Quantum repeaters are the most promising approach for distributing entanglement over long distances. Recent approaches to develop quantum repeaters involve the use of deterministic entangled photon pair sources. On the other hand heralded entangled photon pair sources have been proposed independently both with parametric down conversion source and single photon sources. We modify the latter scheme to implement it as an on demand entangled photon pair source and analyse its performance considering inefficient detectors, inefficient quantum memories and inefficient single photon sources. We conclude that with the current state of art, generation of high fidelity deterministic entangled photon pairs is possible with moderate efficiency. We then compare the results to the deterministic entangled photon pair source obtainable using parametric down conversion source and conclude that the single photon scheme described in this thesis is more practical for such an implementation.
High-rate entanglement generation is essential for the realization of practical quantum repeaters. To this end, frequency multiplexing of the photons employed is an effective approach. In particular, schemes using cavity-enhanced spontaneous parametric down-conversion (cSPDC) as a photon-pair source have been proposed. In this study, toward a theoretical performance evaluation of frequency-multiplexed quantum repeaters based on cSPDC, we derive an approximate expression for the quantum state of the frequency-multiplexed photons, where each frequency mode is treated as an independent two-mode squeezed vacuum (TMSV) state. Using this expression, we calculate the heralding probability and fidelity of entanglement generation for several cases in a single-photon interference scheme using frequency multiplexing with cSPDC. Our results demonstrate that by multiplexing approximately 100 modes, the heralding probability improves to approximately 98% for an elementary link distance
LEL=25km, even in scenarios where the fidelity exceeds 0.9 across all modes. Furthermore, for
LEL=100km, we show that the heralding probability, which was approximately 0.7% in the single-mode case, increases to about 38% under the condition that the fidelity remains 0.9 or higher. These analytical results demonstrate the effectiveness of employing cSPDC as a photon-pair source for quantum repeaters.
Quantum repeater is the key element in quantum communication and quantum information processing. Here, we investigate the possibility of achieving a heralded quantum repeater based on the scattering of photons off single emitters in one-dimensional waveguides. We design the compact quantum circuits for nonlocal entanglement generation, entanglement swapping, and entanglement purification, and discuss the feasibility of our protocols with current experimental technology. In our scheme, we use a parametric down-conversion source instead of ideal single-photon sources to realize the heralded quantum repeater. Moreover, our protocols can turn faulty events into the detection of photon polarization, and the fidelity can reach 100% in principle. Our scheme is attractive and scalable, since it can be realized with artificial solid-state quantum systems. With developed experimental technique on controlling emitter-waveguide systems, the repeater may be very useful in long-distance quantum communication.