We propose OFDM to improve key generation rate of frequency-coded QKD system. Simulation results show raw keylength of 20 kbits/s for link lengths upto 50 km. Frequency guard bands and the choice of orthogonal sub-carrier spacing reduces noise power, leading to lower QBER and increased link length.
We propose a decoy-state method to increase key generation rate of frequency-coded (FC) implementation of B92 quantum key distribution (QKD) protocol. Direct application of decoy-state method to FC-QKD results in a security loophole. We overcome this loophole by optimizing the ratio of decoy-to-signal carrier photon number to unity. Simulation results for a single-channel FC-QKD show 16 kbits of raw key length at a transmission distance of 70 km. We then extend the decoy-state method to subcarrier-multiplexed FC-QKD. We show that key generation rate and secure link length improves by a factor of 5 and 40 km, respectively, compared to single-carrier with decoy-state method.
In this paper, we investigate the performance of orthogonal frequency division multiplexed quantum key distribution (OFDM-QKD) in an integrated quantum-classical wavelength-division-multiplexing system. The presence of an intense classical signal alongside the quantum one generates Raman background noise. Noise reduction techniques should, then, be carried out at the receiver to suppress this crosstalk noise. In this work, we show that OFDM-QKD enables efficient filtering, in time and frequency domains, making it an attractive solution for the high-rate links at the core of quantum-classical networks.
We propose a decoy-pulse protocol for frequency-coded implementation of B92 quantum key distribution protocol. A direct extension of decoy-pulse method to frequency-coding scheme results in security loss as an eavesdropper can distinguish between signal and decoy pulses by measuring the carrier photon number without affecting other statistics. We overcome this problem by optimizing the ratio of carrier photon number of decoy-to-signal pulse to be as close to unity as possible. In our method the switching between signal and decoy pulses is achieved by changing the amplitude of RF signal as opposed to modulating the intensity of optical signal thus reducing system cost. Our simulation results show a key generation rate of 1.5×10-4/pulse for link lengths up to 70 km. Finally, we discuss the optimum value of average photon number of signal pulse for a given key rate while also optimizing the carrier ratio.
Quantum key distribution (QKD), providing a way to generate secret keys with information-theoretic security, is arguably one of the most significant achievements in quantum information. The continuous-variable QKD (CV-QKD) offers the potential advantage of achieving a higher secret key rate (SKR) within a metro area, as well as being compatible with the mature telecom industry. However, the SKR and transmission distance of state-of-the-art CV-QKD systems are currently limited. Here, based on the proposed orthogonal-frequency-division-multiplexing (OFDM) CV-QKD protocol, we demonstrate a high-rate multi-carrier (MC) CV-QKD with a 10 GHz symbol rate that achieves Gbps SKR within 10 km and Mbps SKR over 100 km in the finite-size regime under composable security against collective attacks. The results are achieved by suitable optimization of subcarrier number and modulation variance, well-controlled excess noise induced by both the OFDM mechanism and the efficient DSP scheme, and high-performance post-processing capacity realized by the heterogeneous computing scheme. The composable finite-size SKR reaches 1779.45 Mbps@5 km, 1025.49 Mbps@10 km, 370.50 Mbps@25 km, 99.93 Mbps@50 km, 25.70 Mbps@75 km, and 2.25 Mbps@100 km, which improves the SKR by two orders of magnitude and quintuples the maximal transmission distance compared to the most recently reported CV-QKD results [ Nat. Commun. 13 , 4740 ( 2022 ) NCAOBW 2041-1723 10.1038/s41467-022-32161-y ]. Interestingly, it is experimentally verified that the SKR of the proposed MC CV-QKD can approach five times larger than that of the single-carrier CV-QKD with the same symbol rate without additional hardware costs. Our work constitutes a critical step toward future high-speed quantum metropolitan and access networks.
We introduce a continuous-variable quantum key distribution (CVQKD) protocol that combines orthogonal frequency-division multiplexing (OFDM) with terahertz (THz) carriers to deliver high-throughput and hardware-compatible quantum communication. By distributing quantum states across multiple subcarriers, our approach achieves a noticeable increase in spectral efficiency while mitigating dispersion and atmospheric losses that limit the performance of existing optical and microwave CVQKD systems. A full security analysis under collective Gaussian attacks is presented, incorporating realistic noise models for both terrestrial and inter-satellite channels. In terrestrial free-space links, our simulations show secret key rates approaching 72 bits per channel use, with secure distances up to 4.5 m under strong humidity-induced absorption. For inter satellite links, where propagation losses are minimal, secure transmission is sustained over >100 km. A distinctive feature of our work is its direct link to practical hardware. We evaluate implementation using emerging on-chip coherent THz sources based on superconducting Josephson junctions. These compact, voltage-tunable emitters provide wideband coherent radiation ideally matched to our protocol, enabling scalable, chip integrated quantum networks. By incorporating their measured characteristics into our models, we demonstrate secure communication up to 3 m in ambient conditions and ∼26 km under cryogenic or vacuum conditions. By uniting advanced protocol design, rigorous security modelling, and hardware driven performance analysis, this work establishes THz CVQKD as a viable and scalable route to next-generation terrestrial and space-based quantum communication.
We propose orthogonal frequency-division multiplexing (OFDM), as a spectrally efficient multiplexing technique, for quantum key distribution at the core of trusted-node quantum networks. Two main schemes are proposed and analyzed in detail, considering system imperfections, specifically, time misalignment issues. It turns out that while multiple service providers can share the network infrastructure using the proposed multiplexing techniques, no gain in the total secret key generation rate is obtained if one uses conventional passive all-optical OFDM decoders. To achieve a linear increase in the key rate with the number of channels, an alternative active setup for OFDM decoding is proposed, which employs an optical switch in addition to conventional passive circuits. We show that by using our proposed decoder, the bandwidth utilization is considerably improved as compared to conventional wavelength-division multiplexing techniques.