Control of Open Quantum Systems

Abstract

Control of quantum mechanical systems has become an important topic of research in recent years due to quantum supremacy: quantum mechanical information processing devices have the potential to solve problems that are regarded infeasible to solve by classical information processing devices and thereby surpass them. Qubits, the building block of a quantum computer, is very delicate and can lose information by interacting with the environment. Thus, it is essential to make the qubit more robust against errors due to environmental interactions. In this thesis, we have investigated about improving the GRAPE algorithm that prepares controls in NMR for taking the qubit from one state to another. For the case of pure dephasing, we have modeled the decohering effects of the environment as a Noise Hamiltonian that cause phase accumulation due to local fields. Further we incorporate this noise Hamiltonian into a GRAPE Simulated Annealing Hybrid Algorithm with improved the rate of convergence, to make pulses that are more robust against environmental interactions. We have experimentally tested these Robust Hybrid Pulses to prepare the Long-Lived Singlet States for 2 spin 1/2 system and found that the robust pulse sequences produced more robust states. We have also studied the e ects of dynamical decoupling sequences in extending the relaxation time for the states and have performed noise spectroscopy experiments for different spin systems to understand better the dependence of noise spectrum on the environment of the spin system. Using the noise spectrum, we have constructed a System Specific Dynamical Decoupling Sequence that is better at suppressing decoherence of the spin system.