Initial experiments concerning quantum information processing in rare-earth-ion doped crystals

In this paper initial experiments towards constructing simple quantum gates in a solid state material are presented. Instead of using specially tailored materials, the aim is to select a subset of randomly distributed ions in the material, which have the interaction necessary to control each other and therefore can be used to do quantum logic operations. The experimental results demonstrate that part of an inhomogeneously broadened absorption line can be selected as a qubit and that a subset of ions in the material can control the resonance frequency of other ions. This opens the way for the construction of quantum gates in rare-earth-ion doped crystals.Comment: 24 pages, including 12 figure

Energy levels and magneto-optical transitions in parabolic quantum dots with spin-orbit coupling

We report on the electronic properties of few interacting electrons confined in a parabolic quantum dot based on a theoretical approach developed to investigate the influence of Bychkov-Rashba spin-orbit (SO) interaction on such a system. We note that the spin-orbit coupling profoundly influences the energy spectrum of interacting electrons in a quantum dot. Here we present accurate results for the energy levels and optical-absorption spectra for parabolic quantum dots containing upto four interacting electrons, in the presence of spin-orbit coupling and under the influence of an externally applied, perpendicular magnetic field. We have described in detail about a very accurate numerical scheme to evaluate these quantities. We have evaluated the effects of SO coupling on the Fock-Darwin spectra for quantum dots made out of three different semiconductor systems, InAs, InSb, and GaAs.Comment: expanded version of cond-mat/0501642 to be published in Phys. Rev. Let

Extracting high fidelity quantum computer hardware from random systems

An overview of current status and prospects of the development of quantum computer hardware based on inorganic crystals doped with rare-earth ions is presented. Major parts of the experimental work in this area has been done in two places, Canberra, Australia and Lund, Sweden, and the present description follows more closely the Lund work. Techniques will be described that include optimal filtering of the initially inhomogeneously broadened profile down to well separated and narrow ensembles, as well as the use of advanced pulse-shaping in order to achieve robust arbitrary single-qubit operations with fidelities above 90%, as characterized by quantum state tomography. It is expected that full scalability of these systems will require the ability to determine the state of single rare-earth ions. It has been proposed that this can be done using special readout ions doped into the crystal and an update is given on the work to find and characterize such ions. Finally, a few aspects on the possibilities for remote entanglement of ions in separate rare-earth-ion-doped crystals are considered.Comment: 19 pages, 9 figures. Written for The Proceedings of the Nobelsymposium on qubits for future quantum computers, Gothenburg, May-0

Correlational Origin of the Roton Minimum

We present compelling evidence supporting the conjecture that the origin of the roton in Bose-condensed systems arises from strong correlations between the constituent particles. By studying the two dimensional bosonic dipole systems a paradigm, we find that classical molecular dynamics (MD) simulations provide a faithful representation of the dispersion relation for a low- temperature quantum system. The MD simulations allow one to examine the effect of coupling strength on the formation of the roton minimum and to demonstrate that it is always generated at a sufficiently high enough coupling. Moreover, the classical images of the roton-roton, roton-maxon, etc. states also appear in the MD simulation spectra as a consequence of the strong coupling.Comment: 7 pages, 4 figure

Analysis of radiatively stable entanglement in a system of two dipole-interacting three-level atoms

We explore the possibilities of creating radiatively stable entangled states of two three-level dipole-interacting atoms in a $\Lambda$ configuration by means of laser biharmonic continuous driving or pulses. We propose three schemes for generation of entangled states which involve only the lower states of the $\Lambda$ system, not vulnerable to radiative decay. Two of them employ coherent dynamics to achieve entanglement in the system, whereas the third one uses optical pumping, i.e., an essentially incoherent process.Comment: Replaced with the final version; 14 pages, 6 figures; to appear in Phys. Rev. A, vol. 61 (2000

Ultrafast fluorescent decay induced by metal-mediated dipole-dipole interaction in two-dimensional molecular aggregates

Two-dimensional molecular aggregate (2DMA), a thin sheet of strongly interacting dipole molecules self-assembled at close distance on an ordered lattice, is a fascinating fluorescent material. It is distinctively different from the single or colloidal dye molecules or quantum dots in most previous research. In this paper, we verify for the first time that when a 2DMA is placed at a nanometric distance from a metallic substrate, the strong and coherent interaction between the dipoles inside the 2DMA dominates its fluorescent decay at picosecond timescale. Our streak-camera lifetime measurement and interacting lattice-dipole calculation reveal that the metal-mediated dipole-dipole interaction shortens the fluorescent lifetime to about one half and increases the energy dissipation rate by ten times than expected from the noninteracting single-dipole picture. Our finding can enrich our understanding of nanoscale energy transfer in molecular excitonic systems and may designate a new direction for developing fast and efficient optoelectronic devices.Comment: 9 pages, 6 figure

Spectroscopic properties of a two-level atom interacting with a complex spherical nanoshell

Frequency shifts, radiative decay rates, the Ohmic loss contribution to the nonradiative decay rates, fluorescence yields, and photobleaching of a two-level atom radiating anywhere inside or outside a complex spherical nanoshell, i.e. a stratified sphere consisting of alternating silica and gold concentric spherical shells, are studied. The changes in the spectroscopic properties of an atom interacting with complex nanoshells are significantly enhanced, often more than two orders of magnitude, compared to the same atom interacting with a homogeneous dielectric sphere. The detected fluorescence intensity can be enhanced by 5 or more orders of magnitude. The changes strongly depend on the nanoshell parameters and the atom position. When an atom approaches a metal shell, decay rates are strongly enhanced yet fluorescence exhibits a well-known quenching. Rather contra-intuitively, the Ohmic loss contribution to the nonradiative decay rates for an atomic dipole within the silica core of larger nanoshells may be decreasing when the silica core - inner gold shell interface is approached. The quasistatic result that the radial frequency shift in a close proximity of a spherical shell interface is approximately twice as large as the tangential frequency shift appears to apply also for complex nanoshells. Significantly modified spectroscopic properties (see computer program (pending publication of this manuscript) freely available at http://www.wave-scattering.com) can be observed in a broad band comprising all (nonresonant) optical and near-infrared wavelengths.Comment: 20 pages plus 63 references and 11 figures, plain LaTex, for more information see http://www.wave-scattering.com (color of D sphere in figures 2-6 altered, minor typos corrected.

Applications of atomic ensembles in distributed quantum computing

Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes - stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium - an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single step Q-qubit GHZ states with success probability p(success) similar to eta(Q/2), where eta is the combined detection and source efficiency. This is signifcantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer