© European Physical Society, EDP Sciences, 2025

Quantum communication started with the development of the BB84 protocol for secure distribution of secret keys. By now, quantum cryptography or better quantum key distribution (QKD) is a well developed method with a rapidly increasing number of companies providing commercial products. They integrate seamlessly with standard equipment for secure communication by providing a stream of fresh and secure keys in the style of so called key servers. Yet, as for all quantum communication, the crucial fact originates from the impossibility of amplifying the sent signals. The no-cloning theorem shows that any copying, refreshing or amplification of the signal sent over the quantum channel results in a high amount of noise. This noise is exactly the same as any eavesdropping on the quantum communication process would introduce. For QKD it would directly indicate massive attack, and it would make protocols like quantum dense coding or quantum teleportation impossible as it immediately destroys entanglement. Requiring a direct connection between sender and receiver of course limits the reach of the many new applications. Here an overview of distances which can be reached for secure communication using different strategies is given. Future quantum repeaters will enable quantum networks on the long term as they will be able to distribute entanglement efficiently, that is with only polynomially increasing resources compared to the exponential scaling of todays methods. First steps have been achieved, but significant developments and milestones are still ahead of us.

For QKD, any attack by an eavesdropper can be detected when exchanging single photons or feeble light pulses. The light can be sent over a free-space channel using mutually oriented telescopes or along a glass fiber. The link efficiency, i.e. the rate of possible communication or key generation, is ideally reduced only quadratically with the distance for a free space connection between telescopes, yet, it will be further reduced by atmospheric turbulence. For a fiber connection the efficiency suffers exponential decrease with a factor of 10 every 50 km of fiber length even when using the optimal telecom wavelengths. This reduction of the signal has to be compared with the noise of single photon detectors due to random detection events of the detectors or stray light. Continuous-variable systems are better suited for shorter distances, while longer reach is possible with discrete-variable QKD a la BB84. As a rule of thumb, secure key can be generated only if the critical parameter QBER, the ratio of wrong events compared to all events, is below 10%, which translates to signal-to noise-ratio of much better than 5 to obtain key at all.

Current commercial systems are recommended for distances between 50 km to about 150 km which is limited mainly by the detector noise of semiconductor single photon detectors at the telecom wavelengths. The longest distances have thus been achieved when using superconducting detectors which have very low dark counts below few tens per second and also allow improved time filtering due to their high time resolution. This way the group of Hugo Zbinden and Nicolas Gisin demonstrated a link of 421 km already in 2018. This value can be increased to close to 500 km by even better filtering, the real change came with the introduction of twin-field QKD by the team of Andrew Shields. Provided the sources of both partners are coherent with each other, their light pulses can be interfered at a central location. This detection in the middle thus effectively doubles the distances. A sequence of experiments improved on the remarkable feat to provide the necessary coherence resulting now in generating secure key over a distance of 1000 km by Jian-Wei Pan’s group in 2023.

Longer distances can be bridged only when splitting the whole link in segments. There key can be distributed at a higher rate for segment lengths of about 70 km-100 km between the nodes connecting the segments. All these keys can be used to obtain a common key over the whole link, yet now with classical information accessible at all nodes. It is thus of outmost importance that all the nodes are secure and so called “trusted nodes”. Well known is the impressive 2000 km long connection between Beijing and Shanghai with further trusted node networks in cities along the line. Today similar initiatives are underway, for example the EuroQCI project to establish a trusted node network within the EU which in its planning includes also the next step, the additional space segment.

Satellite-based QKD was proposed by the Los Alamos group of Jane Nordholt and Richard Hughes as a way to enable secure communication on a global scale. In such a scenario, the satellite acts as trusted node and exchanges a secure key with one ground station. On a sun-synchronous orbit the satellite crosses every point on earth twice a day and during night time a link to an optical ground station can be achieved. In a typical height of about 500 km the orbit time is about 90 minutes which gives roughly three to five minutes time to establish an optical link between the fast moving telescope on earth and the satellite over distances up to about 1500 km and consequently also much higher effective thickness of the atmosphere at lower angles (Fig. 1). Higher orbits would give more link time, however, at the cost of much lower link efficiencies which in turn would require larger telescopes on the satellite and on earth. Careful optimization has to be done taking into account a wealth of limiting parameters. MICIUS, designed and operated by Pan’s group, was the first satellite to achieve secure key exchange as well as other quantum communication tasks like distribution of entanglement between two ground stations or quantum teleportation of a quantum state from earth to space. Follow-up missions are already launched by the Chinese groups with many more satellites to follow also from other nations. Already now the satellites established links with ground stations world wide thereby indeed demonstrating secure key exchange on a global skale7.

Fig. 1 The satellite MICIUS demonstrated QkD between ground stations on a global scale. Optical link with the satellite is achieved using beacon laser light controlling fine-pointing of the telescope optics.

Quantum networks will provide all functionalities not only for QKD but also for all other methods of quantum communication. There, quantum nodes are connected via links using quantum repeater methods and will provide the crucial resource, entanglement for the respective applications. It is still a long way to achieve all the necessary milestones. The first one, distributing entangled states along a single link, is now achieved over a few tens of kilometers (Fig. 2). Possible physical systems include impurities and dopants in crystals or trapped neutral atoms and ions. These systems already provide of a natural interface to a photonic quantum channel, and especially the later are excellent systems to perform quantum logic operations necessary for error correction and creation of complex multi-party entanglement. Yet, the rate is very low and requires various improvements. The first is to improve the coupling with the quantum channel, best by using optical resonators. The second concerns the long latency between starting the emission of a photon entangled with the quantum system and receiving confirmation of its arrival from the other node or an intermediate station. Multiplexing with several trapped atoms or ions is a possible solution here and already enabled the observation of entanglement between an ion and a photon after 100 km of glass fiber. Multiple trapped quantum systems offer also the option to solve the third problem, i.e., error correction, first to overcome noise along the links but foremost to provide a reasonable amount of entanglement at all times.

Fig. 2 Schematic layout of long-distance entanglement distribution: Two separated quantum systems (here Rubidium atoms) become entangled in the emission process with a photon, respectively. Quantum frequency converters (QFC) are necessary to change the wavelength of the emitted photon (780 nm) to the telecom wavelengths. Two photon interference enables a Bell-state measurement (BSM) to swap the entanglement to the quantum systems.

On the long run, quantum networks will provide efficient quantum communication over very long distances. This will enable not only to secure our conventional communication but also to link quantum computers and to facilitate multi-party quantum communication. Amazing possibilities, but also amazing challenges ahead, let’s see.

References

All Figures

	Fig. 1 The satellite MICIUS demonstrated QkD between ground stations on a global scale. Optical link with the satellite is achieved using beacon laser light controlling fine-pointing of the telescope optics.
In the text

Fig. 2 Schematic layout of long-distance entanglement distribution: Two separated quantum systems (here Rubidium atoms) become entangled in the emission process with a photon, respectively. Quantum frequency converters (QFC) are necessary to change the wavelength of the emitted photon (780 nm) to the telecom wavelengths. Two photon interference enables a Bell-state measurement (BSM) to swap the entanglement to the quantum systems.

In the text