Two-Qubit Gates for Donor-Based Silicon Quantum Computing

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Copyright: Kranz, Ludwik
We investigate two-qubit operations between precision-placed atom qubits in silicon and resolve key obstacles in achieving high- fidelity two-qubit gates. The three central results presented in this thesis are: I) the development of a donor-bound single electron spin qubit, II) demonstration of a two-qubit SWAP gate between donor qubits, III) reduction of charge noise and quantum state leakage for high- fidelity two-qubit gates. We fi rst construct a theoretical model to optimise the readout fidelity and show that readout capabilities can be improved by atomic-scale optimisation of the charge sensor tunnel gaps. We then use electron spin resonance techniques to control the electron spin states hosted on a donor molecule capable of binding up to five electrons. We show the fi rst demonstration of coherent spin control in precision-placed donor qubits in silicon. The coherent control is achieved for both one and five electrons present in the donor molecule, with pure dephasing time T2* = 158 ns and coherence time T2 = 343 us. Through atomic-scale engineering of each aspect of our devices we demonstrate the first two-qubit SWAP gate between donor electrons with a logical basis fidelity of 90%, limited by charge noise. Measurement of the two-spin states is achieved in both the singlet-triplet basis, with a high readout fidelity of 99.98%, and the single-spin basis by measuring both qubits independently within a timeframe of only 27 us with sequential readout delity of 90.6%. Finally, we study the dynamics of the quantised donor nuclear spin states and show that their impact on two-qubit gate operation can be mitigated by shielding the qubit with additional electrons. Motivated by the coherence time of our two-qubit gate we measure the charge noise spectrum of our device using both the charge sensor and the qubits themselves. We optimise the epitaxial growth to reduce the power spectral density of charge noise at 1 Hz down to 0.0088 ueV2/Hz, an order of magnitude lower than previously reported for silicon qubits. We also construct a theoretical model to study the impact of nuclear spins on the quantum state leakage during two-qubit CNOT gate. We show that leakage can be notably reduced by using multi-donor dots and utilising their nuclear spins as nano-magnets. By optimising the confi guration of donors within the silicon crystal, we show that CNOT gate fidelities as high as 99.97% are theoretically achievable in donor qubits under realistic noise conditions.
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PhD Doctorate
UNSW Faculty