Quantum gate based on Stark tunable nanocrystal interactions with ultrahigh-Q/V field modes in fused silica microcavities

We investigate the use of nanocrystal quantum dots as a quantum bus element for preparing various quantum resources for use in photonic quantum technologies. Using the Stark-tuning property of nanocrystal quantum dots as well as the biexciton transition, we demonstrate a photonic controlled-NOT (CNOT) interaction between two logical photonic qubits comprising two cavity field modes each. We find the CNOT interaction to be a robust generator of photonic Bell states, even with relatively large biexciton losses. These results are discussed in light of the current state of the art of both microcavity fabrication and recent advances in nanocrystal quantum dot technology. Overall, we find that such a scheme should be feasible in the near future with appropriate refinements to both nanocrystal fabrication technology and microcavity design. Such a gate could serve as an active element in photonic-based quantum technologies

Orientation of biological cells using plane-polarized Gaussian beam optical tweezers

Optical tweezers are widely used for the manipulation of cells and their internal structures. However, the degree of manipulation possible is limited by poor control over the orientation of trapped cells. We show that it is possible to controllably align or rotate disc shaped cells - chloroplasts of Spinacia oleracea - in a plane polarised Gaussian beam trap, using optical torques resulting predominantly from circular polarisation induced in the transmitted beam by the non-spherical shape of the cells.Comment: 9 pages, 6 figure

Nanotrapping and the thermodynamics of optical tweezers

Particles that can be trapped in optical tweezers range from tens of microns down to tens of nanometres in size. Interestingly, this size range includes large macromolecules. We show experimentally, in agreement with theoretical expectations, that optical tweezers can be used to manipulate single molecules of polyethylene oxide suspended in water. The trapped molecules accumulate without aggregating, so this provides optical control of the concentration of macromolecules in solution. Apart from possible applications such as the micromanipulation of nanoparticles, nanoassembly, microchemistry, and the study of biological macromolecules, our results also provide insight into the thermodynamics of optical tweezers.Comment: 5 pages, 3 figures, presented at 17th AIP Congress, Brisbane, 200

Forces from highly focused laser beams: modeling, measurement and application to refractive index measurements

The optical forces in optical tweezers can be robustly modeled over a broad range of parameters using generalsed Lorenz-Mie theory. We describe the procedure, and show how the combination of experimental measurement of properties of the trap coupled with computational modeling, can allow unknown parameters of the particle - in this case, the refractive index - to be determined.Comment: 5 pages, 4 figures, presented at 17th AIP Congress, Brisbane, 200

Inorganic Surface Passivation of PbS Nanocrystals resulting in Strong Photoluminescent Emission

Strong photoluminescent emission has been obtained from 3 nm PbS nanocrystals in aqueous colloidal solution, following treatment with CdS precursors. The observed emission can extend across the entire visible spectrum and usually includes a peak near 1.95 eV. We show that much of the visible emission results from absorption by higher-lying excited states above 3.0 eV with subsequent relaxation to and emission from states lying above the observed band-edge of the PbS nanocrystals. The fluorescent lifetimes for this emission are in the nanosecond regime, characteristic of exciton recombination.Comment: Preprint, 23 pages, 6 figure

Optical Measurement of Microscopic Forces and Torques

Many spectacular successes have resulted from the use of laser trapped particles as force-sensing probes. For example, the forces applied to a DNA molecule as an RNA copy is made have been measured, as well as the physical properties of DNA. Optically trapped particles can be used to probe small forces and weak interactions which cannot be readily measured in any other way due to extreme sensitivity to ambient conditions. A number of groups have made measurements of trapping forces, with differing levels of sensitivity and accuracy. However, a serious and fundamental problem common to virtually all measurements of this type is the lack of reliable absolute measurement. Viscous drag forces are generally used for calibration, which immediately presents the problem of changes in viscosity resulting from heating by the trapping beam. Since the optical trapping forces are due to the transfer of momentum from the beam to the particle, it is in principle possible to measure the applied force and torque by measuring the momentum of the scattered light. Direct optical determination of the force and torque gives an absolute measurement, immediately eliminating difficulties with calibration. The theory of direct optical measurement of forces and torques acting on laser trapped non-spherical and birefringent probe particles is presented

Trapping and Alignment of a Microfibre Using the Discrete Dipole Approximation

Optical tweezers can be used to trap, move, and rotate or align microscopic particles. Elongated particles are ideal candidates for alignment and controlled rotation; the behaviour of microfibres within an optical trap is investigated

Laser Trapping of Non-Spherical Particles

Optical trapping, where microscopic particles are trapped and manipulated by light is a powerful technique. The single-beam gradient trap (also known as optical tweezers) is widely used for a large number of biological and other applications. The forces and torques acting on a trapped particle result from the transfer of momentum and angular momentum from the trapping beam to the particle. Despite the apparent simplicity of a laser trap, with a single particle in a single beam, exact calculation of the optical forces and torques acting on particles is difficult, and a number of approximations are normally made. Approximate calculations are performed either by using geometric optics, which is appropriate for large particles, or using small particle approximations. Neither approach is adequate for particles of a size comparable to the wavelength. This is a serious deficiency, since wavelength sized particles are of great practical interest because they can be readily and strongly trapped and can be used to probe interesting microscopic and macroscopic phenomena. The lack of suitable theory is even more acute when the trapping of non-spherical particles is considered. Accurate quantitative calculation of forces and torques acting on non-spherical particles is of particular interest due to the suitability of such particles as microscopic probes. These calculations are also important because of the frequent occurrence of non-spherical biological and other structures, and the possibility of rotating or controlling the orientation of such objects. The application of electromagnetic scattering theory to the laser-trapping of wavelength sized non-spherical particles is presented