Quantum communication: What it is and why it matters now

Introduction

Quantum technologies are expected to significantly impact society in the coming years. These technologies are commonly categorized into three main areas: quantum computing, quantum communication, and quantum sensing. While much is written about their potential – both promising and perilous – it can be difficult to separate what is real from what is fiction. Even foundational figures like Albert Einstein and Niels Bohr disagreed on the core principles of quantum theory, underscoring its complexity.

This blog post offers a short overview to one of the pillars of quantum technology: quantum communication.

Quantum communications: An overview

Quantum communication can be split into two main categories:

1. Quantum Key Distribution (QKD)
2. Beyond-QKD Applications

Quantum Key Distribution (QKD)

The purpose of QKD is to enhance the security of classical communication links by making them more resistant to eavesdropping, as illustrated in Figure 1. It typically uses polarized photons transmitted through optical fibers or free space to distribute the encryption keys. The polarization of the individual photons then represents the state of a qubit. The encrypted data is transmitted over a classical channel, which can be wired or wireless. Once a secure key is distributed, symmetric encryption algorithms like AES are used to secure the classical communication between the sender (Alice) and the receiver (Bob).

The most widely used QKD protocol is BB84, proposed by Charles Bennett and Gilles Brassard in 1984. It is illustrated in Figure 2. It uses polarization filters—similar to those in sunglasses—arranged in either a rectangular (horizontal/vertical) or diagonal (right/left) basis. Alice encodes a random bit sequence into qubits using randomly chosen bases. Each transmitted qubit then has four possible states depending on the value of the generated bit and the basis of the polarization filter. Bob, upon receiving the qubits, also randomly selects measurement bases. After Bob receives the sequence of qubits, they compare bases over a classical channel and retain only the bits where their bases matched. This forms the raw key. A subset of these bits is used to detect eavesdropping. A key quantum property is that qubits cannot be cloned. Any attempt to intercept and measure them alters their state, revealing the presence of an eavesdropper to Alice and Bob.

QKD has been demonstrated in fiber-based experimental testbeds since the early 2000s. Companies like ID Quantique offer commercial QKD systems, and Toshiba is currently running a commercial trial of a QKD network in London. Hence, QKD through optical fibres can be considered as a quite mature technology. In 2017, China’s Micius satellite demonstrated satellite-to-ground QKD with a key rate of 3.5 bps. Europe plans to launch a LEO satellite in 2025 and a GEO satellite by 2030 for similar experiments. Although proofs of concepts were done almost 10 years ago, QKD through free space is less mature than through fibre.

QKD is the most mature quantum communication application for two main reasons. Firstly, it is used for communication between classical devices. The lack of maturity of quantum computers and their applications is therefore not limiting the application of QKD. Secondly, the BB84 algorithm does not depend on quantum entanglement, a key property of quantum technology that many of the advanced quantum communication protocols are based upon. For practical implementations, generating and distributing entangled qubits is challenging and thus limiting practical applications. This is also the reason why more advanced QKD protocols based on entanglement, like the E91 protocol proposed by Arthur Ekert in 1991, are less used, despite better theoretical performance.

While QKD is a way to make a data link secure against eavesdropping, it does not solve all security threats. Authentication of Alice and Bob must be handled by other means. QKD is therefore not a complete security solution, but rather one of the tools available to increase security, together with e.g. Post Quantum Cryptography (PQC). QKD will therefore be applied together with other cyber security algorithms to assure a post quantum secure communication infrastructure.

Moreover, there is a group of threats, called side-channel attacks, that exploit the physical implementation of the QKD system rather than the protocol itself. Examples of side-channel attacks are detecting clicks from the single photon detectors, measuring subtle variations in the intensity from the lasers, or exploiting polarization control imperfections. As QKD systems become more common, it is expected that ways to exploit leaked information from the equipment that is correlated with the key to be generated also will emerge.

Beyond-QKD protocols

Although QKD is the most mature application, quantum communication is more than QKD. Beyond-QKD protocols enable communication between quantum devices like quantum computers and sensors. These protocols often rely on entanglement, where two qubits share a linked quantum state regardless of distance. When a qubit is measured, it collapses into a classical state. Hence, when an entangled qubit is measured, the state of the other entangled qubit also collapses into a classical state — a phenomenon Einstein famously called “spooky action at a distance.” It is, however, important to understand that there is no communication involved in entanglement. Entangled qubits are used as a resource in the quantum algorithms, and not as bearers of information.

Beyond-QKD protocols can be divided between so called Quantum Secure Direct Communication (QSDC) and Quantum Secure Indirect Communication (QSIC).

Quantum Secure Direct Communication (QSDC)

In QSDC, qubits carrying data are transmitted directly through a quantum channel. This is typically photons transmitted over fibre or in free space, similar to QKD. However, the quantum states of qubits are fragile and highly sensitive to environmental noise. For instance, imperfections in the medium and air molecules lead to scattering and absorption and make the photons lose their inherent quantum states, which is called decoherence. As a result, the quantum information is degraded or lost. Current testbeds achieve data rates up to in the order of hundreds of kbps over some kilometres, far below classical communication capabilities. This makes QSDC impractical for most real-world applications.

Quantum Secure Indirect Communication (QSIC)

In QSIC, classical bits are used to transmit quantum information between quantum devices. The most well-known QSIC protocol is quantum teleportation. Using this protocol, one qubit can be teleported by means of two classical bits. The protocol works as follows (see Figure 5): Alice wants to send a qubit Q to Bob. Alice and Bob share two entangled qubits (A, B). Alice has qubit A and Bob has qubit B. How these entangled qubits are generated and distributed to Alice and Bob is not a part of the protocol and is assumed to be arranged somehow beforehand. Alice then performs some quantum operations on the qubits Q and A, first a control NOT (CNOT) gate and then a Hadamard gate on qubit Q, before measuring them. The results are then two classical bits a and b, which are transmitted to Bob via a classical channel. Depending on the values of a and b, Bob applies different quantum gates to qubit B. If ab=00, no operation is done, if ab=10 a bit flip (X) is done, if ab=01, a phase flip (Z) is done, and finally, if ab=11, both a bit flip and a phase flip (ZX) are done. After these operations, the state of Bob’s qubit B is transformed into the original state Q of the qubit that Alice wanted to send.

Teleportation is probably the first quantum protocol that will be implemented and used after QKD. It avoids the challenge of transmitting qubits over a quantum channel and can be used for instance for distributed quantum computing.

Quantum communication in Europe

The European Quantum Communication Infrastructure (EuroQCI) was launched in 2019 to develop a secure quantum infrastructure spanning the whole EU. It consists of a space segment, led by the European Space Agency (ESA) and a terrestrial segment consisting of national QCI centres in each of the 27 member states that have signed the EuroQCI declaration. The terrestrial infrastructure will consist of short-range metropolitan networks and long-distance links interconnecting the metropolitan networks. These may either be free-space links using satellites or terrestrial fibre networks including quantum repeaters. For most of the national QCI centres, the main applications will be secure communication using QKD, in addition to developing an infrastructure for research and education to test protocols such as teleportation. Once the national QCI networks start to be deployed, cross-border links will interconnect the different national networks.

This infrastructure will evolve into what is sometimes called the quantum internet. It will not replace the classical internet that we know today but add a quantum layer to the infrastructure. It will first be used to add security by incorporating QKD, and as the quantum technologies evolve also include other quantum communication protocols for efficient and secure communication between quantum devices.

“Think of our current communication networks as highways and think of classical data as ordinary cars. Quantum technology can be thought of as a flying car. This requires the highway infrastructure to take a leap and support 3D mobility.”
— Petar Popovski, Aalborg University

Conclusions

The primary application of quantum communication is QKD, a tool for enhancing the security of classical communication. Efforts to commercialize quantum communication in the next 5-10 years should focus on integrating QKD with PQC. The concept of “harvest now and decrypt later”, meaning that encrypted data can be stored today and decrypted the day sufficiently powerful quantum computers exist to break current encryptions protocols like RSA, implies that post quantum safe algorithms should be developed and implemented as soon as possible.

In the long term, more advanced protocols beyond QKD will be essential. Imagine a future where quantum processors collaborate across different locations to solve complex problems that classical computers can’t handle – or where a small quantum device securely offloads heavy calculations to a powerful quantum server without revealing either the sensitive data or the nature of the requested calculations. These are just two examples where quantum communication will be required, and for which protocols already exist. However, both the use cases and the protocols for beyond-QKD applications remain significantly less mature than those for QKD. As other quantum technologies —such as quantum computing, quantum sensing, and quantum memory— continue to evolve, new applications and use cases will inevitably emerge. The holy grail of quantum technologies is to obtain the quantum advantage, that is, solve problems that are impossible to solve on classical computers or significantly faster to solve on a quantum computer.

The importance of performing research and innovation within quantum communication is therefore twofold: First, as we edge closer to the quantum era, integrating QKD alongside PQC is not merely a strategic choice—it’s a critical necessity to protect today’s data from tomorrow’s quantum threats. Second, as quantum computing capabilities continue to advance, continuous research into quantum communication protocols and technology is essential to reach the quantum advantage.