ملخص البحث :

Quantum key distribution aims to distribute a secret key among distant parties linked by an optical fiber or free space. Unfortunately, the main problem which limits the long distance direct transmission of qubits is the exponential decay of the signal. This limit can be overcome by introducing quantum repeaters (QRs) between the two distant points. Quantum repeaters suggest a technique for establishing long distance quantum communication by the construction of entangled qubits among the end point of the channel, by dividing the channel into elementary links edged with quantum memory (QM) called nodes. Many schemes use identical QMs (Mastromattei in Assessing the practicality of a simple multi-node quantum repeater. M.Sc. thesis, University of Waterloo, ON, Canada, 2017). These schemes need QMs of long life times and high efficiencies which is not the practical case for QMs. To optimize these requirements, a multi-node sequential quantum repeater of a non-identical QMs is proposed here to interplay between the total channel efficiency and decoherence time T2 and the quantum memories life times and their efficiencies to enhance the execution of the quantum repeater (QR). The outcomes show that the interplay between the decoherence time and efficiency in non-identical QR improves the key rate as compared to that of the identical QR, the distance of the channel can be extended by using non-identical QMs repeater scheme since quantum bit error rate is decreased, and the cost of fabricating non-identical QMs repeater is reduced since we use most QMs with low life time.

ملخص البحث :

One of the remarkable issues of quantum key distribution (QKD) is the subject of how to distribute key over randomly long distances. The transitivity of an optical channel diminishes quickly as the length of the channel develops (exponentially, on account of fiber). To overcome this issue, quantum repeaters (QRs) are first proposed by Briegel et al., where an entanglement connection method is utilized to expand the whole length of entanglement by using multi pairs quantum memories (QMs) components. Previous QRs use identical quantum memories (QMs) to include their nodes . The execution of the quantum channel is influenced by the dephasing parameter dpn f alludes to the degradation of the state stored in a QM after some time. Thus, our scheme of QRs utilizing non - identical QMs of various decoherence (storage) times T2 and efficiencies, of the QRs nodes is proposed here to increase the dephasing parameter dpn f and lessen the errors caused by dephasing. Practically, entangling two nonidentical QMs is demonstrated. The outcomes show that the proposed QRs scheme demonstrates better execution contrasted with the schemes utilizing identical QMs.

ملخص البحث :

In this work, a new scheme for quantum repeater based on multiplexed single photon source is suggested. Long distance quantum communication requires establishment of a distant pair of entangled quantum bits, e.g. photons presented in laser sources, that is the first step towards the more secure message transmission. There are many proposed schemes to achieve such task using many different ideas.which suggest a quantum repeater QR protocol based on pair photon sources PPS. But, pair photon source suffers from a mechanism error due to probability of losing one photon of pairs during its transmission through the fiber or by detector failure, it will not have the desired entangled state consequently these error's effect on entanglement distribution rates. However, single photon sources suggested to implement the new quantum repeater scheme, which makes it possible to eliminate errors inherent in quantum repeater protocols using photon-pair sources.. Here, an efficient architecture for quantum repeaters based on multiplexing single-photon sources MUX-SPS in combination with multiplexing multimode quantum memories MM-QM is suggested to make improvement in entanglement distribution over long distance quantum communications, using only a single photon to entangle an arbitrary photon number of remote quantum memories.