Building a quantum key distribution network in Sweden

On October 4^th, 2022 Alain Aspect, John F. Clauser and Anton Zeilinger received a call from the Royal Swedish Academy of Sciences awarding them the Nobel Prize in Physics for “experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. With this, the scientific community acknowledges, once more, the importance of quantum technologies for society.

We are now witnessing many useful applications emerge related to the trio’s pioneering work on quantum entanglement and superposition, two concepts that are riddled with counterintuitive experimental results. An important first step to enable these applications is to build a quantum internet. A quantum internet consists of a communication network built on quantum links and nodes, where the links are able to distribute quantum entanglement between the nodes. The nodes consist of quantum repeaters/memories that, in turn, are able to store the quantum states that will be distributed by the quantum links. Such networks enable distributed quantum computing, quantum teleportation and quantum key distribution (QKD) amongst other yet-to-be-discovered applications.

QKD addresses the problem of ensuring that a secret key pair is established between two communicating parties. Such key pairs are commonly used in modern cryptographic protocols. But while today the key pair is distributed using a mathematical problem that is hard for a computer to break, QKD instead uses the laws of quantum physics to ensure the secrecy of the keys:

• it is impossible to make exact copies of (unknown) quantum states, read here
• it is impossible (for an eavesdropper) to measure or observe the information conveyed by a quantum state without changing the state in a way that the communicating parties can detect, read here
• The information that an eavesdropper can extract without knowing how to measure the states is too small to be of any value, read here

One of the most common quantum cryptography architectures, where QKD is a component, is illustrated in Figure 1. At the bottom layer we find the QKD system where the encryption key is exchanged according to the chosen protocol. Connected to the QKD system we have the KMS (Key Management System) whose main function is to store and distribute the keys generated by the QKD layer towards the encryptor layer. The encryptor layer is responsible for converting the text into ciphertext to be distributed via a regular classical communication channel. The encryptor function will be connected to the classical transport network. This architecture with two nodes is reproducible along any nodes in the network.

Figure 1. Functional architecture of a telecom service using quantum key distribution technology

In addition, the choice of QKD hardware will determine the protocol to be used. The light sources in a QKD architecture can be classified into Poissonian and sub-Poissonian sources depending on the number of photons emitted. Poissonian sources consist of using groups of photons in light pulses from highly attenuated coherent sources, whereas sub-Poissonian sources are based on single-photon emitters like quantum dots or sources that produce correlated photon pairs using spontaneous parametric down-conversion (SPDC). The receiver side consists of avalanche photodiodes for detecting light pulses, which are the same detectors that are already used for coherent communication in a classical setup, or single photon detectors such as superconducting nanowire single photon sources. The presence of other optical elements such as modulators, polarizers and waveplates will also determine the protocol to be used for the key exchange. The choice of protocol is therefore also determined by the noise level, which is usually related to the distance between the communicating parties.

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The transmission of quantum information can be implemented using different information protocols. These protocols can be divided into categories depending on the detection technique required to recover the key information. Discrete Variable (DV) protocols and distributed phase reference (DPR) protocols rely on information encoded on single photons: polarization for the DV and phase or arrival times of the photons for DPR and thus, both rely on single photon detection techniques. Continuous Variable (CV) protocols encode information on the quadrature of the quantized electromagnetic field using coherent states and thus, homodyne or heterodyne detection techniques are used in this case. Figure 2 shows a sketch of the corresponding transmission channels.

Figure 2. Sketch of continuous variable (CV) and discrete variable (DV) quantum channel transmission.

​After the key is generated and distributed the key management system (KMS) will be responsible to manage and store the keys to the encryptor layer. The encryptor will convert clear text into cipher text with the help of the keys.

Quantum safe encryption is essential for protecting government and military communications, securing financial and banking transactions, preserving the confidentiality of medical records and, of most interest for the telecommunications industry, safeguarding the storage of personal and corporate data in the cloud and restricting access to confidential corporate networks.

NQCIS (National Quantum Communication Infrastructure in Sweden)

NQCIS falls under the umbrella of the EuroQCI initiative, where the Swedish consortium formed by industry (Ericsson AB, Quantum Scopes AB and quCertify AB) and academia (KTH, Chalmers, Linköping and Stockholm University) have been granted 100M SEK to test and deploy quantum key distribution systems tailored to the specific needs of Sweden.  The project is part of a broader European Commission initiative within the Digital Europe Programme and is funded by the EU as well as Vinnova and the Wallenberg Centre for Quantum Technology (WACQT).

The project is granted to start January 2023 and to last for 2.5 years. As part of the EuroQCI (Europe Quantum Communication Infrastructure), the NQCIS will focus on some of these key aspects of the solution:

• Scalability: The infrastructure should be scalable to accommodate a growing number of users, nodes, and quantum devices. It should be capable of handling the increasing demand for secure communication in various sectors highlighted above.
• Interoperability: to ensure interoperability between different quantum communication platforms, enabling seamless communication across national and international networks. This allows users from different regions to communicate securely.
• Infrastructure Resilience: The project focuses on building a robust and resilient quantum communication infrastructure. It considers factors such as reliability, fault tolerance, and redundancy to maintain the continuity of communication services.
• Standards and Protocols: to develop common standards and protocols for quantum communication, ensuring compatibility and facilitating widespread adoption. Standardization is crucial for interoperability and the efficient operation of the infrastructure.

This includes validation of different implementations to identify the most effective solutions that meet Sweden’s needs for secure communications in metropolitan areas, long-distance networks and terrestrial to satellite links.

By joining this consortium, we unlock a business innovation opportunity and lead the way for digital transformation. 

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