The Continuous-variable quantum key distribution (CVQKD) technique is one of the practical uses of quantum information technology. CVQKD system is implemented based on existing optical networks and standard telecommunication technologies. CVQKD can be put into use of the information transmission in city-based areas, within the extent of absolute security and without any restrictions of security. To reduce the experimental system to a commercial prototype, a data acquisition (DAQ) card can be used to replace the DAQ equipment in the laboratory. It can save the cost of the system experiment and commercial application. In this paper, through the high-speed DAQ card, we implement the collection and uploading of the electrical signal, when the light passes through the continuous optical coherent state detector. The whole experiment test system consists of a field-programmable gate array (FPGA), an analog-to-digital converter (ADC) chip, and optical components. This paper provides a method of gain adjustment of the balanced homodyne detector and a way to test the π phase voltage of the optical modulator. The results show that the DAQ card can be used in the development of the CVQKD prototype.
The Continuous-variable quantum key distribution (CVQKD) technique is one of the practical uses of quantum information technology. CVQKD system is implemented based on existing optical networks and standard telecommunication technologies. CVQKD can be put into use of the information transmission in city-based areas, within the extent of absolute security and without any restrictions of security. To reduce the experimental system to a commercial prototype, a data acquisition (DAQ) card can be used to replace the DAQ equipment in the laboratory. It can save the cost of the system experiment and commercial application. In this paper, through the high-speed DAQ card, we implement the collection and uploading of the electrical signal, when the light passes through the continuous optical coherent state detector. The whole experiment test system consists of a field-programmable gate array (FPGA), an analog-to-digital converter (ADC) chip, and optical components. This paper provides a method of gain adjustment of the balanced homodyne detector and a way to test the π phase voltage of the optical modulator. The results show that the DAQ card can be used in the development of the CVQKD prototype.
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.
Summary form only given. Continuous-variable quantum key distribution (CVQKD) is a promising alternative to commonly used discrete-variable QKD protocols, because this technology can be implemented with standard telecom components and recent results suggest that it will be compatible with standard wavelength division multiplexed telecommunication networks [1]. Security proofs against general eavesdropping attacks are available for CVQKD protocols using Gaussian modulation [2], and field implementations over deployed telecommunication networks have been demonstrated over the past few years [3].Long-distance operation of CVQKD systems, however, has been hindered until now by the limited efficiency of the complex error-correction (or reconciliation) algorithms that are required to extract the secret key from the correlated data shared by Alice and Bob. We have designed high-efficiency error correcting-codes [4] allowing to extract an errorless secret key in a CVQKD protocol using a Gaussian modulation of coherent states and a homodyne detection system. These codes are available for a wide range of signal-to-noise ratios on a Gaussian noise channel with a binary modulation and can be combined with a multidimensional reconciliation method [5]. This improved reconciliation procedure considerably extends the secure range of a continuous-variable quantum key distribution with Gaussian modulation.
Compared with discrete variable quantum key distribution, continuous variable quantum key distribution (CVQKD) has the advantages of low cost, good stability, and high channel capacity, in addition to unconditional security and detectability for wiretapping. The basis of CVQKD system operation is the realization of loading information into photon quantum states. In this paper, a laser light source control system has realized the control and adjustment of the photon quantum states and completed the encoding of quantum states. In order to generate synchronous optical signals, FPGA, multi-channel DAC and micro power ADC are used to control the source of the synchronous optical laser. The control and modulation of multiple optical components are realized based on FPGA, multiple 16-bit 1.0 GSPS DACs and high-performance operational amplifiers. The electronic test results show that the system implements the control and adjustment functions of lasers, amplitude modulator, phase modulators, variable optical attenuators, polarization controllers, as well as detection of synchronized light. The performance of the laser light source control system fully meets the requirements of the CVQKD system.
Quantum key distributions, which could make legitimate communication parties Alice and Bob achieve the same random key with unconditional security, will have broad applications in defense, commerce, and communication. The protocol of the continuous variable quantum key distribution (CVQKD) has many advantages, such as easy preparation of the light source, high detector efficiency, and good compatibility with the classic fiber-optic communication systems. In recent years, great progress in the research of CVQKD has been made both theoretically and experimentally. In the protocol, the quadratures of the optical field with Gaussian or Non-Gaussian modulation are employed as the carriers of the key.The quadratures of the pulsed optical quantum states in CVQKD can be detected with a time-domain pulsed homodyne detector. The performance of the detector has great influences on the excess noises and the safe key rate of the quantum communication system. The measurement accuracy, which depends crucially on the common mode rejection ratio and the long-term stability, is the key performance of the detector. In order to improve the accuracy of measurement and avoid saturating the detector, we propose and demonstrate a technique to balance the two output beams of a 50/50 fiber coupler of the homodyne detector automatically. The auto-balance technique, which improves the long-term stability and high common mode rejection ratio, is described in the following.Firstly, the relation between the balance degree and the measurement accuracy is theoretically analyzed in detail. The result shows that a balance degree larger than 10-4 should be reached to ensure a high precision measurement when the intensity of the local oscillator pulse is 108 photons per pulse. Secondly, a fiber-based variable attenuator based on computer-controlled linear stepper motor is designed. The linear stepper motor that is used to drive the fiber coils has a small dimension of 20 cm20 cm28 cm and a minimum step size of 78 nm, and is controlled through the I/O port of a multifunction data acquisition card connected to a computer. The attenuations of the fiber coils of different radii are detected. The precision of attenuation is estimated to be on the order of 10-6 per 100 nm.The principle of the feedback control is described. A method of changing step-size which depends on the balance degree is proposed to fulfill a fast auto-balance process. Using the auto-feedback-control system, a balance degree of about 1.5610-6 can be achieved. The procedure of auto-balance takes about 1 s, and the evolution curves that represent the transformation process from various unbalanced states to the balanced state are presented.The auto-balance apparatus can ensure that the time-domain pulsed homodyne detector run stably in a longterm with a high common mode rejection ratio. The nonlinear and saturation effects due to the drift of the balance point are eliminated. The presented auto-balance time-domain pulsed homodyne detector can be well integrated into the continuous variable quantum key distribution system, and is expected to play an important role in improving the measurement accuracy and reducing the excess noises of the system. We believe that it could also be found to have potential applications in other areas.
Continuous variable quantum key distribution (CVQKD) uses the continuous quadratures of light to share a secret key between a transmitter and receiver. The presence of an eavesdropper can be detected by fundamental quantum mechanics. Compared to the more established discrete variable QKD, CVQKD uses more cost-effective homodyne detectors which are more compatible with current telecommunication technologies. In a future global quantum network, CVQKD could be used to ensure the secure exchange of information. In this work, the feasibility of a variety of CVQKD protocols in different channels that could be used to establish a global quantum network are studied. These are the Gaussian modulated and discrete modulated CVQKD protocols for satellite to optical ground station downlinks and uplinks for Earth-space connections, inter-satellite links for space connections, satellite to ship and underwater links for maritime connections, and fibre network links for terrestrial connections. Secret key distribution between continents and various regions is simulated while accounting for the different adverse effects associated with each link. The combination of these connections with routing and scheduling ensures inter-continental high-speed information exchange with unconditional security that is required to achieve the goal of a global quantum network.
Continuous-variable quantum key distribution (CVQKD) enables remote users to share high-rate and unconditionally secure secret keys while maintaining compatibility with classical optical communication networks and effective resistance against background noise. However, CVQKD experiments have been demonstrated only indoors or over short outdoor distances. Here, by developing channel-fluctuation-independent high-precision manipulation of continuous-variable quantum states, high-accuracy quantum signal acquisition and processing, and high-efficiency free-space acquisition, tracking, and pointing technology, we overcome the excess noise due to atmospheric effects especially in daylight without extra wavelength conversion and narrow-linewidth spectral filtering and demonstrate for the first time long-distance free-space quantum key distribution under the asymptotic condition over 7-km inland and 9.6-km maritime atmospheric channels with Gaussian-modulated coherent states. Given that the CVQKD system is naturally compatible with existing ground fiber telecommunication networks, it marks an essential step for realizing integrated air-ground quantum access networks with cross-domain applications.