Patomäki, Sofia Marjatta;
(2023)
Qubit couplers for silicon spin qubit architectures.
Doctoral thesis (Ph.D), UCL (University College London).
Abstract
The quantum computing paradigm, where bits are replaced with quantum bits (qubits), may offer favourable run-time scaling for certain problems that scale inefficiently using known classical algorithms. This speculated speedup derives from quantum superposition and entanglement, which have been experimen- tally verified in countless research laboratories over the past century. Spin-half states are natural qubits that are insensitive to electrical noise owing to their magnetic nature. Single electron spins can be trapped to quantum dots that act as artificial, semiconducting atomic nuclei. Silicon is dubbed as a semicon- ductor vacuum due to the long spin coherence times the material supports. It also happens to be the number one material for electronics. Semiconducting spin quantum processors need not be larger than microprocessors, while their superconducting brethren are size-limited by their use of microwave waveg- uides. At present, a small number of nearest-neighbour-coupled silicon spin qubits demonstrate fault-tolerant control fidelities. The theme of this thesis are spin qubit couplers; devices enabling scaling to 2D qubit arrays by extending qubit interaction range. Layout complexity is minimized by an elongated quantum dot coupler. Its spin state mediates spin exchange between peripheral electron spins, as has been demonstrated in a GaAs-based device over 0.7 μm. By studying similar, planar metal-oxide- silicon, devices, we experimentally confirm discretised charge states up to 1 μm elongations, coupling to peripheral electrons, and close to zero cross-talk be- tween the separated electrons. Such devices operate as distributed charge sen- sors and local electron reservoirs, exemplifying a high functionality to foot- print ratio. Electron shuttling, realised with a dense quantum dot array, pro- vides an alternative path to 2D via locally 1D quantum dot arrays that meet at T-junctions. We apply the concept of electron shuttling to enable run-time scaling-improving pipelining in a theoretical quantum processor architecture proposal. We find a universal, high-fidelity gate set for the silicon realisation, utilising global-only run-time control in the presence of qubit frequency vari- ations, by exploiting the small controllable g-factor Stark shifts. Longer spin qubit couplers are photonic, requiring electrical spin addressability, typically relying on either a higher-spin qubit encoding, or the presence of spin-orbit coupling, or both. We study a silicon-on-insulator nanowire quantum dot de- vice to find electrically driven coherent singlet-triplet-minus Landau-Zener tran- sitions, corresponding to an estimated spin-orbit coupling of approximately 1 μeV, i.e. electrical control rate of 240 MHz.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Qubit couplers for silicon spin qubit architectures |
| Language: | English |
| Keywords: | quantum computing, silicon spin qubits |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy |
| URI: | https://discovery-pp.ucl.ac.uk/id/eprint/10175948 |
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