Floodlight quantum key distribution (FL-QKD) is a new QKD protocol that can achieve a 2 Gbps secret key rate (SKR) in a 50 km fiber link without multiplexing technology [Q. Zhuang et al., Phys. Rev. A94, 012322 (2016)PLRAAN1050-294710.1103/PhysRevA.94.012322]. In this paper, we propose a wireless FL-QKD at terahertz bands (THz-FL-QKD) in inter-satellite links. THz-FL-QKD is the two-way protocol that sends quantum signals in the forward channel, modulates and amplifies the received signals at the receiver, and then returns to the transmitter through the backward channel for homodyne detection and decoding. We analyze the security of THz-FL-QKD against individual attacks and optimum collective attacks. Numerical simulations show that THz-FL-QKD is capable of a 50 Mbps SKR at 10 THz frequency in a 200 km inter-satellite wireless link. We expect this work will provide an efficient path to build a high-speed global quantum communication network.
Floodlight quantum key distribution (FL-QKD) is a new QKD protocol that can achieve a 2 Gbps secret key rate (SKR) in a 50 km fiber link without multiplexing technology [Q. Zhuang et al., Phys. Rev. A94, 012322 (2016)PLRAAN1050-294710.1103/PhysRevA.94.012322]. In this paper, we propose a wireless FL-QKD at terahertz bands (THz-FL-QKD) in inter-satellite links. THz-FL-QKD is the two-way protocol that sends quantum signals in the forward channel, modulates and amplifies the received signals at the receiver, and then returns to the transmitter through the backward channel for homodyne detection and decoding. We analyze the security of THz-FL-QKD against individual attacks and optimum collective attacks. Numerical simulations show that THz-FL-QKD is capable of a 50 Mbps SKR at 10 THz frequency in a 200 km inter-satellite wireless link. We expect this work will provide an efficient path to build a high-speed global quantum communication network.
Floodlight quantum key distribution (FL-QKD) is a radically different QKD paradigm that can achieve gigabit-per-second secret-key rates over metropolitan area distances without multiplexing [Q. Zhuang et al., Phys. Rev. A 94, 012322 (2016)]. It is a two-way protocol that transmits many photons per bit duration and employs a high-gain optical amplifier, neither of which can be utilized by existing QKD protocols, to mitigate channel loss. FL-QKD uses an optical bandwidth that is substantially larger than the modulation rate and performs decoding with a unique broadband homodyne receiver. Essential to FL-QKD is Alice's injection of photons from a photon-pair source---in addition to the light used for key generation---into the light she sends to Bob. This injection enables Alice and Bob to quantify Eve's intrusion and thus secure FL-QKD against collective attacks. Our proof-of-concept experiment included 10 dB propagation loss---equivalent to 50 km of low-loss fiber---and achieved a 55 Mbit/s secret-key rate (SKR) for a 100 Mbit/s modulation rate, as compared to the state-of-the-art system's 1 Mbit/s SKR for a 1 Gbit/s modulation rate [M. Lucamarini et al., Opt. Express 21, 24550 (2013)], representing $\ensuremath{\sim}500$-fold and $\ensuremath{\sim}50$-fold improvements in secret-key efficiency (bits per channel use) and SKR (bits per second), respectively.
The channel loss incurred in long-distance transmission places a significant burden on quantum key distribution (QKD) systems: they must defeat a passive eavesdropper who detects all the light lost in the quantum channel and does so without disturbing the light that reaches the intended destination. The current QKD implementation with the highest long-distance secret-key rate meets this challenge by transmitting no more than one photon per bit [M. Lucamarini et al., Opt. Express 21, 24550 (2013)]. As a result, it cannot achieve the Gbps secret-key rate needed for one-time pad encryption of large data files unless an impractically large amount of multiplexing is employed. We introduce floodlight QKD (FL-QKD), which floods the quantum channel with a high number of photons per bit distributed over a much greater number of optical modes. FL-QKD offers security against the optimum frequency-domain collective attack by transmitting less than one photon per mode and using photon-coincidence channel monitoring, and it is completely immune to passive eavesdropping. More importantly, FL-QKD is capable of a 2-Gbps secret-key rate over a 50-km fiber link, without any multiplexing, using available equipment, i.e., no new technology need be developed. FL-QKD achieves this extraordinary secret-key rate by virtue of its unprecedented secret-key efficiency, in bits per channel use, which exceeds those of state-of-the-art systems by two orders of magnitude.
We propose a continuous-variable quantum key distribution (CVQKD) scheme at terahertz (THz) bands based on multicarrier multiplexing (MCM) technology. In this scheme, multiple Gaussian modulated subcarriers are coupled to transmit multipath superposed thermal Gaussian states. At the receiver, optical discrete Fourier transform (ODFT) is used to demultiplex the received subcarriers, and the keys can be generated in parallel by homodyne detection and post processing. We analyze the security of the scheme against the optimal collective Gaussian attack under indoor environment and in inter-satellite links respectively. Results indicate that by using MCM technology, each subchannel may be affected by crosstalk, resulting in slight shortening of the maximal transmission distance while the total secret key rate can be greatly increased. We also verify the feasibility of higher key rate and longer distance THz-QKD by using MCM technology in inter-satellite links communication. We expect this work will provide an efficient path to build a global quantum communication network.
Floodlight quantum key distribution (FL-QKD) has realized a 1.3 Gbit/s secret-key rate (SKR) over a 10-dB-loss channel against a frequency-domain collective attack [Quantum Sci. Technol. 3, 025007 (2018)]. It achieved this remarkable SKR by means of binary phase-shift keying (BPSK) of multiple optical modes. Moreover, it did so with available technology, and without space-division or wavelength-division multiplexing. In this paper we explore whether replacing FL-QKD's BPSK modulation with a high-order encoding can further increase that protocol's SKR. First, we show that going to K-ary phase-shift keying with K=32 doubles---from 2.0 to 4.5 Gbit/s---the theoretical prediction from [Phys. Rev. A 94, 012322 (2016)] for FL-QKD's BPSK SKR on a 50-km-long fiber link. Second, we show that $2d\ifmmode\times\else\texttimes\fi{}2d$ quadrature amplitude modulation does not offer any SKR improvement beyond what its d=1 case---which is equivalent to quadrature phase-shift keying---provides.
We propose a novel continuous-variable quantum key distribution (CVQKD) protocol that employs orthogonal frequency-division multiplexing (OFDM) in the terahertz (THz) band to enable high-throughput and secure quantum communication. By encoding quantum information across multiple subcarriers, the protocol enhances spectral efficiency and mitigates channel dispersion and atmospheric attenuation. We present a comprehensive security analysis under collective Gaussian attacks, considering both terrestrial free-space channels, accounting for humidity-induced absorption, and inter-satellite links, incorporating realistic intermodulation noise. Simulations show secret key rates (SKR) reaching ~72 bits per channel use in open-air conditions. While intermodulation noise imposes trade-offs, optimised modulation variance enables resilience and secure communication range. The maximum terrestrial quantum link extends up to 4.5 m due to atmospheric THz absorption, whereas inter-satellite links can support secure communication over distances exceeding 100 km, owing to minimal propagation channel losses in space. We evaluate the practical implementation of our protocol using recently developed on-chip coherent THz sources based on superconducting Josephson junctions. These compact, voltage-tunable emitters produce wideband coherent radiation, making them ideal candidates for integration in scalable quantum networks. By incorporating their characteristics into our simulations, we assess secure key generation under various environmental conditions. Our results show secure communication over distances up to 3 m in open air, and up to 26 km in cryogenic or vacuum environments. This work advances the prospect of compact, high-capacity CVQKD systems for both terrestrial and space-based THz quantum communication.
Continuous variable quantum key distribution (CVQKD) based on a simultaneous quantum and classical communication (SQCC) protocol can use a single coherent communication system for multiple purposes. In this paper, we propose an inter-satellite SQCC-CVQKD scheme in the terahertz (THz) band. This scheme performs classical modulation at the sending end, followed by quantum Gaussian modulation operations. At the receiving end, an amplifier is used to amplify and demultiplex the received signal, and, finally, a homodyne detector or heterodyne detector is selected for decoding. We have demonstrated the security of the SQCC-CVQKD system in the THz band, and a performance analysis of the scheme is given in the finite-size regime. The simulation results show that this scheme has a higher secret key rate and stronger anti-interference ability in practice. This work provides an effective way to construct wireless quantum communication networks.