2012

Ph.D

HBNI

The study of entanglement has gained prominence in recent years due to the advent of fields such as Quantum Optics and Quantum Information Theory — advances that have harnessed such counter-intuitive quantum phenomena into elements of everyday life, improving it in the process. Ideal quantum systems, “closed” to the outside world, remain quantum forever and thus manage to retain entanglement. Real quantum systems, however, are “open” to the environment and are therefore susceptible to the phenomenon of decoherence. The resultant loss of entanglement is a major hindrance to the effectiveness of quantum information tasks. This thesis studies the evolution of entanglement in various types of open quantum systems (OQS) coupled in various ways to local baths. Also studied - existing ways and means of controlling the decay of entanglement and have proposed a new method of doing so. The evolution of entanglement in OQS undergoing Markovian dynamics is studied by using the Lindblad master equation as well as the method of quantum trajectories. The author has analyzed the onset of the phenomenon of entanglement sudden death in finite as well as infinite dimensional OQS, connected either locally to a thermal bath or its squeezed variant, or via a quantum non-demolition-type (QND-type) interaction to a local thermal bath. It is found that the QND-type system-bath interaction works best to conserve entanglement in finite dimensional systems, whereas a squeezed thermal bath causes entanglement sudden death even at zero temperature. Also studied some well-known methods of controlling decoherence in open systems with respect to their ability of preserving entanglement. Some of these procedures include coupling the system to a thermal bath of photonic crystals where the photonic band gap suppresses decoherence, modulation of the system bath frequency in an attempt to contain decoherence, using the method of resonance fluorescence where an external field modulates the transition frequency of the two-qubit systems with each qubit being a two-state atomic system to contain decoherence, and using high-frequency radio waves to decouple the system and bath dynamically and thus reducing decoherence. It is shown through numerical computation, that an ancilla in the ground state extends the lifespan of entanglement in the main system. Increasing the size of the ancilla, that is, increasing the value of n in an n-qubit ancilla slows down entanglement loss for a two-qubit system connected locally to a thermal bath.