Developing a Logical Qubit in Silicon
Zoé Bertrand
Logical qubits are essential for utility scale universal quantum computation and require stable, high-fidelity physical qubits. In this thesis we consider three factors that can affect the performance of phosphorus atom qubits in silicon. Specifically, we determined the number of phosphorus atoms in few-atom qubit registers using the charging energy. We then explored if a new silicon encapsulation recipe could reduce charge noise under realistic qubit operating conditions. Finally, we performed experiments on few qubit devices to identify the dominant causes of qubit decoherence. To attain high-fidelity control requires characterising the underlying atomic structure of qubit registers (≤4P). By combining precise (≤5% error) experimentally measured charging energies with high quality STM images and theoretically modelled charging energies, we show the number of phosphorus atoms present can be determined confidently. Previously, our all-epitaxial nat-Si devices have demonstrated record low charge noise (0.17µeV). In this thesis we consider a new Si-28 growth recipe to assess if introducing an annealed ∼1nm silicon locking layer grown at room temperature can further increase the silicon encapsulation quality, whilst minimising P segregation. We first optimised the geometry of SET charge sensors fabricated in Si-28 to obtain high current contrast (≫1nA). For the locking layer (standard recipe) single electron transistors we found high (low) levels of charge noise at 0V (2.1µeV) that increased with the applied voltage (at 0V (0.29µeV) and 0.5V (0.23µeV)). In addition, we found that the electrical behaviour of the locking layer sensors was highly sensitive to the applied voltage. Since qubits are typically operated at ∼0.5V, further work is needed to assess the viability of this new growth recipe. Finally, we investigated different charge and magnetic noise sources that cause qubit decoherence in a 3P register using a modified Hahn echo experiment. Here, we use resonant drive at the frequencies of different nuclear spin species to identify P-31 in the SET island as causing qubit decoherence. Further, we found that the dominant charge noise source for electron spin qubits was the Stark shift of the g-factor. We also found the spin flip rate of phosphorus nuclei in the spin bath was affected by a change in the mechanical noise coupling into the fridge. For nuclear spin qubits we found the dominant noise source to be the Stark shift of the hyperfine interaction.
•Oct 8, 2026•0 citations