Direct optical excitation of a fullerene-incarcerated metal ion

The endohedral fullerene Er3N@C80 shows characteristic 1.5 micron photoluminescence at cryogenic temperatures associated with radiative relaxation from the crystal-field split Er3+ 4I13/2 manifold to the 4I15/2 manifold. Previous observations of this luminescence were carried out by photoexcitation of the fullerene cage states leading to relaxation via the ionic states. We present direct non-cage-mediated optical interaction with the erbium ion. We have used this interaction to complete a photoluminescence-excitation map of the Er3+ 4I13/2 manifold. This ability to interact directly with the states of an incarcerated ion suggests the possibility of coherently manipulating fullerene qubit states with light

Sensitive Radio-Frequency Measurements of a Quantum Dot by Tuning to Perfect Impedance Matching

Electrical readout of spin qubits requires fast and sensitive measurements, which are hindered by poor impedance matching to the device. We demonstrate perfect impedance matching in a radio-frequency readout circuit, using voltage-tunable varactors to cancel out parasitic capacitances. An optimized capacitance sensitivity of 1.6   aF / √ Hz is achieved at a maximum source-drain bias of 170 − μ V root-mean-square and with a bandwidth of 18 MHz. Coulomb blockade in a quantum-dot is measured in both conductance and capacitance, and the two contributions are found to be proportional as expected from a quasistatic tunneling model. We benchmark our results against the requirements for single-shot qubit readout using quantum capacitance, a goal that has so far been elusive

Circuit Quantum Electrodynamics with Carbon-Nanotube-Based Superconducting Quantum Circuits

Hybrid circuit QED involves the study of coherent quantum physics in solid-state systems via their interactions with superconducting microwave circuits. Here we present a crucial step in the implementation of a hybrid superconducting qubit that employs a carbon nanotube as a Josephson junction. We realize the junction by contacting a carbon nanotube with a superconducting Pd/Al bilayer, and implement voltage tunability of the quantum circuit's frequency using a local electrostatic gate. We demonstrate a strong dispersive coupling to a coplanar waveguide resonator by investigating the gate-tunable resonator frequency. We extract qubit parameters from spectroscopy using dispersive readout and find qubit relaxation and coherence times in the range of 10-200ns

Barkhausen and magneto-acoustic emission from ferromagnetic materials

Barkhausen emission (B.E.) and Magneto-acoustic emission (M.A.E.) can be detected from specimens in a magnetic field varying at a few millihertz. Comparison of the two signals can indicate the nature of the domain walls responsible for the activity at any particular field. In order to characterize a specimen the strength of the emissions around the hysteresis loop are measured together with the distribution of Barkhausen event sizes. This technique has been used to measure the effects of: (A) Microstructure. Both B.E. and M.A.E. are sensitive to dislocations, and the effects of cold-working and its removal by isochronal annealing has been studied in alpha-iron. A simple model of domain wall pinning is presented which enables the dislocation density to be estimated. M.A.E. and B.E. are also sensitive to the growth of precipitates in Incoloy 904 alloy and, for a certain regime of sizes, can potentially be used to monitor the precipitate diameter. B.E. is sensitive to smaller precipitates (-100 nm) than M.A.E. but, unlike M.A.E., its dependence on precipitate size is not monotonic. An understanding of the signal dependence is obtained from Lorentz microscopy. (B) Radiation damage. The sensitivity of B.E. and M.A.E. to radiation damage is quite small by virtue of the small size of defects present. Nevertheless measurements on neutron irradiated alpha-iron specimens in several microstructural states indicate: (a) an accelerated recovery from the cold-worked condition on isochronal annealing and (b) dissolution of nitrides and carbides which formed in preparatory heat treatments. Measurements on a neutron irradiated iron-copper alloy which was subsequently isochronally annealed indicated effects which were consistent with: (a) removal of dislocation loops formed during irradiation at 550°C and (b) growth of precipitates (probably copper) at 600°C which presumably formed during the irradiation, (i.e. the effect was smaller in unirradiated control specimens). These results suggest that B.E. and M.A.E. might be useful tools for the characterization of radiation effects. (C) Tensile stress. Both B.E. and M.A.E. are sensitive to applied tensile stress and measurements on a number of different materials indicate that the dependence of M.A.E. is monotonic (except in nickel) whereas that of B.E. is generally quite complex. Since the microstructural and stress dependences are often interrelated it would be difficult to use the technique to measure say residual stress in a practical material unless the exact condition of the microstructure could be determined. Consequently B.E. and M.A.E. were measured from mild steel specimens (4360 steel) which had recieved a number of different heat treatments. The effects of applied tensile stress on the amplitude and shape of the B.E. and M.A.E. profiles were investigated with a view to be able to use the M.A.E. to measure stresses without prior knowledge of the microstructure. It was found that certain parameters in the signal profile were much more strongly dependent upon the stress than on the microstructure for many of the material conditions. Therefore M.A.E. is potentially useful for residual stress measurements.</p

ACOUSTIC MICROSCOPY TECHNIQUES FOR OBSERVING DISLOCATION DAMPING

A unique advantage of scanning acoustic microscopy lies in the ability to image the interaction of acoustic waves with a specimen. This can be made quantitative, so that the velocity and attenuation of surface waves can be measured over a few microns. This technique can be applied to the detection of dislocation damping in the acoustic microscope

Creation and control of entanglement in condensed matter spin systems

The highly parallel nature of the fundamental principles of quantum mechanics means that certain key resource-intensive tasks --- including searching, code decryption and medical, chemical and material simulations --- can be computed polynomially or even exponentially faster with a quantum computer. In spite of its remarkably fast development, the field of quantum computing is still young, and a large-scale prototype using any one of the candidate quantum bits (or 'qubits') under investigation has yet to be developed. Spin-based qubits in condensed matter systems are excellent candidates. Spins controlled using magnetic resonance have provided the first, most advanced, and highest fidelity experimental demonstrations of quantum algorithms to date. Despite having highly promising control characteristics, most physical ensembles investigated using magnetic resonance are unable to produce entanglement, a critical missing ingredient for a pure-state quantum computer. Quantum objects are said to be entangled if they cannot be described individually: they remain fundamentally linked regardless of their physical separation. Such highly non-classical states can be exploited for a host of quantum technologies including teleportation, metrology, and quantum computation. Here I describe how to experimentally create, control and characterise entangled quantum ensembles using magnetic resonance. I first explore the relationship between entanglement and quantum metrology and demonstrate a sensitivity enhancement over classical resources using molecular sensors controlled with liquid-state nuclear magnetic resonance. I then examine the computational potential of irreversible relaxation processes in combination with traditional reversible magnetic resonance control techniques. I show how irreversible processes can polarise both nuclear and electronic spins, which improves the quality of qubit initialisation. I discuss the process of quantum state tomography, where an arbitrary quantum state can be accurately measured and characterised, including components which go undetected using traditional magnetic resonance techniques. Lastly, I combine the above findings to initialise, create and characterise entanglement between an ensemble of electron and nuclear spin defects in silicon. I further this by generating pseudo-entanglement between an ensemble of nuclear spins mediated by a transient electron spin in a molecular system. These findings help pave the way towards a particular architecture for a scalable, spin-based quantum computer.This thesis is not currently available in ORA

THE INVESTIGATION OF ELECTRODE SURFACES AT WHICH THE ELECTROCHEMISTRY OF PROTEINS OCCURS

Meeting Abstrac