Simultaneous Classical and Quantum Communications: Recent Progress and Three Challenges

TL;DR

The paper surveys the integration of quantum communications into multi-band SAGINs via simultaneous classical and quantum communication (SCQC). It contrasts SCQC with coexistent schemes, outlines optical-band approaches (DV QKD QSDC and CV GMCS) and satellite-forward demonstrations, and discusses extending SCQC to microwave, mmWave, and THz bands under atmospheric and thermal-noise constraints. Key open problems include bridging data-rate and key-rate gaps optically, managing thermal noise and hardware integration in non-optical bands, and addressing THz quantum-technology gaps. The work argues that while optical SCQC is mature, realizing practical multi-band SCQC requires advances in hardware, protocols, and system-level optimization to enable scalable, quantum-secure SAGINs.

Abstract

A critical aspect of next-generation wireless networks is the integration of quantum communications to guard against quantum computing threats to classical networks. Despite successful experimental demonstrations, integrating quantum communications into the classical infrastructure faces substantial challenges, including high costs, compatibility issues, and extra hardware deployment to accommodate both classical and quantum communication equipment. To mitigate these challenges, we explore novel protocols that enable simultaneous classical and quantum communications, relying on a single set of transceivers to jointly modulate and decode classical and quantum information onto the same signal. Additionally, we emphasize extending quantum communication capabilities beyond traditional optical bands into the terahertz, even possibly to millimeter-wave and microwave frequencies, thereby broadening the potential horizon of quantum-secure applications. Finally, we identify open problems that must be addressed to facilitate practical implementation.

Figures (5)

• Figure 1: Illustration of simultaneous classical and quantum communications for wireless futures.
• Figure 2: Rate performance of the SCQC scheme employing DV QKD as described in Pan2025 for a satellite-to-ground FSO downlink. The FSO channel characteristics are adopted from Ghalaii2022, with key parameters including a zenith angle of 80 degrees, an initial spot size of $20$ centimeters (cm), a receiver aperture radius of $70$ cm, and an optical wavelength of $800$ nanometers (nm). The composable key rate evaluation follows the parameter settings in Pan2025Vasylyev2019, with main parameters including a mean signal intensity of $0.6$, a mean weak-decoy intensity of $0.2$, a receiver efficiency of $20\%$, a background error rate of $0.5$, a failure probability of $10^{-10}$, a background yield due to stray light of $2\times10^{-4}$, a background yield due to dark counts of $2.4\times10^{-6}$, and an error-correction efficiency of $1.05$.
• Figure 3: Rate performance of the SCQC scheme employing CV QKD as described in Qi2016 for a satellite-to-ground FSO downlink. The FSO channel characteristics are adopted from Ghalaii2022, with key parameters consistent with those in Fig. \ref{['fig_2']}. The composable key rate evaluation follows the parameter settings in Qi2016Ghalaii2022, with main parameters including a coherent-state variance of $5$, a detector noise of $0.1$, a shot-noise variance of $0.25$, a receiver efficiency of $50\%$, a local LO-induced loss of $0.63$, background thermal photons per mode of $9.31\times10^{-10}$, target classical bit-error rate of $10^{-6}$, a success probability of error correction of $0.9$, a correctness bounding of $10^{-10}$, and a reconciliation efficiency of $0.98$.
• Figure 4: Altitude-dependent atmospheric gas attenuation as a function of frequency and slant distance for an elevation angle of 45 degrees.
• Figure 5: Mean thermal photon number as a function of environmental temperature in Kelvin (K) and frequency.