Quantum Repeaters

Quantum Repeaters for the Novice

Quantum Repeaters

Why  research on quantum repeaters is important?

What are the future applications of quantum repeaters? What are the spin-off effects for fundamental physics?

The distribution of quantum states over long distances is essential for future applications such as quantum key distribution and quantum networks. The direct distribution of quantum states is limited by unavoidable transmission losses of the channel used to transmit these quantum states. The direct approaches are limited to much less than 500 km, even under the most optimistic assumptions for technology evolution.

A promising alternative for long distance quantum states distribution is the use of quantum repeaters. Quantum Repeaters can be thought of as being analogous to the optical amplifiers that provide an economic and compact solution for long distance classical communication. However, whereas the idea of amplifiers is to regenerate the classical optical signal, what these Quantum Repeater links do is to create sections of lossless transmission line over which the quantum state is teleported.

Quantum repeaters are also important from the point of view of fundamental physics. The set of tools – tools for the different domains: theory, simulation, experiment - which will be developed to implement a first quantum repeater, can help to improve our understanding of quantum physics. E.g., the key technology for implementing quantum repeaters are quantum memories. They allow the storage of quantum states. To bring the quantum memory technology to an application ready level of maturity means to better understand and control light-matter interaction in solid-state doped material.

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When will quantum repeaters perform better than direct QKD?

I have heard that a new record of QKD distance has been established recently. Quantum repeater technologies are still in their early stages. When will quantum repeaters really improve QKD performances?

The impact of the development of quantum repeater technology is illustrated in this Figure . Since the first demonstration of a QKD system in the early 90’s, this technology has made tremendous progress. It can be expected that demonstration over distances approaching 300-400 kilometers will be possible in 3 to 4 years. However, without the development of quantum repeater technology, this progress will saturate. This strongly limits the addressable market for QKD. This technology offers extremely high security, but its application is currently restricted to metropolitan area networks, though in the future this should also cover regional networks. Obviously, threats to data confidentiality and integrity also exist for long distance transmission, but currently QKD does not offer any solution. Quantum repeaters have the potential to change this.

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How does a quantum repeater work?

I'm not an expert of quantum communication. Do you have a simple explanation for a quantum repeater?

One way to implement QKD is to use a resource specific to quantum physics: a pair of entangled particles. This resource is described by quantum physics, but has no equivalent in classical physics. In simple words, a pair of entangled particles is a pair of quantum particles. The quantum state of each particle is undefined, but the two states have strong correlations, e.g. the two states are identical. In this case, this means that if we measure both particles separately, we will obtain all the possible results randomly. But if we compare the result of one particle of a pair to the one of the other particle of the same pair, they are identical. It is possible to implement QKD by sending one particle of an entangled pair to each side of the quantum channel - optical fiber. Hence, QKD can be summarized as the distribution of pairs of entangled particles, or of entanglement.

illustration of entanglement

A quantum repeater is composed of two sources of entangled particles. The four particles go in four different fibers arranged in such a way that one particle of each source go to a quantum measurement device, whereas the two remaining particles go in opposite directions. The quantum measurement device asks the question "are you identical?" to the two particles arriving at the same. If the measurement succeeds, the two other particles are entangled together, and the state representing this entanglement depends on the result of the quantum measurement. Using this quantum measurement, we have established the entanglement of two particles which are separated by a distance larger than the one reached using a single source of entangled particles. Hence, a quantum repeater can increase the distance over which  entanglement can be distributed.

illustration of a quantum repeater

In the same way that classical communication uses amplifiers, it is possible to extend the distance over which entanglement is distributed by concatenating several quantum repeaters one after the other. For performance purposes of a chain of quantum repeaters, it has been shown that it is better to add a quantum memory at each side of the quantum repeater to store the quantum states of the two entangled particles.

illustration of quantum repeaters concatenation

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When will quantum networks become a reality?

What will be the evolution of QKD network with quantum repeaters? When will this technology be commercialized?

In spite of this relatively distant outcome, the quantum repeater technology is already part of the product roadmap of QuRep’s industrial partner IDQ, because of its potentially disruptive impact. This roadmap is illustrated by the following figure.

The company currently commercializes point-to-point QKD systems, which can be used to distribute keys over a dedicated fiber and a distance of around 100 kilometers. These systems are typically used by companies to secure links between different sites in a metropolitan area. The technology will continue to improve and it is expected that a bit rate of 1 Mbps over a distance of 50 kilometers will become commercially available in the next 5 years.

In parallel to the improvement of its point-to-point dark fiber technology, the company will also work on the compatibility of its QKD technology with WDM technology – this technology combine different channels with different wavelength (color) in the same fiber - in order to offer the possibility to perform classical data transmission and quantum key distribution on the same fiber. The feasibility of this has already been demonstrated in laboratories, but products do not yet exist. The multiplexing technologies are very common in classical communication, but combining a quantum channel, which carries only one photon per pulse with one or several classical channels with millions of photons without crosstalk is a challenging task. The first commercial QKD systems with WDM compatibility are expected within 2-3 years.

The next technological milestone will be to integrate QKD technology in passive optical networks (GPON or WDM-PON) for Metro Access. Commercial systems are expected in 5 years and will enable telecommunication operators to offer QKD based services to their customers.

The last step of this technology roadmap is the development of quantum repeaters enabling the deployment of long distance and meshed quantum networks for key distribution. This requires the development of quantum repeater technology and its combination with optical cross-connects to route the photons between nodes and to offer reconfigurability. With this last step, it will be possible to deploy QKD technology across all the segments of a telecommunication operator network. We expect this technology to be commercialized in the  next 10 to 15 years.

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What is the actual state of the art for quantum repeaters?

To be completed

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For more technical details, follow the Quantum Communication for Industry link.