Physics - Doctoral Theseshttp://hdl.handle.net/10468/4302017-04-27T07:17:25Z2017-04-27T07:17:25ZQuantum state transfer via invariant based shortcuts to adiabaticityKiely, Anthonyhttp://hdl.handle.net/10468/38772017-04-12T11:00:42Z2017-01-01T00:00:00ZQuantum state transfer via invariant based shortcuts to adiabaticity
Kiely, Anthony
Adiabatic processes in quantum mechanics are very useful to prepare and manipulate quantum states but have the drawback of requiring long operation times. Hence there is a long time for the system to interact with its environment which can lead to a loss of coherence of the final state. This decoherence is problematic for implementing future quantum technologies which require the state's quantum mechanical features. â€œShortcuts to Adiabaticity"(STA) provides a toolbox of methods to improve on adiabatic processes. Using these methods one can derive alternative processes which work for much shorter times with perfect fidelity. Since adiabatic processes are ubiquitous in atomic, molecular and optical physics, there is a broad scope of application for STA. In this thesis, STA (especially those using Lewis-Riesenfeld invariants) are applied to a variety of quantum systems for the purpose of quantum state transfer. In particular I show that STA control schemes in two- and three-level systems can be optimised to be more stable against unwanted uncontrollable transitions than adiabatic methods with the same operation time. I also show that STA methods can be applied in a triple well ring system with complex tunnelling, in optical lattices for the purposes of generating a higher orbital state of neutral atoms and in Penning traps to quickly compress or expand the trapped ion wavefunction. Finally I also investigate the effect of classical Poisson white noise on adiabatic processes.
2017-01-01T00:00:00ZNanofiber control of ultracold quantum gasesHennessy, Tarahttp://hdl.handle.net/10468/24472016-10-17T15:29:53Z2016-01-01T00:00:00ZNanofiber control of ultracold quantum gases
Hennessy, Tara
While a great amount of attention is being given to the development of nanodevices, both through academic research and private industry, the field is still on the verge. Progress hinges upon the development of tools and components that can precisely control the interaction between light and matter, and that can be efficiently integrated into nano-devices. Nanofibers are one of the most promising candidates for such purposes. However, in order to fully exploit their potential, a more intimate knowledge of how nanofibers interact with single neutral atoms must be gained. As we learn more about the properties of nanofiber modes, and the way they interface with atoms, and as the technology develops that allows them to be prepared with more precisely known properties, they become more and more adaptable and effective. The work presented in this thesis touches on many topics, which is testament to the broad range of applications and high degree of promise that nanofibers hold. For immediate use, we need to fully grasp how they can be best implemented as sensors, filters, detectors, and switches in existing nano-technologies. Areas of interest also include how they might be best exploited for probing atom-surface interactions, single-atom detection and single photon generation. Nanofiber research is also motivated by their potential integration into fundamental cold atom quantum experiments, and the role they can play there. Combining nanofibers with existing optical and quantum technologies is a powerful strategy for advancing areas like quantum computation, quantum information processing, and quantum communication. In this thesis I present a variety of theoretical work, which explores a range of the applications listed above. The first work presented concerns the use of the evanescent fields around a nanofiber to manipulate an existing trapping geometry and therefore influence the centre-of-mass dynamics of the atom. The second work presented explores interesting trapping geometries that can be achieved in the vicinity of a fiber in which just four modes are allowed to propagate. In a third study I explore the use of a nanofiber as a detector of small numbers of photons by calculating the rate of emission into the fiber modes when the fiber is moved along next to a regularly separated array of atoms. Also included are some results from a work in progress, where I consider the scattered field that appears along the nanofiber axis when a small number of atoms trapped along that axis are illuminated orthogonally; some interesting preliminary results are outlined. Finally, in contrast with the rest of the thesis, I consider some interesting physics that can be done in one of the trapping geometries that can be created around the fiber, here I explore the ground states of a phase separated two-component superfluid Bose-Einstein condensate trapped in a toroidal potential.
2016-01-01T00:00:00ZA tight-binding analysis of the electronic properties of III-nitride semiconductorsCoughlan, Conor Terencehttp://hdl.handle.net/10468/27612016-10-17T15:30:54Z2016-01-01T00:00:00ZA tight-binding analysis of the electronic properties of III-nitride semiconductors
Coughlan, Conor Terence
This thesis divides into two distinct parts, both of which are underpinned by the tight-binding model. The first part covers our implementation of the tight-binding model in conjunction with the Berry phase theory of electronic polarisation to probe the atomistic origins of spontaneous polarisation and piezoelectricity as well as attempting to accurately calculate the values and coefficients associated with these phenomena. We first develop an analytic model for the polarisation of a one-dimensional linear chain of atoms. We compare the zincblende and ideal wurtzite structures in terms of effective charges, spontaneous polarisation and piezoelectric coefficients, within a first nearest neighbour tight-binding model. We further compare these to real wurtzite structures and conclude that accurate quantitative results are beyond the scope of this model but qualitative trends can still be described. The second part of this thesis deals with implementing the tight-binding model to investigate the effect of local alloy fluctuations in bulk AlGaN alloys and InGaN quantum wells. We calculate the band gap evolution of Al1_xGaxN across the full composition range and compare it to experiment as well as fitting bowing parameters to the band gap as well as to the conduction band and valence band edges. We also investigate the wavefunction character of the valence band edge to determine the composition at which the optical polarisation switches in Al1_xGaxN alloys. Finally, we examine electron and hole localisation in InGaN quantum wells. We show how the built-in field localises the carriers along the c-axis and how local alloy fluctuations strongly localise the highest hole states in the c-plane, while the electrons remain delocalised in the c-plane. We show how this localisation affects the charge density overlap and also investigate the effect of well width fluctuations on the localisation of the electrons.
2016-01-01T00:00:00ZFluorescence properties of rhodamine 101 and Yb3+ in condensed phase systemsChullipalliyalil, Krishnakumarhttp://hdl.handle.net/10468/38642017-04-10T11:00:13Z2016-01-01T00:00:00ZFluorescence properties of rhodamine 101 and Yb3+ in condensed phase systems
Chullipalliyalil, Krishnakumar
The fluorescence properties of two high quantum yield substances, Rhodamine 101 and Yb3+, were studied in different molecular environments. The prompt emission properties of these compounds were spectroscopically characterized using custom-made high resolution fluorescence detection set-ups in the visible and near IR spectral region. The anti-Stokes emission from Rhodamine 101 in ethanol was studied as a function of temperature and concentration. The emission and absorption properties of Yb3+ were investigated in glassy and crystalline matrices. The photophysical properties of Yb3+ phosphate glass nanoparticles, prepared by liquid-phase pulsed laser ablation was compared with those of the bulk material. Emission properties of chemically synthesized nanocrystals of Yb3+ doped tungstate (Yb3+:KGd(WO4)2) were also compared to their bulk counterpart. Nano-glasses and crystals exhibited spectroscopic modifications in comparison to the corresponding bulk materials. These modifications were due to the alterations of the phonon coupling between Yb3+ and the host matrix.
2016-01-01T00:00:00Z