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Coherent Control and Spectroscopy of Valley States of Individual Electrons in Silicon Quantum Dots


The pursuit of a quantum computer has been driven both by the prospect of solving otherwise intractable problems as well as the desire to master the quantum control of mesoscopic systems. Lateral quantum dots in semiconductors, particularly in silicon, have garnered considerable attention as a potential host for a quantum bit formed from the states of confined electrons.

The conduction states of electrons in silicon are characterized by an additional degree of freedom, known as the valley degree of freedom. Valley states have certain properties which represent both a complication and an opportunity for the use of electrons in silicon as a qubit. Valley states are 6-fold degenerate, reducing to being very nearly 2-fold degenerate when confined to a quantum well, meaning they have the potential to disrupt many qubit readout schemes for spin based on the Pauli exclusion principle. Also, they are rapidly oscillating relative to one another, making their interaction highly sensitive to atomic scale properties of the confining potential, which are difficult to control when fabricating devices. However, they share many spin-like properties leading to the possibility of familiar control protocols and long coherence times.

We have developed techniques for the fabrication of lateral double quantum dots and experimental procedures for the coherent manipulation and readout of quantum states of confined electrons. Through the use of fast pulses, quantum oscillations between two valley states are induced, allowing the energy splitting between these valley states to be probed to a resolution not possible with conventional methods of valley spectroscopy. The fabrication techniques and experimental procedures have led to the detection of quantum oscillations in multiple devices, demonstrating the utility and generality of the techniques.

In the course of refining these experimental techniques, we developed software allowing for the linear compensation of multiple experimental parameters. These improvements led to the ability to efficiently adjust the inter-dot tunneling rate, the dot-reservoir tunneling rate and the charge sensing channel sensitivity, control parameters whose fine tuning was vital to the success of the quantum manipulation and readout ultimately achieved.

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