Apostolidis, Pavlos; (2018) High-Frequency Measurements of Carbon Nanotube Quantum Dots. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The implementation of a quantum computer is expected to revolutionise computation in terms of speed and capabilities. Quantum computing is, therefore, an active area of research. This thesis presents experimental attempts towards advances in the field of quantum computation using carbon nanotube-based quantum dot devices and high-frequency techniques. The motivation behind using carbon nanotubes lies with the fact that they provide a priori confinement in two dimensions, making the quantum dot fabrication procedure simpler. In addition, they allow for control of the carbon isotope ratio before growth. As a consequence, isotropically pure carbon-12 nanotubes can serve as ideal hosts for spin qubits due to the lack of hyperfine interactions, providing a clean spin environment and longer coherence times. On the other hand, nanotubes with a controlled percentage of carbon-13 atoms could be used as quantum memories, upon transferring information to the nuclear spin. For the work presented in this thesis, single-walled carbon nanotubes with natural abundance of carbon isotopes were grown using chemical vapour deposition. Quantum dot devices were fabricated on doped and undoped silicon substrates using electrostatic top-gating techniques and studied at ultra-low temperatures, focusing in particular on quantum state readout with high-frequency electric fields. The main topic of this thesis revolves around the fabrication and measurements of carbon nanotube single and double quantum dots, with the potential of being used as charge or spin qubit devices. An overview of the relevant theoretical background is given, including the theory of few-electron quantum dots based on the constant interaction model. The carbon nanotube geometry and electronic structure are outlined, along with relevant perturbations to the band structure. A detailed fabrication procedure for devices is presented and experimental methods are discussed, including customisation of a dilution refrigerator, minimisation of the electron temperature and optimisation of the relevant high-frequency setup. Radio-frequency (RF) reflectometry is exploited as a fast, high-sensitivity, non-invasive technique for quantum dot readout. For readout optimisation, a novel approach for developing voltage-tunable variable capacitors (varactors) that operate reliably at low temperatures is presented. Such varactors are based on strontium titanate and can be used to tune a quantum dot to perfect impedance matching, as well as tune its resonant frequency in situ. It is demonstrated that this approach allows for higher signal-to-noise ratio and results in a charge sensitivity that is among the best reported to date. Furthermore, by investigating iron phthalocyanine molecules, it is demonstrated how RF reflectometry can be used for capacitance spectroscopy of arbitrary molecules or nanoparticles using a simplified device geometry. Electronic transport and high-frequency measurements of carbon nanotube double quantum dots are presented, including characterisation and attempts for high-frequency gate modulation and interdot transition tuning. A video-mode data acquisition technique is introduced as a fast way to perform RF measurements, with an integration time of about 1 μs per point. This technique allows for the effects of gate electrodes to be observed in situ upon varying the corresponding voltage. Relevant challenges and solutions are given, along with future suggestions and directions.

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