Implementing Quantum Information Processing Via The Optical Coherent Pulse

In this thesis, several schemes for the implementing of controlled-phase gate and entanglement purification and quantum cloning are presented by using via the coherent optical pulse as a quantum communication bus (qubus). In the present schemes, the coherent optical pulse sequentially interacts with two qubits separated by distances. The interaction applied for a time t generates a conditional phase-rotation on the qubus coherent state dependent upon the state of qubit. We exploit the coherent optical pulse as qubus to mediate interaction between qubits that never meet, instead of direct qubit-qubit interaction. Next we should measure the coherent state in order to implement the certain tasks of quantum information processing (QIP). For implementing the QIP task, such a measurement would not even be required. Based on such a method for achieving the QIP task, the advantage is that:the quantum information is encoded into discrete variables (DVs). Almost all proposals for realizing a quantum computer rely exclusively on discrete variables. While the CVs mode, the coherent optical pulse, plays the role of a quantum communication bus to mediate interactions between qubits that are separated by distances. There are now also efficient and practical approaches to quantum communication based on continuous variables. The CVs are the phase-space variables of this field mode. This approach brings together the best of both worlds, utilizing DVs for processing and CVs for communication.Using single photon as the carrier of quantum information, we present a scheme for distributed controlled-phase gate, a scheme for bipartite entanglement purification and a scheme for tripartite entanglement purification. Interactions between intense coherent state of qubus beam and distant photons are generated by using weak cross-Kerr nonlinearities. For the three schemes, the strength of the nonlinearities required for the present scheme is orders of magnitude weaker than those required to perform controlled-phase gates naturally between the single photons. Good candidates for weak cross-Kerr nonlinearity are atomic ensemble working under electromagnetically induced transparency (EIT) conditions. We have shown a simple scheme for constructing this efficient CZ gate with few resources. The biggest advantage of our scheme is that ancillary single-photons are not needed. However, two or more ancillary single-photons are essential for the previous protocols. Both of the total success probability and fidelity of the present CZ gate approach unit in principle.We have proposed and demonstrated two efficient and simple schemes for entanglement purification with the help of weak cross-Kerr nonlinearity. Two distant less-entangled photons through interactions with the qubus beams in the coherent states are able to be purified to a maximally entangled state, instead of utilizing classical communication and ancillary photon pairs. The present schemes for entanglement purification can improve the fidelity of less-entangled pairs without the help of performing continuous indefinite iterative purification procedure. Hence this represents a huge saving in the physical resources to implement entanglement purification. For the present each scheme of purification, the total success probability and fidelity of can approach unit in principle. These protocols do not require the sophisticated single-photon detectors and the CNOT gate for the linear-optics-based scheme. For the purification protocol to improve the fidelity of the output state, it is not essential that the fidelity of the resource states being employed is above 25%.In the present cloning scheme, the entangled qubit pair (two electron-spin systems in separate cavities) is constructed deterministically through a sequence of qubit-qubus interactions without requiring any subsequent measurement because the qubus mode is disentangled automatically from the qubits at the end of interactions. Thus the present cloning scheme could effectively avoid measurement-induced errors. Additionally, the measurement-free operation in the present scheme not only simplifies the cloning procedure, but also makes the cloning transformation faster. Due to the deterministic and measurement-free operation in our scheme, the success probability of quantum cloning, which is unit in principle, is superior to the previously reported ones. In contrast to the previous scheme, the coefficient of the output state of our cloning machine is just the product of two trigonometric functions due to introducing the controlled-rotation operation. This ensures that different types of quantum cloning machine could be achieved readily in the same framework by adjusting appropriately the rotated angles.In the four schemes, we investigate photon loss of the qubus beams during the transmission and decoherence effects caused by such a photon loss. Decoherence due to the qubus beam loss in the transmission channel erodes the success probability and fidelity in each scheme. Photon losses of the qubus beam during the transmission should be sufficiently small if we want to obtain both high success probability and fidelity. It is easy to recognize that the parameters should be reasonabe values for the quantum gate to work properly at the end. The fewer we can make photon loss by controlling the decay constant of the qubus beam in optical fiber, the higher the success probability and fidelity of the quantum gate can be achieved. Phase noise in our schemes for controlled-phase gate and bipartite entanglement purification and tripartite entanglement purification can also be eliminated by the double cross-phase modulation method. As long as the degree of the qubus photon loss is known in the scheme for quantum cloning, these effects can be eliminated by increasing the amplitude of the controlled displacement.