RESEARCH NEWS

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 electron’s 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 π-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 µm in diameter. The fiber contains eight airholes around the central core, 12.7 µm 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.