Quantum Optical State Preparation for Quantum Communication

Abstract

Nonclassical states of light are essential for long-distance quantum communication. In this dissertations, we theoretically analyze the state preparation of nonclassical states of light, resource-efficient quantum optical information processing, decoherence in quantum optical communications, and the use of the quantum Zeno effect to protect the phase of a quantum clock. An essential component of a quantum network is entanglement distribution. We study a method that can encode quantum information in entangled macroscopic superposition states, which typically carry a large number of photons. This is based on the generation of phase-entangled Schr�dinger cat states using linear optical elements such as beam splitters for possible application in entanglement distribution. Controlled phase shifts can be used to verify the entanglement of the Schr�dinger cat states. We then show how linear optical elements can be used to implement a controlled phase shift efficiently, with possible applications in quantum repeaters. Decoherence can arise from photon loss in quantum communication applications. Nevertheless, noiselessly attenuating single rail qubits prior to the transmission can suppress the effects of loss in the channel. A linear optical realization of noiseless attenuation is described in phase space by conditional measurements of zero photons in one of the output ports of a beam splitter. We study this approach and analyze the coherence of quantum states that have been attenuated using this operation. Finally, we explore the use of the quantum Zeno dynamics to protect multi-atom clocks from phase drift.