Quantum-Enhanced Communication Systems

• Quantum-enhanced communication systems are advanced networks that utilize quantum phenomena, including entanglement and indefinite causal order, to surpass classical capacity limits.
• They employ innovative receiver protocols such as joint quantum measurements and spin-photon interfaces to achieve superadditive photon efficiency and improved error rates.
• Integration of controlled noise, multiplexed teleportation, and quantum key distribution enables scalable, secure multiuser communication with enhanced performance over conventional systems.

Quantum-enhanced communication systems integrate quantum-mechanical resources—such as quantum logic, entanglement, indefinite causal order, joint measurements, and correlated noise effects—into the architecture of communication links to surpass the performance limits of conventional classical protocols. These systems enable improvements in classical information capacity, quantum information transmission, secure key rates, multi-access scalability, energy efficiency, and physical-layer security, often by leveraging measurements and processing that are not possible with classical receivers.

1. Principles of Quantum-Enhanced Communication

The classical communication paradigm is fundamentally constrained by the Shannon-Hartley theorem, which limits information throughput for a given bandwidth and signal-to-noise ratio. In the quantum regime, new limits arise: the ultimate classical capacity of a quantum channel is fixed by the Holevo bound, while the quantum capacity is determined via regularized coherent information, and private capacities are upper-bounded by various entropic measures (Gisin et al., 2010, Hasan et al., 2022). Exploiting quantum phenomena unlocks new enhancements:

• Joint Quantum Measurements: Receivers that coherently process entire codeword blocks—rather than symbol-by-symbol detection—can approach the Holevo capacity and unlock superadditive photon information efficiency, especially in the low-photon regime (Smith et al., 19 Jun 2025).
• Entanglement and Superposition: Entanglement distribution, quantum teleportation, and entanglement swapping underlie quantum repeaters and facilitate dense coding and secure quantum key distribution (De et al., 2011, Bhaskar et al., 2019).
• Indefinite Causal Order: Coherently controlling the order of noisy channels (quantum SWITCH) enables information transmission capabilities that are classically inaccessible, even over fully depolarizing links (Ebler et al., 2017, Rubino et al., 2020).
• Noise-Assisted Transport: Controlled dephasing or noise correlates can paradoxically enhance transmission rates via suppression of destructive interference or activation of new channel pathways (Caruso et al., 2010).
• Resource-efficient Architectures: Multiplexing across spatial, spectral, or temporal modes allows exponential gains in rate per energy or robustness to channel loss (Christ et al., 2012, Noh et al., 2018).

2. Architectures and Receiver Protocols

Quantum-enhanced communication architectures typically involve sophisticated encoding at the transmitter, a quantum channel, and a quantum-processing-assisted receiver:

• Quantum-Processing-Assisted Classical Receivers: Incoming optical signals are mapped into quantum registers; joint quantum logic (e.g., successive-cancellation decoding over classical-quantum polar codes) enables codeword-level measurements. The quantum receiver requires only $O(\log N)$ matter qubits for a codeword of length $N$ (e.g., $Q_{\rm single}=\lceil\log_2(N+1)\rceil$), achieving the quantum limit with modest hardware (Smith et al., 19 Jun 2025).
• Joint Detection with Spin-Photon Interfaces: State-of-the-art implementations leverage cavity-coupled atomic or solid-state (NV, SiV) spin qubits to realize photon-qubit transduction and quantum-controlled logic for measurements. Gate errors $\sim 0.02\%$ and losses $\sim 2\%$ in four-qubit modules achieve a 10% rate improvement over any classical receiver for $\bar n\sim 10^{-3}$ photons per mode (Smith et al., 19 Jun 2025).
• Multiplexed Teleportation: Multi-mode continuous-variable teleportation across many squeezed optical modes yields an exponentially larger rate per photon, conditional on threshold squeezing (e.g., $r \gtrsim 0.5$ for reliable transfer). Multiplexing is optimized by engineering flat entanglement spectra from a pulsed parametric source (Christ et al., 2012).
• Quantum CDMA: Spectral encoding of continuous-mode field states via random or orthogonal phase masks enables multiple-access quantum channels with vanishing multiaccess interference in the Fock-state regime, exceeding classical CDMA scaling and providing quantum noise-limited sensitivity for point-to-point and radar applications (Rezai et al., 2021).

3. Capacity Gains and Quantum Limits

Quantum-enhanced systems surpass classical limits in several distinct regimes:

• Superadditivity in the Weak-Signal Regime: Symbol-by-symbol (Dolinar-type) receivers are limited to $$C_1(\bar n)=1-h_2(\tfrac{1-\sqrt{e^{-4\bar n}}}{2})$$ bits per mode, while joint quantum decoders saturate the Holevo bound $C_\infty(\bar n)=h_2(\tfrac{1-e^{-2\bar n}}{2})$, with photon information efficiency gains sharply realized for $N=4,8$ codewords at low mean photon number (Smith et al., 19 Jun 2025).
• Capacity Enhancement via Refocusing and Multiplexing: Properly designed optical refocusing (thin-lens systems) and spatial-mode multiplexing raise the Fresnel number, increasing both mode count and total lossless throughput compared to free-space propagation. Nearfield lens systems (${\cal F}\gg1$) can accommodate multiple lossless modes, yielding capacity $C_{\rm near}=\nu g(N/\nu)$ (Lupo et al., 2011).
• Energy-Constrained Rate Enhancement: On bosonic Gaussian channels, introducing correlated multi-mode thermal states (via Gaussian Fourier unitaries) convexifies the coherent-information function and yields strictly higher lower bounds on the quantum and private capacities compared to single-mode strategies, especially in the low-energy or high-loss regime (Noh et al., 2018).
• Indefinite Causal Order Advantages: Combining two completely depolarizing channels in a coherent superposition of orders ("quantum SWITCH") yields a nonzero classical capacity $\chi \approx 0.048$ bits per double use in the qubit case, strictly impossible in any fixed order arrangement (Ebler et al., 2017). Series composition with quantum-controlled operations can further enhance coherent information in broad classes of Pauli channels (Rubino et al., 2020).

4. Quantum-Enhanced Multiuser, Secure, and Hybrid Systems

Quantum resources enable new functionality for multiuser communication, key distribution, and hybrid quantum-classical systems:

• Quantum Key Distribution Integration: Quantum-enhanced links provide information-theoretic security via protocols such as BB84, entanglement-based QKD, and QSDC. Quantum-processing-assisted receivers are robust to various attacks and fundamental eavesdropper bounds are tied to the Holevo information (Gisin et al., 2010, Wang et al., 11 Nov 2025).
• Quantum Semantic Communication: Combining quantum secure direct communication (QSDC) with semantic compression achieves equivalent data rates exceeding Shannon and Wyner's classical secrecy capacities for data objects such as 3D point clouds. Experimentally, $R_{\rm eq}(10)=1591.52$ kbps at $n=10$ bits/block, surpassing both Shannon capacity $C=1496.53$ kbps and secrecy capacity $C_s=560.20$ kbps at 50 km (Wang et al., 11 Nov 2025).
• Simultaneous Quantum-Classical Transmission: Minimal modifications to classical free-space optical links enable coexisting high-rate classical and quantum (QKD) communication. Dual-measurement receivers, possibly assisted by non-deterministic noiseless linear amplifiers, achieve repeaterless scaling for QKD and maintain classical bit error rates at high loss (Zaunders et al., 15 May 2024).
• Multi-Access Scaling via QCDMA: Code Division Multiple Access using quantum state encoding allows scaling the number of users $M$ and code length $N_c$ such that in the Fock-state regime, multi-user interference is eliminated by phase uncertainty, and in the coherent-state regime, joint detection enables approaching bosonic channel capacity (Rezai et al., 2021).

5. Noise Engineering, Physical Layer Security, and Implementation

Quantum-enhanced communication systems often utilize controlled noise or specific device-level innovations to surpass or secure performance:

• Noise-Assisted Capacities: Controlled dephasing in quantum networks can suppress destructive interference and increase effective transfer probability $\eta$, thereby enhancing both classical (Holevo) and quantum (coherent information) capacities beyond the noiseless scenario (Caruso et al., 2010).
• Physical Layer Security: Thermo-optical power limiters provide passive, wavelength- and pulse-width-robust limits on incident optical power, safeguarding quantum links against Trojan-horse or blinding attacks without degrading signal integrity (QBER-opt $<0.1\%$), and enabling secure deployment in practice (Zhang et al., 2020).
• Memory-Assisted Repeater Architectures: Integration of high-cooperativity spin-photon interfaces and asynchronous Bell-state measurement enables secret key rates exceeding direct-transmission limits by factors of $>4$ at fiber-equivalent losses up to 69 dB (~350 km), paving the way toward practical repeater chains (Bhaskar et al., 2019).

6. Scaling, Engineering Challenges, and Future Directions

Significant open challenges and avenues for further enhancement include:

• Hardware Error Rates and Loss Budgets: Achieving sub-percent photon storage loss and gate error rates $<10^{-4}$ is necessary for quantum-processing-assisted capacity gains; improvements in cavity-QED and material integration are essential (Smith et al., 19 Jun 2025).
• Efficient Resource Allocation and Networking: AI/ML frameworks are increasingly deployed for real-time parameter tuning, phase stabilization, scheduling, and anomaly detection in quantum networks (e.g., LSTM, RF, DRL, GA approaches), enabling autonomous and optimized networking (Xu et al., 12 Nov 2025).
• Integration with Quantum Internet Architectures: Satellite-ground links leveraging CV-QKD, teleportation via downlink TMSV resources, and entanglement swapping are under active demonstration. Adaptive optics, frequency conversion, photonic integration, and cross-layer protocol stacks will be critical for end-to-end scalability (Elser et al., 2015, Villaseñor et al., 2021).
• Robust Protocol Design: Hybrid quantum-semantic coding, universal quantum multiple-access frameworks, and protocols exploiting superadditivity and non-classical causal orderings represent research targets for joint performance, security, and robustness demands.

Quantum-enhanced communication systems thus constitute a rapidly advancing field, structured around rigorous information-theoretic limits, enabled by quantum-device engineering, and aimed at ultimate performance in rate, security, and interconnectivity for both short- and global-scale networks (Gisin et al., 2010, Hasan et al., 2022, Smith et al., 19 Jun 2025).