A qubit about Photonics: What is Quantum Communication and will it be the future of communications?

Secure communication is therefore fundamental, as these online procedures involve sensitive data ^[1]. In the classical encryption, the share of the key begins with a common public information that is manipulated mathematically several times. This also goes to the receiver and sender several times until the point they can extract the key. An eavesdropper is able to record the process and therefore obtain the key ^[2]. Although there are some efforts to improve the security of classical communication ^[2], the capability to detect eavesdropping on site is pointed as the main difference between the two types of communication ^[3]. This happens because if there is measurement or cloning of quantum information this will disturb the system, as in accordance with quantum physics laws ^[4]. 

Here, we will explore how quantum communication could help us improve the security of the communication, but first we need to know what this communication is and how it works.

What is quantum communication?

It is a type of communication that allows sending the encryption key information in a secure way using the principles of quantum physics to ensure security ^[5].

So, what is quantum physics?

Classical physics deals with the everyday scale of things. Quantum physics refers to the very small world, the basic components of matter (the particle size, such as atoms). At this scale, the equations of the classical physics are no longer valid or useful and, instead of having a specific place and time for an object to exist, there is a haze of probability (the object has a certain probability of being in two places) ^[6].

It works with light, right?

Yes. Light has a duality wave and particle behaviour. With the classic theory, it is possible to observe the particle’s path or the wave behaviour, but not both at the same time. However, if we know the wave and the particle behaviour we can then block or allow the light to pass. To do this, it is needed to isolate the photons with the single-photon sources ^[7]. Quantum communication makes use of the particles of light to develop a system that ensures secure transfer of encryption key information. In this context, the photons are referred to quantum bits or qubits ^[5].

One important light property is the polarisation. If the light is polarised this means that the direction and magnitude of the vibrating electric field are behaving in a certain way ^[8]. The sun, for example, is unpolarised light, since the light wave vibrates in many directions. It is also possible to transform unpolarized light into polarised one (light waves in which the vibrations occur in a single plane) ^[9].

So, how does quantum communication work?

Being Quantum Communication, the communication using quantum laws to ensure security, Quantum Key Distribution (QKD) is a security protocol that generates the encryption key material. It was in late 1960s that the encoding information with photons was reported and in 1984, Bennett and Brassard introduced the first quantum protocol to generate a shared secret key between two parties using single-photons, the BB84. This protocol is popular, it can generate unlimited amounts of encryption keys for use with the only known encryption algorithm that achieves good secrecy – The One-Time-Pad ^[5].

Let’s say Alice wants communicate with Bob without anybody eavesdropping. Alice is the sender and Bob the receiver. Alice has a laser source, to generate single-photons and Bob has a quantum decoder and single-photon detectors. They communicate through a quantum channel and a classical one (conventional networked connection) ^[5].The quantum channel can be optical fibre or direct line of sight free space. Although the research at the moment using optical fibre is more developed, regarding its commercialization ^[10], the main disadvantages in using optical fibre for this application is that the fibre limits its range, it is currently a very expensive setup ^[11], so efforts are being done using the free space approach ^[11].

Alice works with the property of light we have seen above, the polarisation of light. She encodes single-photons in one of four polarisation states (horizontal, vertical and two diagonal) in accordance with one randomly selected bit value (0 or 1) and a basis (vertical and horizontal or the two diagonal). Alice is the working at the quantum scale, selecting and preparing the state bases and at the same time encoding classical bits as well ^[12]. When Bob receives the single-photons, he measures each one using a randomly selected basis. When the encoding by Alice and the decoding by Bob match, the photon’s bit value is read correctly. If the encoding and decoding don’t match, a random result happens ^[5]. The random results are removed during the communication because of the steps in the protocol and do not contribute to the shared key ^[5].

Why is it secure? What happens if someone is listening to them?

If someone tries to eavesdrop, usually named Eve, she would need to guess the information of the encryption key. However, she would increase the error of the protocol, a known variable, the Quantum Bit Error rate, QBER, making it easy to detect the eavesdrop’s presence. How is this done? A simple example can be an intercept-resend attack: Eve is on the quantum channel and she will randomly select a basis to measure the photons. This will inevitably increase the errors and as they know the protocol´s security threshold, something above that will be doubtful and the distribution of the key will stop ^[5].

 What are the challenges in Quantum Communication?

There are some challenges such as limitations or vulnerabilities that allow Eve the possibility to eavesdrop the conversation between Alice and Bob. For example, to have perfect single-photon sources. It is currently easier to detect the photons than to generate them and most experiments use pseudo-single-photon sources called weak coherent pulses, where we obtain not one photon, as it would be ideal, but a statistic number ^[7]. Other examples of challenges can be to have a lossless quantum transmission, a perfect alignment between the transmitter and receiver and the perfect single-photon detection ^[5].

However, research efforts are being done to overcome these difficulties ^[5]. For example, there is already a way to detect a type of Eves attack to the security, Photon Number Splitting attack, that is being employed in commercially viable QKD systems: the Decoy State Protocol. This is easy to implement, effective and practical, low cost and improves implementation security. It is based on the fact that Alice might send different types of pulses and, as Eve does not know which pulse she is reading, only the number of photons sent ^[5], she is no longer able to listen to Alice and Bob’s conversation.

Applications of quantum communication and technology

There is no doubt we all would like to have our personal data secure. Some foreseen applications of quantum communication could be bank transactions or even health records. Efforts in quantum communication are being done and some significant progress is being made, as across Asia, Europe and North America ^[5]. China has launched a satellite for quantum communication, and in 2017, it helped doing a secured video conference between Beijing and Vienna ^[13]. In the UK there is the EPSRC Quantum Communications Hub ^[14], a partnership of ten UK universities, and private and public companies. The aim is to develop and commercialise secure quantum communication technologies and services ^[14].

Other applications of quantum technology could be quantum computing devices, quantum enhanced imaging, quantum acceleration and navigation devices, quantum gravity sensing devices or even quantum timing devices ^[15]. Quantum computing has been demonstrated to process some problems and information more effectively than the conventional computer and it has been considered a very important tool for problem solving ^[16].

The immense potential of this technology and the efforts that are being done to overcome current limitations will certainly flourish and this makes me very enthusiastic about the upcoming years.