The quest to build a quantum computer has been inspired by the recognition of the formidable computational power such a device could offer. In particular silicon-based proposals, using the nuclear or electron spin of dopants as qubits, are attractive due to the long spin relaxation times involved, their scalability, and the ease of integration with existing silicon technology. Fabrication of such devices however requires atomic scale manipulation -an immense technological challenge. We demonstrate that it is possible to fabricate an atomically-precise linear array of single phosphorus bearing molecules on a silicon surface with the required dimensions for the fabrication of a siliconbased quantum computer. We also discuss strategies for the encapsulation of these phosphorus atoms by subsequent silicon crystal growth. (To appear in Phys. Rev. B Rapid Comm.) 03.67. Lx, 68.37.Ef, A quantum bit (or qubit) is a two level quantum system that is the building block of a quantum computer. To date the most advanced realisations of a quantum computer are qubit ion trap 1 and nuclear magnetic resonance 2-4 systems. However scaling these systems to large numbers of qubits will be difficult 5 , making solidstate architectures 6 , with their promise of scalability, important. In 1998 Kane proposed a novel solid state quantum computer design 7 using phosphorus 31 P nuclei (nuclear spin I = 1/2) as the qubits in isotopically-pure silicon 28 Si (I = 0). The device architecture is shown in Fig. 1a, with phosphorus qubits embedded in silicon approximately 20 nm apart. This separation allows the donor electron wavefunctions to overlap, whilst an insulating barrier isolates them from the surface control A and J gates. These A and J gates control the hyperfine interaction between the nuclear and electron spins and the coupling between adjacent donor electrons respectively. For a detailed description of the computer operation refer to Kane 7 . An alternative strategy using the electron spins of the phosphorus donors as qubits has also been proposed 8 .One of the major challenges of this design is to reliably fabricate an atomically-precise array of phosphorus nuclei in silicon -a feat that has yet to be achieved in a semiconductor system. Whilst a scanning tunnelling microscope (STM) tip has been used for atomic scale arrangement of metal atoms on metal surfaces 9 , rearrangement of individual atoms in a semiconductor system is not straightforward due to the strong covalent bonds involved. As a result, we have employed a hydrogen resist strategy outlined in Fig. 1b. Here the array is fabricated using a resist technology, much like in conventional lithography, where the resist is a layer of hydrogen atoms that terminate the silicon surface. An STM tip is used to selectively desorb individual hydrogen atoms, exposing the underlying silicon surface in the required array. STM induced hydrogen desorption has been developed and refined over the past ten years 10 and has been proposed 11 for the assembly of atomically-ordered device structure...

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The sensitivity of explosives is controlled by factors that span from intrinsic chemical reactivity to mesoscale structure, and has been a topic of extensive study for over 50 years.

PBX 9502 is a plastic‐bonded explosive that contains 95 wt.‐% TATB, a graphitic‐structured high explosive known to undergo “ratchet growth,” i.e., irreversible volume change that accompanies temperature excursions. Earlier studies have reported changes in TATB‐based composites as a function of thermal cycling and density change, however, a clear distinction between density and ratchet‐growth effects has not been made. In the work reported here, an “as‐pressed density” baseline for the mechanical response of recycled PBX 9502 is established over a density range of interest, then high‐density specimens are thermally cycled between −55 and 80 °C to achieve “ratchet‐grown” parts in the same low‐density region. As‐pressed and ratchet‐grown specimens with identical densities are then analyzed using microX‐ray computed tomography and USANS techniques to obtain information about pore‐size distributions. Data show that after ratchet‐growth, PBX 9502 specimens contain, in general, more numerous and smaller voids than specimens that were pressed with lower compaction pressures to match the same density. The mechanical response of the ratchet‐grown material is consistent with damage, showing lower tensile stress and modulus, lower compressive modulus, and higher tensile and compressive strain, than as‐pressed specimens of the same density.