Design and Implementation of Quantum Repeaters:Insights on Quantum Entanglement Purification

Quantum communication is a groundbreaking technology that is driving the future of information transmission and communication technologies to a new paradigm.It relies on quantum entanglement to facilitate the transmission of quantum states between parties.Quantum repeaters are crucial for facilitating long-distance quantum communication.These quantum devices act as intermediaries between adjacent communication channel segments within a fragmented quantum network,allowing for entanglement swapping between the channel segments.This entanglement swapping process establishes entanglement links between the endpoints of adjacent segments,gradually creating a continuous entanglement connection over the entire length of the transmission channel.The established quantum link can be utilized for secure and efficient quantum communication between distant sender and receiver nodes.This study focuses on quantum entanglement purification,a protocol aimed at maintaining high fidelity entangled states above the operational threshold of the communication channel.This study investigates the optimal stage for executing the purification protocol and applies optimization schemes to evaluate various purification protocols.We use IBM Qiskit for circuit implementation and simulation.The results offer valuable insights into future approaches to implementing practical quantum repeaters and shed light on existing and anticipated challenges.

Efficient entanglement purification in quantum repeaters

We present an efficient entanglement purification protocol （EPP） with controlled-not （CNOT） gates and linear optics. With the CNOT gates, our EPP can reach a higher fidelity than the conventional one. Moreover, it does not require the fidelity of the initial mixed state to satisfy 1 2. If the initial state is not entangled, it still can be purified. With the linear optics, this protocol can get pure maximally entangled pairs with some probabilities. Meanwhile, it can be used to purify the entanglement between the atomic ensembles in distant locations. This protocol may be useful in long-distance quantum communication.

Building a Large-Scale and Wide-Area Quantum Internet Based on an OSI-Alike Model

The theory and experiment of quantum information have been studied extensively in recent years,and the feasibility of quantum communication has been proved.Although the fundamental technology is not yet mature,research on quantum internet should be conducted.To implement quantum internet,an architecture that describes how quantum nodes are linked to form networks and how protocol functions are vertically composed need to be developed urgently.In this paper,we present a novel design of a clusterbased structure to describe how quantum nodes are interconnected,and how the structure can improve the performance of qubit transmission and reduce the network complexity.The idea of the quantum local area network(QLAN)is proposed as an essential component of the quantum internet.Besides,each quantum repeater links to neighboring repeaters to form a core network,and multiple QLANs are connected through the core network.The core network can be grouped into different hierarchical quantum repeater networks according to needed service requirements.For the sake of interoperability and fast prototyping,we adopt the idea of OSI layering model of the current Internet in the design of quantum internet.Finally,we elaborate on the composition of quantum nodes and the realization of end-to-end communication.

An efficient and long-lived quantum memory for quantum repeater

A research team led by Prof.Pan Jianwei(潘建伟)and Prof.Bao Xiaohui(包小辉)at the University of Science and Technology of China,reported the successful realization of an efficient quantum light-matter interface with sub-second lifetime,which can be used as an elementary unit to extend the distance of quantum communication through quantum repeater.This result was recently published in Nature

Experimental one-step deterministic polarization entanglement purification

Entanglement purification is to distill high-quality entangled states from low-quality entangled states.It is a key step in quantum repeaters,determines the efficiency and communication rates of quantum communication protocols,and is hence of central importance in long-distance communications and quantum networks.In this work,we report the first experimental demonstration of deterministic entanglement purification using polarization and spatial mode hyperentanglement.After purification,the fidelity of polarization entanglement arises from 0.268±0.002 to 0.989±0.001.Assisted with robust spatial mode entanglement,the total purification efficiency can be estimated as 10^(9) times that of the entanglement purification protocols using two copies of entangled states when one uses the spontaneous parametric down-conversion sources.Our work may have the potential to be implemented as a part of full repeater protocols.