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Title: Charge dynamics in superconducting double dots
Author: Esmail, Adam Ashiq
ISNI:       0000 0004 7225 3170
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2017
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The work presented in this thesis investigates transitions between quantum states in superconducting double dots (SDDs), a nanoscale device consisting of two aluminium superconducting islands coupled together by a Josephson junction, with each dot connected to a normal state lead. The energy landscape consists of a two level manifold of even charge parity Cooper pair states, and continuous bands corresponding to charge states with single quasiparticles in one or both islands. These devices are fabricated using shadow mask evaporation, and are measured at sub Kelvin temperatures using a dilution refrigerator. We use radio frequency reflectometry to measure quantum capacitance, which is dependent on the quantum state of the device. We measure the quantum capacitance as a function of gate voltage, and observe capacitance maxima corresponding to the Josephson coupling between even parity states. We also perform charge sensing and detect odd parity states. These measurements support the theoretical model of the energy landscape of the SDD. By measuring the quantum capacitance in the time domain, we observe random switching of capacitance between two levels. We determine this to be the stochastic breaking and recombination of single Cooper pairs. By carrying out spectroscopy of the bath responsible for the pair breaking we attribute it to black-body radiation in the cryogenic environment. We also drive the breaking process with a continuous microwave signal, and find that the rate is linearly proportional to incident power. This suggests that a single photon process is responsible, and demonstrates the potential of the SDD as a single photon microwave detector. We investigate this mechanism further, and design an experiment in which the breaking rate is enhanced when the SDD is in the antisymmetric state rather than the symmetric state. We also measure the quantum capacitance of a charge isolated double dot. We observe 2e periodicity, indicating the tunnelling of Cooper pairs and the lack of occupation of quasiparticle states. This work is relevant to the range of experiments investigating the effect of non-equilibrium quasiparticles on the operation of superconducting qubits and other superconducting devices.
Supervisor: Lambert, Nicholas J. ; Ferguson, Andrew J. Sponsor: EPSRC ; Hitachi Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Superconducting devices ; Quantum computing ; Quantum information processing ; Josephson junctions ; Superconducting single electron transistors ; Quasiparticles ; Superconducting double dots ; Quantum dots ; Electron transport ; Electron dynamics ; Cooper pair ; Cooper pair splitting ; Superconducting qubits ; Qubits ; Low temperature measurements ; Dilution refrigerator ; Shadow mask evaporation ; Quantum capacitance ; Single microwave photon detection ; Non-equilibrium quasiparticles ; Charge isolated double dot