For the Novice

For the Novice

This section will evolve over the duration of the project to bring you explanations of common concepts for quantum communication that are suited for the general public....

Quantum Cryptography

Everything you need to know about Quantum Cryptography...

This page is a non-technical introduction to Quantum Cryptography. If you are interested to learn more about the way Quantum Cryptography works, see our Quantum Communication for Industry section. We also have a more specific area on Quantum Repeaters and related technologies.


Quantum Cryptography or Quantum Key Distribution?

I have heard about quantum cryptography and quantum key distribution. Is it the same thing?

Both terms describe the same technology. However the more accurate name is Quantum Key Distribution. This technology allows one to distribute sequence of random bit whose randomness and secrecy are guaranteed by the laws of quantum physics. These sequences can then be used as secret keys with conventional cryptography techniques to guarantee the confidentiality of data transmissions.

The name Quantum Key Distribution — sometimes abbreviated as QKD — is preferred

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What Problem does QKD solve?

What advantage does QKD really bring in comparision to classical cryptographic techniques?

Cryptography offers — among other things — confidentiality of data transmissions. Before being transmitted, data is encrypted using an encryption algorithm (or process) and a secret key. After transmission, data is decrypted by reversing the encryption algorithm using the same secret key. The security of this scheme is based on the premise that the key is distributed only to the legitimate parties. This implies that the key transmission is a central problem (more information under Key Distribution).

Conventional cryptographic techniques rely on mathematical approaches to secure key transmission. However the security they offer is based on unproven assumptions and depends on the technology available to an eavesdropper.

Quantum Key Distribution is a technology that allows transmission of a sequence of random bits across an optical network and also verifies if this sequence was intercepted or not. This verification is based on the laws of quantum physics.

In practice, QKD is combined with conventional key distribution techniques (dual key agreement) to produce a key that is as secure as the strongest of the two original keys. With this approach, one can be sure to get the best of the classical and quantum world.

In summary, QKD provides long-term data transmission secrecy, which is not vulnerable to technological progress. On the contrary, classical cryptography provides secrecy only for a limited period of time.

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Is it complex to deploy QKD?

QKD is a new technology and it exploits quantum physics. It is certainly complex to deploy and operate, isn’t it?

No, QKD is not complex to deploy. It has reached a level of maturity such that it only takes a typical network engineer a few minutes to install a QKD system. Moreover it can be managed using standard network administration tools.

Recently some of us were involved in setting up the SwissQuantum network, QKD equipment was deployed in less than a day, with most of the time spent in traffic jams whilst travelling from one site to the other.

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Do commercial QKD products exist?

Is QKD technology really so mature that commercial products already exist?

QKD products are available from at least two companies:
- QuReP’s partner ID Quantique
- MagiQ Technologies

ID Quantique’s QKD product was used in conjunction with layer 2 Ethernet encryption to secure elections in Geneva, in what is the world’s first application of QKD.

Other companies claim to offer or to be developing QKD products, but limited information is publicly available. It is however likely that the situation will evolve in the near future.

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What are the practical limitations of QKD?

In what scenarios can QKD be used? What are the limitations of the technology?

QKD requires a direct optical channel — usually an optical fiber — between the emitter and the receiver. Standard optical fibers can be used. Contrary to a widespread misconception, it is not necessary to use a single fiber. Connections, splices and patching can be used.

Furthermore, QKD is also compatible with wavelength division multiplexing (WDM), although it does require more careful network design.

The amount of light propagating through an optical fiber decreases with the covered distance. Classical communication over the world, i.e. over long distance, is only possible thanks to optical amplifiers which are used to regenerate the optical signal regularly. The only constraint for QKD is that no amplifiers are used on the optical fiber carrying the quantum signal. Such devices would perturb the communication in the same way an eavesdropper does. This implies in turn that the range of QKD is limited. Current commercial products can cover 100 km, while research prototypes were shown to work up to 250 km.

In order to overcome this distance limitation, there exist two possibilities. The distance can be extended by using either trusted repeaters or quantum repeaters. Trusted repeaters are secured locations containing QKD devices and a key management layer to route and relay secret keys. The SwissQuantum project is a good example of this technology. Whereas the security of trusted repeaters relies on the security of the locations, the security of quantum repeaters relies on the laws of quantum Physics and is the focus of the QuReP project.

Overall QKD is perfectly compatible with the requirements of metropolitan area networks (MAN) communications. Applications include enterprise backbones and disaster recovery networks.

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Does QKD really offer absolute security?

From time to time, I come across pieces of news that claim that QKD has been broken. Is it true?

Generally speaking, there are two conditions for a system to be secure:

  1. it must be based on sound principles
  2. its implementation must be correct and must not open up vulnerabilities

Contrary to classical key distribution techniques, which rely on unproven assumptions and thus do not fulfil the first criterion, the security of QKD is based on the laws of quantum physics and can be rigorously proven.

This having said that, it is then important to make sure that the practical embodiment of a QKD system also fulfils the second criterion and does not have any implementation flaws.

All the announcements about QKD having been cracked actually dealt with implementation flaws. These flaws are important but are inherent to any technological system.

In summary, the security of QKD is based on sound principles and, if properly implemented, it guarantees absolute security for key distribution.

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