Spins based in silicon provide one of the most promising architectures for quantum computing. Quantum dots are an inherently scalable technology. Here, we combine these two concepts into a workable design for a silicon-germanium quantum bit. The novel structure incorporates vertical and lateral tunneling, provides controlled coupling between dots, and enables single electron occupation of each dot. Precise modeling of the design elucidates its potential for scalable quantum computing. For the first time it is possible to translate the requirements of faulttolerant error correction into specific requirements for gate voltage control electronics in quantum dots. We demonstrate that these requirements are met by existing pulse generators in the kHzMHz range, but GHz operation is not yet achievable. Our calculations further pinpoint device features that enhance operation speed and robustness against leakage errors. We find that the component technologies for silicon quantum dot quantum computers are already in hand.Quantum computing offers the prospect of breaking out of the classical von Neumann paradigm that dominates present-day computation. It would enable huge speedups of certain very hard problems, notably factorization 1 . Constructing a quantum computer (QC) presents many challenges, however. Chief among these is scalability: the 10 6 qubits needed for simple applications 2 far exceed the potential of existing implementations. This requirement points strongly in the direction of Si-based electronics for QC. Silicon devices offer the advantage of long spin coherence times, fast operation, and a proven record of scalable integration.Specific Si-based qubit proposals utilize donor-bound nuclear 3,4 or electronic 5 spins as qubits. However, quantum dots can also be used to house electron spins 6-8 , and they have the advantage that the electrostatic gates controlling qubit operations are naturally aligned to each qubit. These proposals describe an intriguing possibility. Our aim here is to describe a new SiGe qubit design, and, just as importantly, to carry out detailed modeling of a specific design for the first time. Modeling provides a proof of principle, pinpoints problem areas, and suggests new directions.The fundamental goal of our design is the ability to reduce the electron occupation of an individual dot precisely to one, as in vertically coupled structures. It may be possible to use the spin of multi-electron quantum dots as qubits, but single occupation is clearly desirable. The spin state "up" = 0 or "down" = 1 , stores the quantum bit of information. At the same time, it is necessary to have tunable coupling between neighboring dots. This is achieved by controlled movement of electrons along the quantum well that contains two dots. The solution is to draw on two distinct quantum dot technologies: lateral and vertical tunneling quantum dots 9 .

The nanomechanical properties of thin silicon films will become increasingly critical in semiconductor devices, particularly in the context of substrates that consist of a silicon film on an insulating layer (known as silicon-on-insulator, or SOI, substrates). Here we use very small germanium crystals as a new type of nanomechanical stressor to demonstrate a surprising mechanical behaviour of the thin layer of silicon in SOI substrates, and to show that there is a large local reduction in the viscosity of the oxide on which the silicon layer rests. These findings have implications for the use of SOI substrates in nanoelectronic devices.

Mechanical bending is ubiquitous in heteroepitaxial growth of thin films where the strained growing film applies effectively an "external" stress to bend the substrate. Conventionally, when the deposited film is much thinner than the substrate, the bending increases linearly with increasing film thickness following the classical Stoney formula. Here we analyze the bending of ultrathin ͑nanometer range͒ substrates induced by growth of coherently strained thin films. The behavior deviates dramatically from the classical linear dependence: when the film thickness becomes comparable to the substrate thickness the bending decreases with increasing film thickness. This complex bending behavior can be understood by considering evolution of strain sharing between the film and substrate. We demonstrate experimentally such counterintuitive bending of a nanoscale thin Si substrate induced by a coherently strained Ge film, in the form of islands, grown on silicon-on-insulator substrate. Larger dome islands, representing a thicker film, induce much less bending of the substrate than smaller hut islands, representing a thinner film, in direct contrast to their behavior on thick Si. We explain these observations by properly considering the island shape and strain relaxation within the island.

Pseudomorphic three-dimensional Ge nanocrystals (quantum dots) grown on thin silicon-on-insulator substrates can induce significant bending of the silicon template layer that is local on the nanometer scale. We use molecular dynamics simulations and analytical models to confirm the local bending of the Si template and to show that its magnitude approaches the maximum value for a freestanding membrane. The requisite greatly enhanced viscous flow of SiO2 underneath the Si layer is consistent with the dependence of the viscosity of SiO2 on shear stress.

We report a comparative study of the efficacy of immobilizing living bacteria by means of seven antibodies against bacterial surface antigens associated with Salmonella enterica Serovar Typhimurium. The targeted bacterial antigens were CFA/I fimbriae, flagella, lipopolysaccharides (LPS) and capsular F1 antigen. The best immobilization of S. Typhimurium was achieved with the antibody against CFA/I fimbriae. The immobilization of bacteria using anti-flagellin showed significant enhancement if the flagella rotary motion was paralyzed. Of the four antibodies targeting LPS structures, only one, the antibody against the O-antigen polysaccharides, showed a relatively efficient bacterial immobilization. No bacterial immobilization was achieved using the antibody against F1 antigen, presumably because F1 protein can detach from the bacterial surface easily. The results suggest that an antibody for bacterial immunoimmobilization should target a surface antigen which extends out from the bacterial surface and is tightly attached to the bacterial cell wall. The microarrays of living S. Typhimurium cells immobilized in this manner remained viable and effective for at least two weeks in growth medium before a thick biofilm covered the whole surface.