Definition of quantum repeater

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Photonics Dictionary

quantum repeater: A quantum repeater is a crucial component in quantum communication networks, designed to extend the range over which quantum information can be transmitted. Quantum information, typically carried by quantum bits (qubits), is extremely fragile and can easily be corrupted by noise or loss during transmission. In classical communication, repeaters (amplifiers) are used to boost signals over long distances, but the direct amplification of quantum signals is not possible due to the no-cloning theorem, which prohibits copying an unknown quantum state.

Quantum repeaters overcome this challenge through a combination of techniques:

Entanglement swapping: Quantum repeaters use entanglement swapping to extend the entanglement between qubits over long distances. In entanglement swapping, two pairs of entangled qubits (A-B and C-D) are used to create a new pair (A-D) without direct interaction between A and D.

Quantum error correction: Quantum error correction schemes are employed to protect the qubits from errors introduced during transmission and storage.

Quantum memory: Quantum repeaters utilize quantum memory to temporarily store qubits. This allows for the synchronization of entanglement distribution, which is necessary for entanglement swapping and error correction processes.

Teleportation: Quantum teleportation is used to transfer quantum states between distant locations using pre-shared entanglement and classical communication. This process is integral to the operation of quantum repeaters.

Functionality of a quantum repeater:

Initial entanglement generation: The repeater generates entanglement between two quantum nodes that are within a relatively short distance from each other.

Segmentation: The total distance between the source and destination is divided into shorter segments. Entanglement is generated and stored at intermediate nodes along these segments.

Entanglement swapping and purification: At each intermediate node, entanglement swapping is performed to extend the entanglement over larger segments. Additionally, entanglement purification techniques are applied to improve the quality of the entangled states by reducing noise and errors.

Repeating the process: The process of entanglement swapping, purification, and storage is repeated along the chain of nodes until entanglement spans the entire distance between the source and destination.

By using these techniques, quantum repeaters enable long-distance quantum communication and are essential for building a global quantum internet, which would facilitate secure communication, distributed quantum computing, and other applications of quantum technology.