Decoherence Effects on All-optical Quantum Information Processing

Abstract

It has been thought for long that information is something useful but does not have physical reality, irrelevant to physical laws. Landauers principle concerning the entropy increase accompanying the erasure of a bit implies that this belief is not true. The recent developments in quantum information processing (QIP) have brought the advantages inaccessible for classical means, such as unconditionally secure communication, exponential speed-ups in factoring integers and database search.

Optical QIP, using light for information carrier, has been a popular choice among many candidates due to the significant developments in photon manipulation. In addition, it is easy to combine communication and computation using light. Linear optics using only passive optical elements (e.g. beam splitter) which conserve energy of the states is of interest, as naturally occurring non-linearity is very small.

Decoherence caused by the openness of the system of interest is nowadays considered as a major factor of the occurrence of classicality out of quantum physics. Decoherence destroys coherence inevitable for quantum aspect of information, and is a big obstacle for QIP.

In this thesis, the focus will be put on the effect of decoherence on the optical QIP, particularly on the quantum teleportation proposed by Bennett \textit{et al.}, which is one of the core ingredient of linear optical QIP. Quantum teleportation can be a very efficient way to implement quantum gate operations, and thus the degradation on it will affect the efficiency of total quantum circuit.

I will introduce the two works related to this topic. First, we study entangled coherent states versus entangled photon pairs for practical quantum-information processing. We compare effects of decoherence and detection inefficiency on entangled coherent states (ECSs) and entangled photon pairs (EPPs), both of which are known to be particularly useful for quantum information processing. When decoherence effects caused by photon losses are heavy, the ECSs outperform the EPPs as quantum channels for teleportation both in fidelities and in success probabilities. On the other hand, when inefficient detectors are used, the teleportation scheme using the ECSs suffers undetected errors that result in the degradation of fidelity, while this is not the case for the teleportation scheme using the EPPs. Our study reveals the merits and demerits of the two types of entangled states in realizing practical QIP under realistic conditions.

Secondly, we study quantum teleportation between two different types of optical qubits, one of which is ``particle-like'' and the other ``field-like,'' via hybrid entangled states under the effects of decoherence. We find that teleportation from particle-like to field-like qubits can be achieved with a higher fidelity than that in the opposite direction. However, teleportation from field-like to particle-like qubits is found to be more efficient in terms of the success probabilities. Our study shows that the direction of teleportation should be considered an important factor in developing optical hybrid architectures for quantum information processing.