Preserving Information in Quantum Systems via Coherent Driving

Abstract

Among the fundamental phenomena that characterize a quantum system is coherent superposition between states, which we can write for a two-level system as: |ψ⟩ = α|0⟩ + e^(iφ) β|1⟩ A superposition is characterized by a phase φ which can be difficult to conceptualize with our physical intuition, trained only by experience in a classical world. Perhaps we can best approach it by considering the phase of classical waves, which we can concretely visualize, and which lead to some analogous phenomena like interference. In effect there is additional information in the system beyond the probability that it is measured in one state vs. the other. This phase is both a resource and a vulnerability of quantum systems. On one hand, it gives the system another degree of freedom which we can control. On the other hand, it can be easily lost, since the phase of a superposition tends to be very sensitive to fluctuations in the system's surrounding environment. The length of time that the phase remains known in a system (or equivalently, that the system maintains its information) is called the coherence time. In this thesis I will discuss two examples in which a coherent superposition of states is used to preserve information in a quantum system. In both cases the key laboratory tool used to access the quantum system is an AC field of classical amplitude which coherently drives the states, and in both cases we will see the beneficial properties of the stationary solutions of the combined system plus field, which are superposition states (in the basis that does not include the field). Chapter 1 will deal with an example of a coherent field extending the coherence time of a two-level system. I will show experimental data demonstrating this effect in the electron spin of the silicon vacancy in diamond, where the coherent driving field is a strain field in the carbon lattice. Chapter 2 will explain how the decay channels of a single atomic system can be controlled using superpositions of states, a phenomena which could be used to increase the fidelity of a state measurement. These results are computational rather than experimental, but I will suggest a real quantum system in which this phenomena could be demonstrated and calculate expected results.