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The scalar theory of image formation for a strongly spherical object in ref lection confocal optical microscopy is considered. The image contrast is derived from the far-field scattering amplitude. Experimentally obtained images of the sphere are seen to be in good agreement with the theoretical prediction.

Acoustic microscopy enables one to image the interactis:, ef acoustic waves with the elastic properties of a specimen with microscopic reso1u;ion. A lens with good focusing properties on axis can be used for both transmitting and receiving the signal, and an image is formed by scanning the lens mechanically over the specimen. In pressurised superhid helium with nonlinear coupling to harmonics a resolution of 15 nm has been achieved, but for routine use 2 GHz is the highest practical frequency, which offers a resolution of about 0.7 pm. The information is contained in the way that the acoustic wave is retlected from the specimen. For subsurface imaging, especially in polymer based materials such as composites and electronic packaging, the enhanced depth resolution of a confocal imaging system can he exploited to give good contrast from the plane of interest even when the specimen contains many scatters. In higher stilhess specimens, including most metals, semiconductors and ceramics, a dominant role in the contrast can he played by Rayleigh waves in the surface. If the specimen has a surface layer, then the propagation of the Rayleigh waves is sensitive to the perturbing action of the layer. If the specimen is anisotropic, then there will be dependence on the orientation of the surface and the direction of propagation in it. If there are surface cracks or boundaries, then there will he strong contrast when they scatter the Rayleigh waves. Detailed theory is available to relate the elastic properties of the surface to the conmst. and these enable informed interpretation of the acoustic images to be made, and also provide a foundation for more quantitative acoustic microscopy.

Resonant energy transfer mechanisms have been observed in the sensitized luminescence of solids, in quantum dots and in molecular nanostructures, and they also play a central role in light harvesting processes in photosynthetic organisms. We demonstrate that such mechanisms, together with the exciton-exciton binding energy shift typical of these nanostructures, can be used to perform universal quantum logic. In particular, we show how to generate controlled exciton entanglement and identify two different regimes of quantum behaviour.PACS numbers: 03.67. Lx, 78.67.Hc, 73.20.Mf The Förster energy transfer was first studied in the context of the sensitized luminescence of solids [1,2], in which an excited sensitizer atom can transfer its excitation to a neighbouring acceptor atom, via an intermediate virtual photon. This mechanism is also responsible for photosynthetic energy processes in antenna complexes, biosystems (BSs) that harvest sunlight [3]. More recently, interest has focussed on such a transfer in quantum dot (QD) nanostructures [4] and within molecular systems (MSs) [5]. In this Letter we give a general scheme for quantum computation that can be implemented in different nanostructures (NSs) by exploiting the Förster and exciton-exciton interactions. Thus, methods for controlled generation of exciton entanglement that use both diagonal and off-diagonal interactions are given.Consider two coupled generic NSs (scalability will be addressed later). We assume that the excitations of each NS are charge neutral (i.e., of an excitonic nature) and that they can be produced by optical means [6]. We also assume that tunnelling processes between them may be neglected, but that there is a strong exciton-exciton electromagnetic coupling. Our two-level-qubit system is represented in each NS by a single low-lying exciton (qubit state |1 ) and the ground state (qubit state |0 ). Then the interaction Hamiltonian can be written in the computational basis ({|00 , |01 , |10 , |11 }, with the first digit referring to NS I and the second to NS II) as follows:where the diagonal interaction V XX is the direct Coulomb binding energy between the two excitons, one located on each NS, and the off-diagonal V F denotes the Coulomb exchange (Förster) interaction which induces the transfer of an exciton from one NS to the other. These are the only Coulomb interaction terms which act between the qubits. ω 0 denotes the ground state energy and we define ∆ 0 ≡ ω 1 − ω 2 as the difference between the exciton creation energy for NS I (ω 1 ) and that for NS II (ω 2 ).Energy contributions due to spin singlet-triplet splittings do not significantly affect the present gating scheme and such effects are dealt with elsewhere [7]. The eigenenergies and eigenstates of the interacting qubit system are E 00 = ω 0 , Single qubit operations can be achieved by inducing Rabi oscillations in the excitonic system (e.g., see Ref.[8]). The V XX and V F interactions lead to two regimes for achieving quantum entanglement. First, if the ratio V F /∆ 0 ≫ 1, t...

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