Fig.1: (a) Scanning electron micrographs comparing typical GaAs and Si/SiGe double quantum dot gate patterns. (b) Few-electron charge stability diagram measured in an accumulation mode Si/SiGe double quantum dot device. Data in the inset demonstrate tunable interdot tunnel coupling. (c) Using pulsed microwaves, we determine state occupation probabilities in order to extract T1 for different device configurations.
Silicon is a very attractive candidate for quantum information processing due to the long spin coherence times that can be achieved. In particular, 28Si is nuclear spin-free, which suppresses hyperfine interaction induced decoherence. Moreover, the spin-orbit interaction is weaker in Si compared with that in GaAs, InAs, and InSb. In order to explore the potential of this material, we study quantum dot devices operated in the few electron regime. Quantum dot devices need to be scaled down in size compared to conventional GaAs devices due to the larger effective mass of electrons in Si [see Fig. 1(a)].
We are currently focusing on dual-gated Si/SiGe quantum dot devices in which a global top gate is used to accumulate carriers in the underlying Si quantum well. Additional fine depletion gates are used to control the quantum dot confinement potential and manipulate the trapped electrons. These devices can access the few-electron regime and demonstrate a tunable interdot tunnel coupling, important pre-requisites for spin and charge qubit operations[see Fig. 1(b)].
Using pulsed-gate techniques, we can operate a quantum dot device as a single-electron charge qubit and systematically measure the charge relaxation time T1 as a function of detuning and interdot tunnel coupling [see Fig. 1(c)]. Our results demonstrate that T1 is tunable over four orders of magnitude, with a maximum of 45 μs [see Fig. 1(d)]. Future work will focus on spin qubits in such devices.
The existence of a six-fold valley degeneracy in the conduction band of Si presents an additional complication that is absent in the GaAs system. Valley degeneracy introduces an unwanted orbital degree of freedom that leads to fast decoherence. The development of ultra-coherent Si spin qubits will require an understanding of how device specifics, such as defects and quantum well step edges, impact the valley splitting. Our future research will explore valley physics and its consequences on electron spin dynamics and coherence.
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