No abstract

We calculate the first-order energy shifts for the N-dimensional hydrogen atom exposed to a static electric field. The results are compared with numerical diagonalization of the Hamiltonian in a finite basis. Using simple scaling relations, we show how corrections to arbitrarily high order may be obtained from known results for the three-dimensional Coulomb problem.

The dynamics of two electrons in a 2-dimensional quantum dot molecule in the presence of a time-dependent electromagnetic field is calculated from first principles. We show that carefully selected microwave pulses can exclusively populate a single state of the first excitation band and that the transition time can be further decreased by optimal pulse control. Finally we demonstrate that an oscillating charge localized state may be created by multiple transitions using a sequence of pulses.

We investigate optimal control strategies for state to state transitions in a model of a quantum dot molecule containing two active strongly interacting electrons. The Schrödinger equation is solved nonperturbatively in conjunction with several quantum control strategies. This results in optimized electric pulses in the terahertz regime which can populate combinations of states with very short transition times. The speed-up compared to intuitively constructed pulses is an order of magnitude. We furthermore make use of optimized pulse control in the simulation of an experimental preparation of the molecular quantum dot system. It is shown that exclusive population of certain excited states leads to a complete suppression of spin dephasing, as was indicated in Nepstad et al (2008 Phys. Rev. B 77 125315).

We derive an accurate molecular orbital based expression for the coherent time evolution of a two-electron wave function in a quantum dot molecule where the electrons interact with each other, with external timedependent electromagnetic fields and with a surrounding nuclear spin reservoir. The theory allows for direct numerical modeling of the decoherence in quantum dots due to hyperfine interactions. Calculations result in good agreement with recent singlet-triplet dephasing experiments by Laird et al. ͓Phys. Rev. Lett. 97, 056801 ͑2006͔͒, as well as analytical model calculations. Furthermore, it is shown that using a much faster electric switch than applied in these experiments will transfer the initial state to excited states where the hyperfine singlet-triplet mixing is negligible.

We demonstrate that conditional as well as unconditional basic operations which are prerequisite for universal quantum gates can be performed with almost 100% fidelity within a strongly interacting two-electron quantum ring. Both sets of operations are based on a quantum control algorithm that optimizes a driving electromagnetic pulse for a given quantum gate. The demonstrated transitions occur on a time scale much shorter than typical decoherence times of the system. PACS numbers: 85.35. Be, Quantum computing requires a set of fundamental single-qubit operations which can address and manipulate each qubit regardless of the state of the others. In addition at least one conditional operation must be defined which can address any chosen qubit based on the status of another [1]. This poses a major challenge in all logical devices composed of strongly interacting single particle qubits: The interaction then creates entangled multi-particle states which hide the single particle character completely, e.g. regarding the energy spectrum or the spatial particle distribution. Nevertheless, several ba- [6] demonstrated single qubit control using the total spin state of a two-electron quantum dot molecule. Conditional operations in coupled quantum dots have also been experimentally demonstrated [7], where excited states are a part of the information carrier. Another suggestion has been to include two qubits in a single quantum dot molecule with the total spin as one qubit and charge localization as the other [8].Relative to coupled quantum dot-molecules and quantum dot-arrays, the quantum-ring structure possesses a high-degree of symmetry, implying the existence of conserved quantities, e.g. persistent currents [9], related to the conservation of total electron angular momentum. The use of conserved quantities for the buildup of a quantum processor may be advantageous, compared to e.g. charge localized states, since the former are timeindependent as long as weak decoherence mechanisms, such as spin-orbit or hyperfine interactions, can be neglected. In compliance with this we recently proposed the two-electron quantum ring total angular momentum and total electron spin as a pair of independent qubits [10]. Since the total angular momentum is truly multivalued, M L = 0, ±1, ±2, ... we coined this system a "quMbit".In this Letter we show that the total orbital angular momentum and the total electron spin in the two-electron quantum ring, in spite of the strong electron-electron interaction, can be coherently and independently manipulated and that the intended quantum state is obtained with almost 100% probability. Hereby successful gate operations are achieved, for both the unconditional (NOT) and the conditional (CNOT) inversion operation. An alternative route to scalability can then be foreseen since the information content of each quantum ring increases with the number of controllable states. After a short introduction we demonstrate conditional and unconditional manipulations of the angular momenta and finally the uncondit...

We demonstrate a feature of the Rydberg blockade mechanism which occurs between two initially excited circular Rydberg atoms. When both atoms are exposed to weak time-dependent electric fields, it is shown that the intrashell dynamics of each atom is strongly modified by the presence of the other. Three characteristic dynamical regimes are identified with separating radii which both scale linearly with principal quantum number n for otherwise constant field parameters. A region of conditional entangled electron dynamics is separated from the outer asymptotic region of independent atom dynamics through a conditional radius, Rc. An inner region, where both atoms becomes locked in their initial state, is again separated from the conditional region by a smaller blocking radius, Rb. About 10 years ago it was discovered that the large dipole moment of Rydberg states of interacting atoms can induce a detuning which effectively prohibits more than a single atom to become optically excited within a given volume [1,2]. Thus, in an atomic cloud exposed to a driving optical excitation scheme, the dipole-dipole interaction sets up an entangled multiparticle state with special correlation properties. Recently, the dipole blockade mechanism has been measured in controlled two-atom experiments with high-lying Rydberg states of rubidium [3,4] as well as in cold gases [5][6][7]. In addition to the fascinating exploration of exotic quantum dynamics in mesoscopic systems, the dipole blockade opens for applications within quantum information [8]. Here a number of quantum gates based on single-atom gates and conditional two-atom gates and protocols involving a large number of atoms have been proposed [9].Isolated Rydberg atoms can be experimentally prepared in almost any given linear combinations of spherical l,m states of a given principal quantum number n, including circular states (magnetic quantum number m = ±(n − 1)), coherent elliptical states (corresponding to classical states of fixed eccentricity [10]), or strongly polarized Stark states (Stark quantum number k ∼ n) [11][12][13]. Experiments where recent progress has realized trapping and probing of conditional dynamics of two single atoms may therefore also probe intrashell dynamics of two initially excited Rydberg atoms. From the point of view of optical driving frequencies between ground-state atoms |g and a single Rydberg level |e , the dynamics of this setup seems at first sight only to amount to a trivial phase development, as the combined initial state in fact is the dipole blocked dark state |ee of the optical excitation scheme. However, when considering the response of Rydberg atoms to weak, time-dependent electric and magnetic fields, it is clear that for these interactions the initial state couple effectively to a manifold of intrashell states |e i ,e j . In fact, the isolated atom intrashell dynamics can be completely controlled and driven between certain initial and final states with 100% transition probability for any n level [14].In this Brief Report we explor...

We present a detailed analysis of the interference effects observed for ionization in collisions of fast highly charged projectiles with molecular hydrogen. We propose a nonperturbative semiclassical approach to describe the process under consideration by solving the time-dependent Schrödinger equation fully numerically on a 3D spatial grid. We present results for Kr 34+ -H 2 collisions at 63 MeV/u impact energy and discuss different structures observed experimentally in doubly differential cross sections. The presence of Young-type minima and the absence of high-frequency oscillations are especially addressed. We also report unexpected interference patterns which can be observed for fixed-in-space molecular targets.