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Lessons learned from our journey in QKD

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3 mins read
Continuous and discrete Quantum Key Distribution (QKD)

Continuous and discrete Quantum Key Distribution (QKD)

02 / 07 / 2021

In general, there are two different QKD methods to generate a secret key, each putting the emphasis on different quantum characteristics of the light. As a result, they both have different performances and applications.
Learn the basic differences and applications of these two types of QKD technologies.

5 mins read
Get the quantum lock

Get the quantum lock

What does quantum mechanics have to do with making sure that some Alice secretly speaks to certain Bob?

Follow us on this mini-webinar about quantum cryptography and its crucial role on today’s communications

Coming next
3 mins mins read
Meet the European Quantum Flagship

Meet the European Quantum Flagship

04 / 27 / 2022

This year at Mobile World Congress Barcelona we had the pleasure to be part of the booth from the Quantum Flagship initiative that was run in this occasion by the outstanding communications team from ICFO.
Don't miss these 3 mins video with key opinions from the true experts behind these Quantum Technologies industries... Enjoy!

Frequently asked questions

What is quantum mechanics and why is getting so much attention?

Quantum mechanics is the set of principles used to explain the behaviour of particles at the atomic and subatomic scale. It all began around late 1800s and early 1900s, when scientist realized from a series of experimental observations that the behaviour of atoms didn’t agree with the rules of classical mechanics, where everyday objects exist in a specific place at a specific time. This changed the traditional concept of an atom with a nucleus surrounded by electrons, to orbitals that represent the probability of the electrons being in a given range at any given time. Electrons can jump from one orbital to another as they gain or lose energy, but they cannot be found between orbitals. From this idea, and over many decades, the rules of quantum mechanics were unveiled allowing scientists to build devices that followed those rules. This led to the first quantum revolution with the invention of the transistor, the laser, and the atomic clock that gave us computers, optical fibre communications and the global positioning system, respectively.

The reason why is getting again so much attention is because we are in the early stages of a second quantum revolution with scientists now being able to control individual atoms, electrons and photons. This is allowing our scientific community to build extremely fast quantum computers, interception-proof quantum communication and hyper-sensitive quantum measurement methods. All harnessed by strong technological companies across the world that are now in a frenetic race to redefine the limits of our technology and, with it, the very fabric of our everyday lives.

How do quantum computers work and how are they built?

Classical computers have billions of transistors that turn on or off to represent a value that is a 0 or a 1. Hence, in classical computing we talk about binary digit or bits. In contrast, quantum computers process data using quantum bits or qubits that, unlike classical bits, can exist in simultaneous states or superposition at the exact same point of time thanks to the laws of quantum mechanics. This allows each qubit to be 1, or 0, or both states simultaneously.

The magic of quantum computers happens when these qubits are entangled. Entanglement is a type of correlation that ties qubits together so the state of one qubit is tied to another. Hence, by leveraging both superposition and entanglement, quantum computers can speed up computation and do things that classical computers can’t do.

Entangled qubits can be created in many different ways for example with superconductors electronic circuits, by trapping ionized atoms or by squeezing particles of light (photons). Each technology is currently trying to preserve the quantum effects for as long as possible as they scale-up in the number of qubits from the current hundreds to the targeted Million that will forever redefine the boundaries of computing technology.

Is quantum cryptography and post-quantum cryptography the same?

Post-quantum cryptography (also known as quantum-proof, quantum-safe or quantum-resistant) refers to cryptographic algorithms that are thought to be secure against the attack of quantum computers in the future. These algorithms are called post-quantum because the security of most standard algorithms today relies on solving very difficult mathematical problems, sufficient for defending against modern computers but unable to resist the attack of a quantum computer once they reach certain computational power in number of Qubits. Quantum cryptography, on the other hand, also known as Quantum Key Distribution (QKD), describes he use of quantum effects to enable unconditionally secure key distribution between two legitimate users, guaranteed by the fundamental laws of quantum physics. Although some people tend to think that these two technologies are exclusive, they are in fact meant to be allies on securing future communications.

In a few words, how does quantum key distribution (QKD) system work?

Quantum Key Distribution is a method for two parties, in cryptography referred as Alice and Bob, to securely establish a shared key to encode messages through optical fibre or space. To create the key, first Alice encrypts random bits into quantum signals (extremely weak photons) and transmits them through the channel. Bob measures the state of the arriving photons and obtains data that is partially correlated to the data encoded by Alice. These data can be used to distil a secret key by means of error correction and privacy amplification. When a hacker tries to look at the information encoded into the quantum photons sent by Alice, he or she will irreversibly change their properties because quantum states cannot be cloned or copied. This means that Bob receives quantum signals that are not correlated to Alice’s as they should be, therefore letting them know that someone has tried to intercept the message. Alice and Bob discard this key that has been compromised and a new one following the same process is generated until it is guaranteed that is free from attacks.

What is the fundamental difference between discrete and continuous quantum key distribution (QKD) approaches?

In Discrete Variable QKD (DV-QKD) the emitter (Alice) prepares and sends to a receiver (Bob) quantum signals which consist of single photons with encoded random data. The encoding is done following a specific QKD protocol by using a discrete-valued degree of freedom of the photons such as polarization, time-bin or linear momentum. In the receiver, Bob measures the state of the arriving photons using single-photo detectors to distil a secret key. In Continuous Variable QKD (CV-QKD), the quantum signals typically consist of coherent states of light with information encoded in the quadrature of electromagnetic fields. Instead of single photon detectors, CV-QKD uses coherent homodyne or heterodyne detection (known in telecommunication phase-diversity homodyne detection) to retrieve the quadrature value of the signal and thus distil a secret key. 

What is the status of standardization and certification of quantum key distribution (QKD) technologies?

Standardization and certification of QKD technology is vital to enable market penetration, ensure equipment interoperability and a strong supply chain. For that, the standards are quite comprehensive as they define frameworks that consider all aspects of the technology as well as the implementation into a complete system, performance, best operational practices, or security specifications to name some. All the key standard organizations across the world (national, European, and world wide) already began years ago to write their specifications on QKD systems, which is an indicator of both increased maturity and a strong interest in the application and commercialization of QKD technology. For further information on QKD standardization, you can read the comprehensive analysis ran by OpenQKD here.

If classical or mathematical and post-quantum cryptographies advance rapidly, will they pose a serious threat for the adoption of Quantum Cryptography?

Rather than competing, both mathematical and post-quantum cryptographies are complementary to quantum cryptography. This is emerging when talking to encryption and telecom providers and supported by the fact that European Commission plans to deploy EuroQCI. The idea is to continuously monitor the evolution of all these technologies and put together a roadmap for leveraging both physical against mathematical complexity security-based protocols.

Aren’t Quantum Key Distribution (QKD) technologies too complex and expensive, far from telecom grade usability?

That is what we are working on. There are numerous projects in Europe and private companies investing and researching on making the technology affordable by involving production experts and the know-how of both end user companies and network infrastructure owners.

Europe seems to be behind the rest of the world on photonic components manufacturing, will this delay the introduction of Quantum Key Distribution (QKD) systems?

European-made technology is desired/preferred for these kinds of systems to ensure European sovereignty. For that reason the European Union itself through many fund programs, as well as industry consortiums such as the European Quantum Industry Consortium (QuIC), are stimulating potential makers and suppliers to develop and produce all key components in Europe over the next years.

Frequently asked questions