Locally Optimal Control of Quantum Systems with Strong Feedback

For quantum systems with high purity, we find all observables that, when continuously monitored, maximize the instantaneous reduction in the von Neumann entropy. This allows us to obtain all locally optimal feedback protocols with strong feedback, and explicit expressions for the best such protocols for systems of size N <= 4. We also show that for a qutrit the locally optimal protocol is the optimal protocol for a given range of control times, and derive an upper bound on all optimal protocols with strong feedback.Comment: 4 pages, Revtex4. v2: published version (some errors corrected

A numerical study of two-photon ionization of helium using the Pyprop framework

Few-photon induced breakup of helium is studied using a newly developed ab initio numerical framework for solving the six-dimensional time-dependent Schroedinger equation. We present details of the method and calculate (generalized) cross sections for the process of two-photon nonsequential (direct) double ionization at photon energies ranging from 39.4 to 54.4 eV, a process that has been very much debated in recent years and is not yet fully understood. In particular, we have studied the convergence property of the total cross section in the vicinity of the upper threshold (54.4 eV), versus the pulse duration of the applied laser field. We find that the cross section exhibits an increasing trend near the threshold, as has also been observed by others, and show that this rise cannot solely be attributed to an unintended inclusion of the sequential two-photon double ionization process, caused by the bandwidth of the applied field.Comment: 7 pages, 3 figure

Physical model for the generation of ideal resources in multipartite quantum networking

We propose a physical model for generating multipartite entangled states of spin-$s$ particles that have important applications in distributed quantum information processing. Our protocol is based on a process where mobile spins induce the interaction among remote scattering centers. As such, a major advantage lies on the management of stationary and well separated spins. Among the generable states, there is a class of $N$-qubit singlets allowing for optimal quantum telecloning in a scalable and controllable way. We also show how to prepare Aharonov, W and Greenberger-Horne-Zeilinger states.Comment: 5 pages, 2 figures. Format revise

Dilute Bose gases interacting via power-law potentials

Neutral atoms interact through a van der Waals potential which asymptotically falls off as r^{-6}. In ultracold gases, this interaction can be described to a good approximation by the atom-atom scattering length. However, corrections arise that depend on the characteristic length of the van der Waals potential. We parameterize these corrections by analyzing the energies of two- and few-atom systems under external harmonic confinement, obtained by numerically and analytically solving the Schrodinger equation. We generalize our results to particles interacting through a longer-ranged potential which asymptotically falls off as r^{-4}.Comment: 7 pages, 4 figure

Clusters under strong VUV pulses: A quantum-classical hybrid-description incorporating plasma effects

The quantum-classical hybrid-description of rare-gas clusters interacting with intense light pulses which we have developed is described in detail. Much emphasis is put on the treatment of screening electrons in the cluster which set the time scale for the evolution of the system and form the link between electrons strongly bound to ions and quasi-free plasma electrons in the cluster. As an example we discuss the dynamics of an Ar147 cluster exposed to a short VUV laser pulse of 20eV photon energy.Comment: 8 pages, 9 figure

Attosecond probing of instantaneous AC Stark shifts in helium atoms

Based on numerical solutions of the time-dependent Schr\"odinger equation for either one or two active electrons, we propose a method for observing instantaneous level shifts in an oscillating strong infrared (IR) field in time, using a single tunable attosecond pulse to probe excited states of the perturbed atom. The ionization probability in the combined fields depends on both, the frequency of the attosecond pulse and the time delay between both pulses, since the IR field shifts excited energy levels into and out of resonance with the attosecond probe pulse. We show that this method (i) allows the detection of instantaneous atomic energy gaps with sub-laser-cycle time resolution and (ii) can be applied as an ultrafast gate for more complex processes such as non-sequential double-ionization

Fermion Dynamics by Internal and Space-Time Symmetries

This manuscript is devoted to introduce a gauge theory of the Lorentz Group based on the ambiguity emerging in dealing with isometric diffeo-morphism-induced Lorentz transformations. The behaviors under local transformations of fermion fields and spin connections (assumed to be ordinary world vectors) are analyzed in flat space-time and the role of the torsion field, within the generalization to curved space-time, is briefly discussed. The fermion dynamics is then analyzed including the new gauge fields and assuming time-gauge. Stationary solutions of the problem are also analyzed in the non-relativistic limit, to study the spinor structure of an hydrogen-like atom.Comment: 10 pages, no figur

Perfect State Transfer, Effective Gates and Entanglement Generation in Engineered Bosonic and Fermionic Networks

We show how to achieve perfect quantum state transfer and construct effective two-qubit gates between distant sites in engineered bosonic and fermionic networks. The Hamiltonian for the system can be determined by choosing an eigenvalue spectrum satisfying a certain condition, which is shown to be both sufficient and necessary in mirror-symmetrical networks. The natures of the effective two-qubit gates depend on the exchange symmetry for fermions and bosons. For fermionic networks, the gates are entangling (and thus universal for quantum computation). For bosonic networks, though the gates are not entangling, they allow two-way simultaneous communications. Protocols of entanglement generation in both bosonic and fermionic engineered networks are discussed.Comment: RevTeX4, 6 pages, 1 figure; replaced with a more general example and clarified the sufficient and necessary condition for perfect state transfe

The Accelerated Kepler Problem

The accelerated Kepler problem is obtained by adding a constant acceleration to the classical two-body Kepler problem. This setting models the dynamics of a jet-sustaining accretion disk and its content of forming planets as the disk loses linear momentum through the asymmetric jet-counterjet system it powers. The dynamics of the accelerated Kepler problem is analyzed using physical as well as parabolic coordinates. The latter naturally separate the problem's Hamiltonian into two unidimensional Hamiltonians. In particular, we identify the origin of the secular resonance in the accelerated Kepler problem and determine analytically the radius of stability boundary of initially circular orbits that are of particular interest to the problem of radial migration in binary systems as well as to the truncation of accretion disks through stellar jet acceleration.Comment: 16 pages, 9 figures, in press at Celestial Mechanics and Dynamical Astronom

Heterointerface effects on the charging energy of shallow D- ground state in silicon: the role of dielectric mismatch

Donor states in Si nanodevices can be strongly modified by nearby insulating barriers and metallic gates. We report here experimental results indicating a strong reduction in the charging energy of isolated As dopants in Si FinFETs relative to the bulk value. By studying the problem of two electrons bound to a shallow donor within the effective mass approach, we find that the measured small charging energy may be due to a combined effect of the insulator screening and the proximity of metallic gates.Comment: 7 pages, 6 figure