Continuous-variable (CV) quantum communication is an important branch of quantum information technology and promises information-theoretical secure communication. High-performance and lightweight CV quantum devices are one of the keys to realizing quantum networks. Among them, the Gaussian-modulation module still has problems such as cumbersome photoelectric calibration and inconvenient use in the experimental stage, which introduces modulation noise and brings additional excess noise to the system, reducing the signal-to-noise ratio of the system and thus reducing system performance. To this end, by designing a field-programmable gate array and integrating all optical components (except the laser source), we achieved a miniaturized and low-cost Gaussian-modulation unit compatible with the existing fiber-optic communication infrastructure, realizing quantum signal modulation under adaptive photoelectric calibration. Taking CV quantum key distribution as the verification object, we analyzed the signal characteristics and the stability of the unit experimentally. The experimental results show that the proposed scheme exhibits quality Gaussian properties and correlations. Moreover, the temporal stability of the device is relatively high (the mean fluctuation is on the order of ${10}^{\ensuremath{-}31}$ and the variance fluctuation is 0.0363), which provides more possibilities for the subsequent popularization of quantum communication and the development of quantum information networks.
Continuous-variable (CV) quantum communication is an important branch of quantum information technology and promises information-theoretical secure communication. High-performance and lightweight CV quantum devices are one of the keys to realizing quantum networks. Among them, the Gaussian-modulation module still has problems such as cumbersome photoelectric calibration and inconvenient use in the experimental stage, which introduces modulation noise and brings additional excess noise to the system, reducing the signal-to-noise ratio of the system and thus reducing system performance. To this end, by designing a field-programmable gate array and integrating all optical components (except the laser source), we achieved a miniaturized and low-cost Gaussian-modulation unit compatible with the existing fiber-optic communication infrastructure, realizing quantum signal modulation under adaptive photoelectric calibration. Taking CV quantum key distribution as the verification object, we analyzed the signal characteristics and the stability of the unit experimentally. The experimental results show that the proposed scheme exhibits quality Gaussian properties and correlations. Moreover, the temporal stability of the device is relatively high (the mean fluctuation is on the order of 10\ensuremath−31 and the variance fluctuation is 0.0363), which provides more possibilities for the subsequent popularization of quantum communication and the development of quantum information networks.
Quantum secure direct communication (QSDC) realizes the transmission of secret messages directly in a quantum channel. A continuous-variable- (CV) based quantum-communication system allows high-speed, large-capacity information transmission in optical telecommunication systems. In this work, we propose a QSDC protocol based on Gaussian mapping. As for Gaussian modulation, the designed Gaussian-mapping scheme effectively solves the problem, which results from the nonuniformity of the secret-message bitstream, and is applicable to different modulation variances. The system performance analysis shows that the proposed scheme can realize inerrant Gaussian modulation of secret messages, and secure transmission of messages under information theory. Moreover, our work hopes to stimulate discussion on experimental realization and practical advance of CV-QSDC protocol.
Quantum key distribution (QKD) offers a promising solution by providing information-theoretic security based on the principles of quantum mechanics. Beyond theoretical experiments, QKD network construction and verification are actively progressing in various locations worldwide. Continuousvariable QKD (CVQKD) has emerged as a strong candidate for integration into optical networks, particularly due to its compatibility with coherent detection schemes used in existing optical communication systems. Gaussian-modulated (GM) CVQKD enables simpler security analysis through covariance matrices, but its practical implementation typically requires highperformance optical components, leading to increased cost and complexity. In contrast, discrete-modulated (DM) CVQKD using modulation formats, such as quadrature amplitude modulation (QAM), can reduce implementation costs by utilizing the existing optical components. However, its security analysis requires more advanced techniques due to the non-Gaussian nature of the quantum states involved. This study evaluates the performance of GM- and DM-CVQKD under a common optical fiber channel model. Our results show that the secure key rates (SKRs) of DM-CVQKD with probabilistic constellation shaping (PCS) 64QAM and PCS-256QAM drop below zero at transmission distances of 277 km and 399 km, respectively. The results confirm the feasibility of DM-CVQKD for long-distance secure communication over optical fiber networks when appropriate modulation formats and PCS are employed.
Compared with discrete variable quantum key distribution, the Continuous Variable Quantum Key Distribution (CVQKD) system has the advantages of simple equipment, low technical difficulty, and high channel capacity, in addition to unconditional security and detectability for wiretapping. In this paper, the laser light source control system realizes the control and adjustment functions of the photon quantum state, which was encoded with information. In order to generate the synchronous optical pulse signals, the Field Programmable Gate Array (FPGA), multi-channel Digital to Analog Converter (DAC), and micro power Analog to Digital Converter (ADC) were used to control the synchronous optical laser. The control of multiple amplitude modulators and phase modulators is achieved based on the FPGA and high-speed DAC. At present, the electronics performance test of the laser light source control system has been completed, and the results show that the requirements of the CVQKD system index are met.
Abstract Quantum key distribution (QKD) is one quantum technology that can provide secure encryption keys for data transmission. The secret key rate (SKR) is a core performance indicator in QKD, which directly determines the transmission rate of enciphered data. Here, for the first time, we demonstrate a high-key-rate Gaussian-modulated continuous-variable QKD (CV-QKD) using telecom optical components. The framework of CV-QKD over these components is constructed. Specifically, the high-rate low-noise Gaussian modulation of coherent states is realized by a classical optical IQ modulator. High-baud low-intensity quantum signals are received by an integrated coherent receiver under the shot-noise limit. A series of digital signal processing algorithms are proposed to achieve accurate signal recovery and key distillation. The system can yield a high asymptotic SKR of 10.37 Mbps within 20 km standard telecom fiber, and the secure distance can exceed 100 km. This result confirms the feasibility of CV-QKD with state-of-the-art performance using telecom optical components. Besides, due to the ease of integrating these discrete components, it provides a high-performance and miniaturized QKD solution for the metropolitan quantum network.
Quantum information processing uses the laws of quantum physics to perform tasks that are infeasible or impossible classically. Within this broader programme, quantum communication has emerged as one of the most practically mature sub-fields of quantum information science, with several proposed protocols having already been demonstrated over deployed optical fibre and free-space links. Of the various platforms available for encoding quantum information for communication, the continuous-variable (CV) framework encodes information in the quadratures of the electromagnetic field and is therefore directly compatible with the existing machinery of classical coherent optical communications. The most prominent CV protocol is continuous-variable quantum key distribution (CV-QKD), which establishes an information-theoretically secure cryptographic key between two spatially separated parties. Because CV-QKD reuses much of the hardware of classical coherent telecommunications, including off-the-shelf lasers, modulators, and balanced detectors, it is a leading candidate for the deployment of quantum-secured communication over standard optical fibre networks. The development of CV-QKD has typically proceeded with the underlying physics dictating the engineering. Security proofs, often relying on the Gaussian optimality of collective attacks, set the structure of the protocol, and the practical system is then built to satisfy the assumptions on which those proofs depend. Contributions of this thesis adopt the opposite view, taking the practical engineering of high-rate CV-QKD systems as the starting point and developing the theoretical machinery required to support it. The first contribution is the formalisation of one-round sliced reconciliation with guard-band post-selection as a key distillation scheme designed for real-time, high-throughput operation. Originally introduced experimentally as a means to stabilise bit error rates and enable the use of fixed-rate error correction codes at high symbol rates, guard-band post-selection is shown here to identify the guard-band span as a free parameter that can be tuned to hold the quantum bit error rate approximately constant across a range of channel conditions. However, conditioning on accepted data produces non-Gaussian post-selected statistics that lie outside the reach of standard Gaussian-optimality security arguments, which motivates the remaining two contributions. The second contribution is an asymptotic security analysis of guard-band post-selected CV-QKD against arbitrary collective attacks, no longer restricted to Gaussian adversaries. By using a quantum channel estimate as side information and by lifting reduced-state distances to purification distances via entropic continuity bounds, security is certified against the class of channels lying close to a Gaussian thermal-loss reference, with a key-rate penalty that scales with the energy-constrained diamond distance to that reference. The third contribution is the experimental basis required to apply this framework in practice. A lightweight three-stage protocol for quantum channel estimation is proposed and demonstrated, which repurposes the Gaussian probe constellation already present in the CV-QKD signal and requires no additional hardware or modifications to the underlying protocol. Over single-mode optical fibre at distances from 10 km to 50 km, the protocol produces physically valid process tensor estimates whose energy-constrained diamond distances to a Gaussian reference channel remain on the order of 10^(-3) throughout. Together, these three contributions provide a treatment of post-selected Gaussian-modulated CV-QKD in which the engineering, the security theory, and the channel characterisation are designed to operate together within a deployable system.
Abstract Discrete-modulation continuous-variable (CV) quantum key distribution has the potential for large-scale deployment in secure quantum communication networks owing to its low implementation complexity and compatibility with the current coherent optical telecommunication. However, current discrete-modulation protocols require relatively large constellation sizes to achieve a key rate comparable to that of the Gaussian modulation. Here, we show that a high key rate comparable to the Gaussian modulation can be achieved using only ten or so coherent states by implementing suitable key map and numerical convex optimization techniques. Specifically, the key rate of the two-ring constellation with 12 coherent states (four states in the inner ring and eight states in the outer ring) can reach 2.4 times of that of original quadrature phase shift keying and 70% of the Gaussian modulation protocol at 50 km. Such an approach can easily be applied to existing systems, making the discrete-modulation protocol an attractive alternative for high-rate and low-cost applications in secure quantum communication networks.