Controlling the evolution of a quantum system with dynamical decoupling methods
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Abstract
This thesis discusses ways to manipulate the dynamics of chains of spins-vi, The evolution of these systems can be altered by applying sequences of magnetic pulses to specific sites within the chain. The control of the system evolution is interesting, for example, in relation to the development of quantum computers. Classical computers represent information by strings of binary digits, or bits. Quantum computing would be based on quantum bits, or qubits, which can be in any superposition of the classical bit's two possible values. As a result of this superposition, a string of qubits can hold more information than a string of classical bits. Quantum computers would be at least as fast as classical computers on some classes of problems and would speed up finding solutions for many other problems. However, challenges lie ahead for the pioneers of quantum information before we can usefully take advantage of the power of quantum systems. One difficulty is to create a physical quantum system with which scientists can implement the theories that have developed around quantum computing. Another main challenge is the presence of decoherence, where quantum systems are easily disturbed by their environment and the information stored in the quantum states can leak out. This thesis will examine a promising method to prevent decoherence by decoupling the system from its environment with a sequence of magnetic pulses. These so-called quantum control methods have also been used to freeze the system evolution by cancelling out its internal interactions. But here, instead of preventing decoherence or freezing the system evolution, the application of these methods is extended to analyze how they can be used to induce a desired dynamics in the quantum systems under investigation.