Electron Spin Resonance on Si/SiGe Quantum Dots

Abstract

Qubits in a solid state environment are promising candidates for large scale quantum computing, but they usually suffer short decoherence times. Electron spin qubits in silicon quantum dots are expected to have long coherence because of the weak spin-orbit coupling and hyperfine interaction in silicon. In this thesis we study the spin relaxation and decoherence in Si/SiGe heterostructures using ensemble electron spin resonance (ESR), and report the first measured long coherence time of 250 μs of a single electron spin in a Si quantum dot.

The first experiment was performed on a single gated Si/SiGe heterostructure device. We observed a residual ESR signal originating from the Si quantum well at gate voltages below the conduction threshold. The signal exhibits a Curie temperature dependence, leading us to attribute it to isolated electrons trapped by intrinsic disorder in the quantum well, referred as natural quantum dots. From these confined electron spins we measured the relaxation time, T1 &sim 0.5 ms, and coherence time, T2 ∼ 16 μs. The long T1 and the fact that T1 > T2 suggest a strong suppression of the Dyakonov-Perel relaxation mechanism, which arises from spin-orbit coupling and is dominant in two dimensional electron gases. The high density of natural quantum dots in this wafer suggests spin coherence is most likely limited by the electron exchange interaction between neighbouring dots.

In the second experiment we fabricated a dual-gated structure on an ultra-high mobility, undoped Si/SiGe heterostructure wafer. The device accommodates &sim; 10<super>8</super> electrostatically gated quantum dots for ensemble ESR. Our data show that approximately one unpaired electron spin is confined per dot. At 0.35 K this single electron spin has a T1 of 280 &mu;s and a T2 of 250 &mu;s, demonstrating the expected long coherence of an electron spin in a silicon quantum dot. Both T1 and T2 have weak temperature dependencies from 0.35 K to 0.8 K, and show a non-monotonic dependence on the gate voltages. These results indicate that several possible mechanisms may be responsible for the relaxation and decoherence in the gated quantum dots, including multi-electron occupancy, valley degeneracy and spin-orbit coupling.

Finally, a microwave circuit based on a cryogenic low noise amplifier is demonstrated as a pre-amp for ESR experiments. The circuit operates at 4.2 K and increases the signal to noise ratio by an order of magnitude for pulsed ESR. It can be integrated with a <super>3</super>He cryostat or a dilution refrigerator for future experiments in detecting weak microwave signals.