Short answer: A quantum repeater is a device meant to extend how far quantum information can be sent over fibre, by dividing a long link into shorter segments and joining them with entanglement swapping. It is needed because quantum signals cannot be copied and amplified the way classical ones can. A 2023 Reviews of Modern Physics article calls realizing one perhaps the most crucial challenge before a quantum internet can exist, and describes experimental progress as work in progress.
Why classical amplification fails in quantum networks
In classical communication, signals can be copied and amplified to counteract attenuation. In quantum communication, the signal is impossible to copy due to the basic principles of quantum mechanics, specifically quantum state collapse and the no-cloning theorem. Consequently, the communication range is limited by inevitable photon loss. Unlike classical systems, one cannot simply boost the signal strength without destroying the quantum information. This fundamental constraint necessitates a different architectural approach to extend the range of quantum communication.
The scattering of photons in optical fibers and the difficulties in creating full-fledged quantum nodes impede the construction of large quantum networks. These physical limitations mean that point-to-point quantum communication faces strict limits on communication rates. To address these issues, researchers have proposed inserting several repeater stations between two remote end nodes. This strategy divides the full distance into several shorter segments, allowing for the successive operation of entanglement swapping between adjacent repeater stations. A 2021 review says this enables long-distance quantum communication.
Architectural generations and error reduction
A 2021 review says quantum repeaters can be classified into three generations based on the methods of reducing errors resulting from photon loss and imperfect operations. That review discusses the advantages and challenges of each generation.
The 2023 review also discusses various near-term proposals to overcome the limits on communication rates set by point-to-point quantum communication.
Role in the quantum internet and cryptography
The 2023 review calls a quantum internet the holy grail of quantum information processing, enabling the deployment of a broad range of quantum technologies and protocols on a global scale. It says that, of the challenges to be addressed before it can become a reality, perhaps the most crucial is realizing a quantum repeater. They fit within the broader architecture of quantum networks, which allow generating entangled states between distant qubits.
A 2021 review says such networks have enormous scientific and applied potential. They can be used for secure quantum cryptography and the teleportation of quantum states between cities and countries. Additionally, they support applications in high-resolution astronomy and distributed quantum computing. Quantum repeaters were proposed in 1998 to compensate for losses due to the absorption of photons. Their implementation would allow not only implementing quantum key distribution systems at long distances but also creating a quantum internet. That review also discusses ways to increase the range of quantum cryptography without repeaters.
Current status and experimental progress
The realization of a quantum repeater is an essential component in the long-distance transmission of quantum information. The 2023 review covers the conceptual frameworks and architectures as well as experimental progress toward realization. It describes work toward repeaters, not deployed systems.
The 2021 Physics-Uspekhi review says the scattering of photons in optical fibre and the difficulties in creating full-fledged quantum nodes impede the construction of large quantum networks. This is research-stage work, and we make no claim about production availability.
FAQ
Why can't quantum signals just be amplified?
A 2021 review says quantum signals cannot be copied, because of quantum state collapse and the no-cloning theorem, so the communication range is limited by photon loss.
What does a repeater do?
It divides the full distance into shorter segments with repeater stations and successively performs entanglement swapping between adjacent stations, enabling long-distance quantum communication.
How are repeaters classified?
The 2021 review says into three generations, based on the methods used to reduce errors from photon loss and imperfect operations.
When were they proposed?
In 1998, according to a 2021 review in Physics-Uspekhi.
Do working quantum repeaters exist?
The 2023 review covers experimental progress toward realizing them. This article makes no claim of operational or commercial deployment.
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