Generation estimation and protection of novel quantum states of spin systems

Abstract

This thesis deals with the generation, estimation and preservation of novel quantum newlinestates of two and three qubits on an NMR quantum information processor. Using the newlinemaximum likelihood ansatz, a method has been developed for state estimation such that newlinethe reconstructed density matrix does not have negative eigenvalues and the errors are newlinewithin the space of valid density operators. Due to interactions with the environment, newlineunwanted changes occur in the system, leading to decoherence. Controlling decoher- newlineence is one of the biggest challenges to be overcome to build quantum computers. To newlinedecouple the quantum system from its environment, several experimental strategies newlinehave been used. These strategies are based on our knowledge of system-environment newlineinteraction and states that need to be preserved. Considering the and#64257;rst case, where the newlinesystem state is known but there is no knowledge about its interaction with the envi- newlineronment. To tackle decoherence in this case, the super-Zeno scheme is used and its newlineefand#64257;cacy to preserve quantum states is demonstrated. The next situation considered is newlinethat where only the subspace to which the system state belongs is known. To address newlinesuch a situation, the nested Uhrig dynamical decoupling scheme has been used. The newlinelater part of the thesis deals with situations where the state of the system as well as its newlineinteraction with the environment is known. In such situations, since the noise model newlineis known, decoupling strategies can be explicitly designed to cancel this noise. Using newlinethese decoupling strategies, the lifetime of time-invariant discord of two-qubit Bell- newlinediagonal states has been experimentally extended. The decay of three-qubit entangled newlinestates namely the GHZ state, the W state and the WWand#772; state are studied, and the noise newlinemodel is constructed for the spin system. The experimentally observed and theoretical newlineexpected entanglement decay rates of these states are compared. Then, the dynamical newlinedecoupling scheme is applied to these states and remarkable protection is observed in newlinethe cas