Bridging the Quantum Divide: Integrated Classical and Quantum Networks

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Author: Denis Avetisyan

This review examines the potential of combining classical and quantum communication channels to build more efficient and secure Space-Air-Ground Integrated Networks.

The envisioned wireless networks of the future demand a convergence of classical and quantum communication channels, acknowledging that any architecture built today inevitably seeds the conditions of its own eventual limitations.
The envisioned wireless networks of the future demand a convergence of classical and quantum communication channels, acknowledging that any architecture built today inevitably seeds the conditions of its own eventual limitations.

The article analyzes recent advancements and key challenges in Simultaneous Classical and Quantum Communication (SCQC) across various frequency bands, including optical, microwave, mmWave, and THz.

While next-generation networks demand enhanced security, integrating quantum communication infrastructure alongside existing classical systems presents significant logistical and economic hurdles. This is addressed in ‘Simultaneous Classical and Quantum Communications: Recent Progress and Three Challenges’, which explores a resource-efficient approach-simultaneous communication-leveraging shared transceivers across optical, terahertz, and even microwave frequencies. The authors demonstrate the feasibility of this technique within Space-Air-Ground Integrated Networks, paving the way for truly quantum-secured data transmission. What further innovations are needed to overcome remaining challenges and fully realize the potential of this integrated communication paradigm?

The Inevitable Limits of Broadcast

The escalating demand for data transmission, fueled by applications like high-definition video streaming, virtual reality, and the Internet of Things, is rapidly exceeding the capabilities of traditional radio frequency (RF) communication systems. Simultaneously, RF signals are inherently vulnerable to eavesdropping and jamming, posing significant security risks – a concern amplified by increasingly sophisticated cyber threats. These vulnerabilities stem from the broadcast nature of RF waves, which propagate widely and can be intercepted with relative ease. Moreover, the limited and increasingly congested RF spectrum necessitates complex and often insecure modulation schemes to maximize bandwidth, further compromising data privacy. Consequently, reliance on RF communication alone presents both practical limitations and substantial security challenges in a world demanding both greater connectivity and robust data protection.

Free-space optical (FSO) communication presents a compelling alternative to radio frequencies, promising significantly higher bandwidth capabilities for data transmission. Unlike radio waves, FSO utilizes light to carry information through the atmosphere, enabling data rates far exceeding conventional methods. However, this reliance on light also introduces substantial challenges; atmospheric disturbances – including fog, rain, snow, and even atmospheric turbulence – can scatter, absorb, or distort the optical signal. These effects lead to signal attenuation and increased error rates, limiting both the range and reliability of FSO links. Consequently, robust system designs must incorporate advanced techniques like adaptive optics, diversity schemes, and powerful error correction codes to mitigate these atmospheric effects and ensure consistent, high-performance communication.

The pursuit of truly secure communication in open environments necessitates a departure from conventional methods, given the inherent weaknesses of radio frequencies and the atmospheric vulnerabilities of free-space optical (FSO) systems. Researchers are actively exploring advanced techniques, such as quantum key distribution (QKD), to establish cryptographic protocols impervious to eavesdropping. These innovative approaches don’t merely encrypt data; they leverage the laws of physics to guarantee security. Recent demonstrations of secure communication using QKD have extended practical distances to over 480 kilometers, pushing the boundaries of what’s achievable with optical technology. These advancements aren’t simply incremental improvements; they represent a fundamental shift towards communication systems designed with inherent security, offering a path to protect sensitive information against increasingly sophisticated threats and ensuring data integrity in a world reliant on constant connectivity.

Recent advancements in secure communication technologies have demonstrated the feasibility of Space-Certified Quantum Communication (SCQC) over unprecedented distances. Utilizing quantum key distribution (QKD), specifically discrete-variable (DV) and continuous-variable (CV) protocols, researchers have successfully extended secure communication links to 583 kilometers using DV QKD and 487 kilometers with CV QKD, both achieved under asymptotic conditions-meaning performance approaches theoretical limits with increasing transmission distance. These results represent a significant leap toward establishing a global quantum communication network, bypassing the limitations of traditional cryptography and offering a fundamentally secure means of data transmission. The extended range is crucial for satellite-based quantum networks, enabling secure communication across continents and bolstering data privacy in an increasingly interconnected world.

The SCQC scheme employing device-independent quantum key distribution achieves a quantifiable key rate for satellite-to-ground free-space optical communication with an 80-degree zenith angle, utilizing parameters including a 20x20 cm initial spot size, a 70 cm receiver aperture, 800 nm wavelength, and optimized settings for signal/decoy intensities, efficiencies, and error correction.
The SCQC scheme employing device-independent quantum key distribution achieves a quantifiable key rate for satellite-to-ground free-space optical communication with an 80-degree zenith angle, utilizing parameters including a 20×20 cm initial spot size, a 70 cm receiver aperture, 800 nm wavelength, and optimized settings for signal/decoy intensities, efficiencies, and error correction.

Quantum Security: An Inherent, Not Imposed, Guarantee

Quantum Key Distribution (QKD) achieves theoretically unbreakable encryption by encoding information in the quantum states of particles, typically photons. Unlike classical encryption algorithms which rely on computational complexity, QKD’s security is rooted in the fundamental laws of physics, specifically the principles of quantum mechanics. Any attempt to intercept or measure the quantum key during transmission inevitably disturbs the quantum state, alerting the legitimate parties to the eavesdropper’s presence. This disturbance is detectable due to the Heisenberg uncertainty principle and the no-cloning theorem, which prevents the perfect duplication of unknown quantum states. Consequently, a secure key can be established with the certainty that it has not been compromised, offering a level of security unattainable with classical methods.

Quantum Key Distribution (QKD) establishes a secure key between parties by transmitting quantum states, most commonly photons, which encode information. These photons are prepared and transmitted in non-orthogonal states – for example, utilizing polarization – ensuring that any attempt to intercept or measure the quantum states inevitably disturbs them, alerting the legitimate parties to the eavesdropper’s presence. Entangled photons represent a specific quantum state where the properties of two photons are intrinsically linked, regardless of the distance separating them; measuring the state of one instantly reveals information about the other, allowing for correlation-based key generation. The resulting key, established through this quantum channel, can then be used with a classical encryption algorithm, like Advanced Encryption Standard (AES), to securely encrypt and decrypt messages.

Free-space quantum key distribution (QKD) systems are susceptible to signal attenuation and distortion due to atmospheric effects. Signal loss, primarily caused by absorption and scattering, reduces the received photon count, limiting transmission distance and key generation rates. Atmospheric turbulence introduces variations in the refractive index, causing beam wander, spreading, and scintillation, which degrade the quantum state fidelity and increase the quantum bit error rate (QBER). These effects necessitate adaptive optics, higher transmission power, and sophisticated error correction protocols to maintain secure communication over practical distances. The severity of these challenges is wavelength-dependent, with shorter wavelengths experiencing greater scattering but potentially higher key rates.

Entanglement links establish the basis for quantum communication protocols by utilizing the quantum mechanical phenomenon of entanglement, where two or more particles become correlated in such a way that they share the same fate, no matter how far apart they are. In the context of Quantum Key Distribution (QKD), these links are created through the transmission of entangled photons; any attempt to intercept or measure the quantum state of these photons inevitably disturbs the entanglement, alerting the communicating parties to a potential eavesdropper. The integrity of this entanglement is thus directly tied to the security of the key generated, providing a theoretical guarantee against unauthorized access. Successful implementation relies on maintaining the entanglement fidelity over the communication distance, which is affected by factors such as signal loss and decoherence.

Space-based Quantum Communication (SCQC) systems, utilizing both Discrete Variable (DV) and Continuous Variable (CV) Quantum Key Distribution (QKD) protocols, demonstrate varying classical data rates dependent on the QKD method employed. Specifically, DV QKD, when implemented optically, achieves data rates ranging from $5.9 \times 10^{-4}$ to $1.2 \times 10^{-3}$ bits per use. Conversely, CV QKD, also utilizing optical transmission, exhibits significantly higher data rates, ranging from $6.3 \times 10^{-2}$ to $1.08 \times 10^{-1}$ bits per use. These rates are achieved at designated secure operating altitudes, representing the system’s performance envelope for quantum communication.

Employing continuous-variable quantum key distribution over a satellite-to-ground free-space optical link, the SCQC scheme achieves a key rate consistent with parameters mirroring those in Figure 2, utilizing a coherent-state variance of 55, detector noise of 0.1, and other settings detailed in references [6, 14].
Employing continuous-variable quantum key distribution over a satellite-to-ground free-space optical link, the SCQC scheme achieves a key rate consistent with parameters mirroring those in Figure 2, utilizing a coherent-state variance of 55, detector noise of 0.1, and other settings detailed in references [6, 14].

Beyond Point-to-Point: The Inevitable Integration of Diverse Networks

Space-Air-Ground Integrated Networks (SAGIN) represent a heterogenous network architecture designed to provide continuous connectivity across vast geographical areas. These networks integrate space-based assets – including Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and Geostationary Earth Orbit (GEO) satellites – with aerial platforms such as High Altitude Platform Stations (HAPS) and Unmanned Aerial Vehicles (UAVs), and terrestrial infrastructure. This layered approach allows for dynamic resource allocation and seamless handover between network elements, increasing resilience and capacity. The scalability of SAGINs is achieved through the modular deployment of satellite constellations and aerial platforms, enabling coverage expansion and adaptation to fluctuating demand. Furthermore, the integration of diverse technologies, including free-space optical (FSO) and radio frequency (RF) communications, facilitates optimized link performance based on environmental conditions and user requirements.

Optical Reconfigurable Intelligent Surfaces (ORISs) function as planar arrays of electronically controllable optical elements that manipulate light propagation in free-space optical (FSO) communication links within Space-Air-Ground Integrated Networks (SAGINs). By dynamically adjusting the phase and amplitude of reflected or transmitted optical signals, ORISs can compensate for atmospheric turbulence, beam divergence, and multipath fading – all significant degradation factors in FSO systems. Strategic deployment of ORISs, either on ground stations, airborne platforms, or satellites, allows for beam steering, focusing, and shaping, effectively extending the communication range and improving signal-to-noise ratio (SNR). This capability is particularly valuable in scenarios where direct line-of-sight communication is obstructed or unreliable, enabling the establishment of robust and high-bandwidth connections across heterogeneous network segments.

The integration of Optical Reconfigurable Intelligent Surfaces (ORISs) with Free-Space Optical (FSO) systems addresses key atmospheric limitations impacting FSO communication. Atmospheric turbulence, absorption, and scattering traditionally degrade FSO signal quality and reduce link range. ORISs, through controlled reflection and refraction of light, can dynamically reshape the wavefront, mitigating these distortions and refocusing the signal onto the receiver. By strategically deploying ORISs within a network, signal-to-noise ratio (SNR) is improved, extending the effective communication distance and increasing data rates. This combination allows for more reliable and higher-bandwidth connectivity compared to standalone FSO systems, particularly in challenging atmospheric conditions. Furthermore, ORISs can be configured to steer beams, enabling non-line-of-sight (NLOS) communication paths and enhancing network flexibility.

The performance of Space-Air-Ground Integrated Networks (SAGIN) is directly dependent on spectrum availability across multiple frequency bands. Specifically, the Microwave Band (0.3-300 GHz), millimeter-Wave (mmWave) Band (30-300 GHz), and Terahertz (THz) Band (0.1-10 THz) provide the necessary bandwidth for high-data-rate communications between space, air, and ground segments. While the Microwave Band offers established infrastructure and propagation characteristics, the mmWave and THz bands are crucial for achieving the capacity required for future applications. Effective spectrum allocation and utilization within these bands, alongside the development of associated technologies like beamforming and multiple-input multiple-output (MIMO) systems, are essential for maximizing the efficiency and scalability of SAGIN architectures. Limitations in available bandwidth within these bands can significantly constrain network throughput and coverage.

The operational efficacy of quantum communication channels within Space-Air-Ground Integrated Networks (SAGINs) is maintained by consistently low thermal photon numbers, quantified as remaining below $10^{-5}$. This negligible level of background noise is critical, as even a small increase in thermal photons can overwhelm the weak quantum signals used for key distribution and secure communication. Maintaining this low noise floor ensures the fidelity of quantum key distribution (QKD) protocols and the overall integrity of data transmitted via quantum channels, allowing for reliable and secure communication links despite atmospheric and space-based transmission challenges.

Beyond Coexistence: A Unified Framework for Secure Communication

Simultaneous Classical and Quantum Communication (SCQC) represents a significant advancement in communication technology by enabling the transmission of conventional digital data alongside quantum keys within the same communication channel. This integration bypasses the need for dedicated infrastructure for quantum key distribution (QKD), substantially reducing the complexity and cost associated with deploying quantum-secured networks. By multiplexing classical and quantum signals, SCQC systems can leverage existing fiber optic infrastructure while simultaneously enhancing security through the use of unbreakable quantum encryption. This co-existence not only streamlines network implementation but also opens possibilities for hybrid communication systems that balance bandwidth demands with stringent security requirements, promising a practical pathway towards widespread quantum-secured communication.

Coexisting Classical and Quantum Communication (CCQC) establishes a fundamental architecture for Simultaneous Classical and Quantum Communication (SCQC) by enabling the transmission of both conventional data and quantum keys over the same communication channel. This approach doesn’t simply layer quantum signals onto classical infrastructure; instead, it designs a unified framework where these signals harmoniously coexist, minimizing interference and maximizing efficiency. The core principle involves carefully allocating resources-such as time, frequency, or polarization-to separate and distinguish between classical and quantum components. By meticulously managing these resources, CCQC protocols ensure that the quantum key distribution (QKD) process doesn’t degrade the classical data transmission, and vice versa. This foundational framework is critical, as it provides a scalable and practical pathway for integrating quantum security features into existing communication networks without requiring a complete overhaul of the infrastructure, offering a cost-effective solution for enhanced data protection.

One-way Quantum Secure Direct Communication (QSDC) protocols represent a significant advancement in the field of simultaneous classical and quantum communication, fortifying data transmission with inherent security features. Unlike traditional key distribution methods that require a separate key exchange, these protocols integrate key establishment directly into the data transmission process itself. This is achieved by encoding both the classical information and the quantum key within the same quantum state, allowing for secure communication without the need for prior key agreement or trusted third parties. The inherent link between data and key generation drastically reduces vulnerabilities to eavesdropping attacks, as any attempt to intercept the quantum signal will inevitably disturb the transmitted data and alert the communicating parties. This streamlined approach not only enhances security but also improves the efficiency and practicality of integrating quantum cryptography into existing communication infrastructures, offering a robust solution for safeguarding sensitive information.

The practical implementation of simultaneous classical and quantum communication (SCQC) faces significant hurdles related to Size, Weight, Power, and Cost – collectively known as SWaP-C. These constraints are particularly acute when scaling SCQC systems for real-world deployment, as quantum technologies often demand specialized and resource-intensive hardware. Minimizing SWaP-C is crucial for integrating SCQC into existing infrastructure, such as satellite architectures or software-defined networking (SDN) environments, where space and energy are limited. Advancements in integrated photonics, miniaturized components, and efficient modulation techniques are therefore vital to overcoming these challenges and enabling widespread adoption of secure, coexistent classical-quantum communication networks. Reducing these factors will directly impact the feasibility of deploying robust quantum key distribution (QKD) alongside conventional data transmission, unlocking the potential for highly secure and versatile communication systems.

Recent advancements in simultaneous classical and quantum communication (SCQC) have yielded a functional framework capable of establishing secure connections over significant distances. Demonstrations utilizing optical channels have achieved secure key distribution of up to 583 kilometers with discrete-variable quantum key distribution (DV QKD), and 487 kilometers employing continuous-variable QKD (CV QKD). This performance is particularly relevant for integrating quantum security into Software-Defined Area Global Information Networks (SAGINs), as the framework’s design prioritizes cost-effectiveness without compromising security. The extended range and reduced resource demands represent a practical step towards deploying quantum-enhanced communication infrastructure, paving the way for secure data transmission across wider geographical areas and diverse network topologies.

The pursuit of Simultaneous Classical and Quantum Communication (SCQC) within Space-Air-Ground Integrated Networks (SAGINs) reveals a familiar pattern: the attempt to impose order on inherently probabilistic systems. It’s not that these systems fail to deliver quantum security; rather, they evolve in ways unanticipated by initial design. As Werner Heisenberg observed, “The more precisely the position is determined, the less precisely the momentum is known.” This holds true for communication networks as well. The very act of attempting to perfectly define and control quantum signals within a complex, multi-frequency architecture – optical, mmWave, THz – introduces unforeseen disturbances and limitations. Long stability isn’t the goal; it’s a deceptive prelude to eventual, emergent behavior. The challenges identified within the paper – synchronization, atmospheric turbulence, and practical implementation – are not roadblocks, but indicators of the system’s inevitable adaptation.

The Horizon Beckons

The pursuit of Simultaneous Classical and Quantum Communication (SCQC) within Space-Air-Ground Integrated Networks (SAGINs) does not solve a problem so much as relocate it. Each frequency band – optical, microwave, mmWave, THz – offers a temporary reprieve from limitations, but only trades one form of decay for another. The paper correctly identifies the challenges, yet fails to fully internalize the lesson: every efficient architecture is merely a deferral of inevitable compromise. The assumption of seamless integration implies a belief in perfect control, a fiction entropy readily dispels.

Future work will inevitably focus on mitigating the identified challenges – polarization drift, atmospheric turbulence, the ever-present noise floor. However, the true frontier lies not in refining the signal, but in accepting the inherent fragility of the system. A more fruitful path may be to explore architectures that embrace imperfection, designing for graceful degradation rather than striving for unattainable fidelity.

The current emphasis on extending the range of SCQC feels… optimistic. It assumes a static environment, a fixed set of threats. But the universe does not negotiate. The real challenge is not how to send a quantum key further, but how to build a system that can adapt, evolve, and ultimately, survive the unpredictable currents of time and interference. The next iteration will likely reveal that the most robust solution isn’t a better signal, but a more forgiving network.

Original article: https://arxiv.org/pdf/2512.10176.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

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2025-12-12 11:01