiency optical detectors that can determine the number of photons in a pulse of monochromatic light have applications in a variety of physics studies, including post-selection-based entanglement protocols for linear optics quantum computing and experiments that simultaneously close the detection and communication loopholes of Bell’s inequalities. Here we report on our demonstration of fiber-coupled, noise-free, photon-number-resolving transition-edge sensors with 88% efficiency at 1550 nm. The efficiency of these sensors could be made even higher at any wavelength in the visible and near-infrared spectrum without resulting in a higher dark-count rate or degraded photon-number resolution.