Generation, estimation, and protection of novel quantum states of spin systems

TL;DR

This thesis explores methods for generating, estimating, and protecting novel quantum states of spin systems, employing advanced decoupling techniques to mitigate decoherence and preserve quantum correlations in NMR-based quantum information processing.

Contribution

It introduces a maximum likelihood-based state estimation method and applies various dynamical decoupling schemes to effectively control decoherence in quantum spin systems.

Findings

Decoherence can be significantly reduced using super-Zeno and nested Uhrig dynamical decoupling schemes.

The lifetime of time-invariant discord was experimentally extended.

Entanglement preservation in three NMR qubits was achieved to a remarkable extent.

Abstract

This thesis deals with the generation, estimation and preservation of novel quantum states of two and three qubits, on an NMR quantum information processor. Using the maximum likelihood ansatz, we have developed a method for state estimation such that the reconstructed density matrix does not have negative eigenvalues and the errors are within the space of valid density operators. Due to interactions with the environment, unwanted changes occur in the system, leading to decoherence. Controlling decoherence is one of the biggest challenges to be overcome to build quantum computers. We have used several experimental strategies to decouple the quantum system from its environment. These strategies are based on how much we know about system-environment interaction and what states we want to preserve. We first consider a case where we are aware of the system state but have no knowledge about its interaction with the environment. We demonstrate the efficacy of the super-Zeno scheme to tackle decoherence in this case. Then we consider a situation where only the subspace is known to which the system state belongs. To address such a situation we used a nested Uhrig dynamical decoupling scheme. Next, we consider situations where we have knowledge of the state of the system as well as its interaction with the environment. In such situations, since the noise model is known, decoupling strategies can be explicitly designed to cancel this noise. Using these decoupling strategies, we can experimentally extend the lifetime of time-invariant discord of two-qubit Bell-diagonal states and are also able to preserve the entanglement of three NMR qubits, to a remarkable extent.