Bridging the gap: Nanotechnology

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    October 200318

    A molecular bridge could provide a

    means of transferring spin between

    quantum dots (QDs), according to Min

    Ouyang and David D. Awschalom of the

    University of California, Santa Barbara

    (UCSB) [Science (2003) 301, 1074].The setup could form the basis of a

    scalable solid-state quantum computer.

    QDs provide an ideal way of achieving

    the isolation between an electrons spin

    and external influences that is required

    for quantum computation. But, until

    now, there has been no reliable means

    of transferring spin in and out of QDs.

    The UCSB researchers use a

    controlled, layer-by-layer, bottom-up

    approach to create structures

    consisting of CdSe QDs bridged by

    conjugated molecules. The structure is

    built up on a silica substrate

    functionalized with amine or thiol

    groups. When immersed in a CdSe QD

    solution, the first monolayer forms.

    The QD ligands are then modified to

    form thiol end groups. The process is

    repeated to build up a three-

    dimensional structure of CdSe QDs

    linked together by the dithiol

    conjugated molecules. Simply changing

    the QD solution during the fabrication

    process allows a multilayer structure

    to be created. In this case, Ouyang and

    Awschalom used two differently sized

    CdSe QDs. All-optical spin resonance

    methods reveal spin transfer between

    the QDs at room temperature, with an

    efficiency of 20%.

    Various mechanisms have been

    proposed to explain spin coupling, but

    the researchers suggest an alternative.

    They believe that the dithiol conjugated

    molecules binding the CdSe QDs

    together allow spin communication.

    The delocalized -orbitals of the thiol

    molecules allow spin carrier transport

    between the dots. They could serve as

    a means for transferring quantum

    information and could be a step

    towards spintronics manufacturing.

    Bridging thegapNANOTECHNOLOGY

    A semiconductor quantum dot (QD)

    structure can be used to carry out

    simple quantum logic operations,

    according to new research from The

    University of Michigan, Naval Research

    Laboratory, Michigan State University,

    and The University of California, San

    Diego [Science (2003), 301, 809].The first scalable quantum computers

    are likely to be based on ions or

    atoms, but there is much interest on

    solid-state versions. The problem with

    such solid-state systems, explains

    Duncan G. Steel of The University of

    Michigan, is that at the quantum

    mechanical level these systems are

    very complex compared to isolated

    atoms. However, the researchers show

    that quantum logic gates can be

    realized by controlling the excitation of

    two electron-hole pairs (biexcitons) in a

    QD using a coherent laser system. The

    QDs are formed in a 4.2 nm GaAs

    layer in between two 25 nm

    Al0.3Ga0.7As barriers. Quantum

    confinement in the QD enhances the

    higher order quantum Coulomb

    interaction, leading to the formation of

    a bound state by two orthogonally

    polarized excitons. The excitation of

    one exciton affects the resonant energy

    of the other a key characteristic for

    quantum computing.

    By working in a single QD, the system

    behaves very much like an isolated

    atom and the complexities of most

    semiconductor systems and

    decoherence are not an issue, explains

    Steel. The system is a demonstration

    that coherent, optically-controlled

    quantum computing could be realized

    with multidot systems. Using excitons

    as qubits limits the scale up of system,

    so the researchers are working on

    using spin as the qubit instead.

    A semiconductor QD system... may

    function as a good basis for a solid-

    state quantum computing system,"

    Steel told Materials Today.

    Controlling theexcitationOPTICAL MATERIALS

    Drawing a quantum computerNANOTECHNOLOGY

    A new technique could allow the drawing and erasing ofquantum electronic components [Nature (2003) 442244, 751].Researchers from the University of Cambridge have developed atechnique, erasable electrostatic lithography (EEL), which usesa negatively biased scanning probe under low temperature, highvacuum conditions, to draw patterns of charge on the surfaceof a GaAs/AlGaAs heterojunction. Electrons are locally depletedfrom a subsurface two-dimensional electron system (2DES) inthe charged regions, forming quantum components. Thepatterns can be erased by positively biasing the probe orexposing the surface to red light.The ability to draw quantum components with novel geometriesis an advantage. Our studies of low-dimensional quantumdevices have revealed many effects that seem to be dependentupon device geometry, explains Charles Smith. At themoment, changing the geometry requires the fabrication of acompletely new device using electron beam lithography. Thedevice then has to be cooled to 50 mK for detailedmeasurements. This turn around time can take weeks or oftenlonger, says Smith. Using EEL can reduce this optimizationtime to a few hours. The resolution of the novel lithographictechnique is limited by the distance between the surface andthe conducting plane of electrons, currently ~100 nm. Butthere is room for improvement. By using differentheterostructure materials, where the electrons are close to thesurface, we hope to improve the resolution, explains Smith. The technique could be particularly useful in the construction ofscalable solid-state quantum computers, where a high level ofuniformity between quantum components is required. Theresearchers envisage a scanning probe that could move to eachcomponent in turn, characterize it, repair it if necessary, useEEL to tune it, and produce an array of identical components.We have recently fabricated a submicron semiconductor deviceat the corner of the chip, which we can use to detect single-electron movements in devices on the substrate, says Smith.

    Different at the coreOPTICAL MATERIALS

    Researchers at Corning Inc. have fabricated an air-core photonic bandgap fiber (PBGF) with losses two orders of magnitude lower thanpreviously reported fibers [Nature (2003), 424, 657].The team use the stack-and-draw technique to fabricate long lengths(100 m) of PBGF 125 2 m in diameter. The fiber contains eight airholes around the central core, 12.7 m in diameter. The transmissionwindow ranges is 1395-1700 nm, with a region of high attenuation at1550-1650 nm. The researchers believe that the 1395-1700 nmtransmission window represents one bandgap, with the higher lossregion produced by interaction of the core and surface modes, ratherthan a bandgap edge. Not only do these results make a significant advance in low-loss air-core PBGFs, say the researchers, but also indicate the potential toachieve interaction over hundreds of meters at reduced pump power.