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Journal of Optical Communications and Networking

Quantum key distribution with spectral amplitude coding OCDMA for a multiple access network

P. O. Amadi, Syed Alwee Aljunid, N. Ali, Syed Mohammad Ammar, Nursalasawati Rusli, R. Endut, A. M. Alhassan

Published Oct 29, 2025• 1 citation
Publisher

Abstract

This study presents the design and analysis of a hybrid multi-user quantum key distribution (QKD) system utilizing spectral amplitude coding optical code division multiple access (SAC-OCDMA) encoding techniques. By assigning unique optical codes to each user, SAC-OCDMA enables spontaneous, asynchronous data transmission without any strict synchronization, which makes it scalable and flexible for quantum networks. In the architecture, each user’s quantum signal, initially prepared as weak coherent pulses and encoded using phase or polarization bases, is further spectrally sliced by a SAC-OCDMA encoder in a zero-based cross-correlation code. The physical impairments, comprising spontaneous Raman scattering, four-wave mixing, and crosstalk, were modeled and analyzed. We report a maximum secret key rate of ∼10 5 bps over a transmission distance of ∼58km . Furthermore, our analysis demonstrates that careful selection of launch power, simultaneous users, code weight, and spectral bin width is necessary for optimizing the trade-off between multi-user capacity and security performance. In comparison with wave division multiplexing QKD, which has much better spectral efficiency but strict channel allocation, the flexibility and asynchronous access to secure communication of our OCDMA-QKD design with zero cross-correlation coding are more aligned with the needs of quantum security.

DOI
10.1364/jocn.574271

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This study presents the design and analysis of a hybrid multi-user quantum key distribution (QKD) system utilizing spectral amplitude coding optical code division multiple access (SAC-OCDMA) encoding techniques. By assigning unique optical codes to each user, SAC-OCDMA enables spontaneous, asynchronous data transmission without any strict synchronization, which makes it scalable and flexible for quantum networks. In the architecture, each user’s quantum signal, initially prepared as weak coherent pulses and encoded using phase or polarization bases, is further spectrally sliced by a SAC-OCDMA encoder in a zero-based cross-correlation code. The physical impairments, comprising spontaneous Raman scattering, four-wave mixing, and crosstalk, were modeled and analyzed. We report a maximum secret key rate of ∼10 5 bps over a transmission distance of ∼58km . Furthermore, our analysis demonstrates that careful selection of launch power, simultaneous users, code weight, and spectral bin width is necessary for optimizing the trade-off between multi-user capacity and security performance. In comparison with wave division multiplexing QKD, which has much better spectral efficiency but strict channel allocation, the flexibility and asynchronous access to secure communication of our OCDMA-QKD design with zero cross-correlation coding are more aligned with the needs of quantum security.
Quantum Key Distribution (QKD) has remained the provable technique of ensuring secure communication amidst the impending security threat by the emerging quantum computers. Its security guarantee is based on superposition and entanglement, which are fundamental principle of quantum mechanics. As efforts are currently underway toward actualization of QKD network, researchers have explored a number of techniques though which this could be achieved. Some of the techniques proposed are; subcarrier based QKD network, Orthogonal Frequency Division Multiplexed (OFDM) based QKD network and spectral amplitude coding optical code division multiplexed (SAC-OCDMA) based QKD network. Among the aforementioned technique, SAC-OCDMA based system has history of efficient channel management, asynchronous nature, as well as channel security. Nevertheless, the adopted Optical Orthogonal Code (OOC) which is a family of OCDMA code, has a couple of challenges which include complex architecture, as well as longer code length, both of which are capable of resulting to reduction in the secure key rate and number of simultaneous users. Thus, this work presents a new one-weight authentication code, which are assigned, uniquely to individual user in SAC-OCDMA based QKD network. The performance of the code was explored in QKD network using simulation, mathematics and laboratory experiment. Firstly, the proof of concept of plug and play (p&p) in QKD network was carried out and the concept of phase reorientation was clearly established using Optisystem simulation tool. This was followed by mathematical modelling of the secure key rate and QBER for different number of users. As observed in the obtained results, at an average mean photon number of 0.48, a 21-user network was achieved at a secure key rate of 24 bps over a distance of 5 km. The result of comparison of the proposed code with a Wavelength Division Multiplexed (WDM) system shows its security capability above the later. The code was further compared with an OOC based network and found to generate key at higher rate when the number of users is below 14. A two-user experimental result shows the real life realization of the proposed system as a transmission distance up to 40 km was achieved with secure key rate of 320 bps. Application of the proposed code in Differential Phase Shift Key – Quantum Key Distribution (DPS-QKD) environment was also established. The obtained results have shown that aside presenting a secure mechanism, the design flexibility and moderate code length exhibited by the proposed code are excellent factors that could be explored in enhancing the performance of QKD network.
Spectral Amplitude Coding Optical Code Division Multiple Access (SAC OCDMA) is an advanced technique in asynchronous environments. This paper proposes design and implementation of a novel quantum logic gate (QLG) code, with code construction algorithm generated without following any code mapping procedures for SAC system. The proposed code has a unitary matrices property with maximum overlap of one chip for various clients and no overlaps in spectra for the rest of the subscribers. Results indicate that a single algorithm produces the same length increment for codes with weight greater than two and follows the same signal to noise ratio (SNR) and bit error rate (BER) calculations for a higher number of users. This paper further examines the performance of a QLG code based SAC-OCDMA system with NAND and direct detection techniques. BER analysis was carried out for the proposed code and results were compared with existing MDW, RD and GMP codes. We demonstrate that the QLG code based system performs better in terms of cardinality, which is followed by improved BER. Numerical analysis reveals that for error free transmission (10−9), the suggested code supports approximately 170 users with code weight 4. Our results also conclude that the proposed code provides improvement in the code construction, cross-correlation and minimization of noises.
We present a quantum code-division multiple-access (q-CDMA) framework for multiuser continuous-variable quantum key distribution (CV-QKD) over a shared quantum channel. The proposed architecture employs chaotic phase shifters to encode and decode quantum states, enabling efficient multiplexing and demultiplexing of signals generated by multiple transmitters. In this scheme, quantum states from different users are chaotically phase-encoded and combined through a beam splitter network before transmission. At the receiver, synchronized chaotic phase shifters are used for decoding, followed by an inverse beam splitter structure to recover the individual user signals. This chaotic synchronization allows reliable state recovery and secure key establishment between each sender-receiver pair. For an arbitrary number of users, we derive the input-output quadrature relations describing the multiuser q-CDMA CV-QKD system. Using this model, we evaluate the achievable secret key rate under collective attacks with reverse reconciliation. We further investigate the impact of key system parameters including the correction factor, multiuser interference noise, environmental noise, and channel transmittance. A comparison between the asymptotic and finite-size regimes is also presented to highlight the associated performance trade-offs. These results provide a theoretical framework for assessing the performance of q-CDMA-based CV-QKD and support the development of scalable and secure multiuser quantum communication networks.
This paper addresses multi-user quantum key distribution networks, in which any two users can mutually exchange a secret key without trusting any other nodes. The same network also supports conventional classical communications by assigning two different wavelength bands to quantum and classical signals. Time and code division multiple access (CDMA) techniques, within a passive star network, are considered. In the case of CDMA, it turns out that the optimal performance is achieved at a unity code weight. A listen-before-send protocol is then proposed to improve secret key generation rates in this case. Finally, a hybrid setup with wavelength routers and passive optical networks, which can support a large number of users, is considered and analyzed.
We propose a quantum-classical access network architecture based on multicore fiber to scale up the number of quantum key distribution (QKD) users. Quantum signals are integrated with classical signals in both feeder and drop fibers to save deployment cost since access networks are cost-sensitive. To increase the secure key rate (SKR) of QKD, we show that wavelength-time division multiplexing is suitable for quantum signals, which are required to support a large number of quantum users. Also, we propose a core and wavelength assignment scheme to alleviate spontaneous Raman scattering and intercore cross talk noise, which are the main impairment sources to QKD in this quantum-classical network. Finally, we experimentally show the performance of QKD in the proposed quantum-classical access network in terms of noise and channel loss. The experiment results are consistent with our analysis. The characteristics of the SKRs prove the superiority of wavelength-time division multiplexing. The properties of spontaneous Raman scattering in the feeder and drop fibers are shown in the experiments through the quantum bit error rates in different experimental conditions, which verifies the effectiveness of the proposed core and wavelength assignment scheme.
Theoretical research and numerical simulation of the noise influence caused by spontaneous Raman scattering, four-wave mixing, and linear channel cross talk on the performances of quantum key distribution (QKD) systems were conducted. Three types of QKD systems were considered: a coherent one-way QKD protocol, subcarrier-wave QKD system, and continuous-variable QKD integrated with classical dense wavelength division multiplexing (DWDM) channels. We calculate the secure key generation rate for the systems mentioned addressing different channel allocation schemes (i.e., configurations). A uniform DWDM grid is considered with a quantum channel located in the C-band and O-band (at 1310 nm) of a telecommunication window. The systems’ performances are analyzed in terms of the maximal achievable distance values. Configurations for further analysis and investigation are chosen optimally, i.e., their maximal achievable distances are the best.