Abstract The time-domain balanced homodyne detector (TBHD) is critical component in continuous-variable quantum key distribution (CVQKD). The shot-noise-limited attribute of a TBHD depends on the whole spectrum of detector, in contrast to a frequency-domain balanced homodyne detector that focuses only on part of the spectrum. We propose a method for accurate calibration of the shot-noise-limited attribute of the TBHD by overlapping the gain lines of the TBHD and an integrated photodetector. The overlapping of two lines indicates that the detector is indeed shot noise limited, which is crucial to the security of CVQKD. Experimentally, we use this method to obtain a calibration error of ±1‰. Theoretical analysis showed that the CVQKD performance is very sensitive to calibration errors of shot noise, and proper countermeasures should be employed to eliminate this issue. The presented methods are expected to contribute to CVQKD security and TBHD standardization.
Abstract The time-domain balanced homodyne detector (TBHD) is critical component in continuous-variable quantum key distribution (CVQKD). The shot-noise-limited attribute of a TBHD depends on the whole spectrum of detector, in contrast to a frequency-domain balanced homodyne detector that focuses only on part of the spectrum. We propose a method for accurate calibration of the shot-noise-limited attribute of the TBHD by overlapping the gain lines of the TBHD and an integrated photodetector. The overlapping of two lines indicates that the detector is indeed shot noise limited, which is crucial to the security of CVQKD. Experimentally, we use this method to obtain a calibration error of ±1‰. Theoretical analysis showed that the CVQKD performance is very sensitive to calibration errors of shot noise, and proper countermeasures should be employed to eliminate this issue. The presented methods are expected to contribute to CVQKD security and TBHD standardization.
We propose a rigorous calibration method for homodyne detection efficiency, which combines all the factors that affect detection efficiency to calibrate together through the actual homodyne detection. With this method, the transmittance converted from electronic noise in the one-time calibration method of the shot noise can be attributed to the detection inefficiency. Thus, a trusted detection noise-free model for continuous-variable quantum key distribution (CV-QKD) can be established, which simplifies the calibration of shot noise while having the same performance as the trusted detection noise model. We demonstrate this calibration method with a balanced detector based on a transimpedance amplifier. Experimental results show that detection efficiency will be overestimated if the integration factor of the detector is overlooked. The overestimation of the detection efficiency leads to an underestimation of modulation variance and excess noise when the modulation variance is monitored by the balanced detector, which opens security loopholes. Our method may prove a necessary method in the calibration of detection efficiency for CV-QKD.
In Gaussian-modulated coherent-state quantum key distribution, the measurement of quadratures of coherent states is performed by using a homodyne detector. However, conventional detectors usually suffer from narrow bands. We present a method to design a high-speed shot-noise-limited balanced homodyne detector. A 300-MHz bandwidth detector is experimentally tested and the level of shot noise is 14 dB higher than the electronic noise. The results show that a detector with this method is potential to design a GHz bandwidth detector for continuous variable quantum key distribution at a low level of ratio of shot noise to electronic noise.
Abstract We designed and experimentally demonstrated a silicon photonics-integrated time-domain balanced homodyne detector (TBHD), containing an optical part of dimensions of 1.5 mm × 0.4 mm. To automatically and accurately balance the detector, new variable optical attenuators were used, and a common mode rejection ratio of 86.9 dB could be achieved. In the quantum tomography experiment, the density matrix and Wigner function of a coherent state were reconstructed with 99.97% fidelity. The feasibility of this TBHD in a continuous-variable quantum key distribution (CVQKD) system was also demonstrated. Our TBHD technologies are expected to be used in silicon photonics-integrated CVQKD system and silicon photonics-integrated BB84 heterodyne system.
The shot-noise unit in continuous-variable quantum key distribution plays an important and fundamental role in experimental implementation, as it is used as a normalization parameter that contributes to performing security analyses and distilling key information. However, the traditional calibration procedure and detector model cannot cover all system noise in practical applications, which results in loopholes and influences the practical security. What is more, the traditional procedure is also rather complicated and has difficulty in providing compatibility with automatic operating systems. In this paper, we propose a calibration model based on a proposed trusted detector model, which could naturally close the loopholes in practical applications. It can help to identify the shot-noise unit in only one step, which can not only effectively simplify the evaluation process but also reduce the statistical fluctuations, while two steps are needed in the traditional method. We prove its feasibility and derive a complete version of the corresponding entanglement-based model. A detailed security analysis against arbitrary collective attacks and numerous simulation results in both the asymptotic-limit regime and the finite-size regime are provided. A proof-of-principle experiment is implemented, and the results indicate that our one-time calibration model can be employed as a powerful alternative method for calibrating the shot-noise unit. Our method paves the way for the deployment of continuous-variable quantum key distribution with real-time calibration and automatic operation.
Imperfections in practical detectors, including limited detection efficiency, and inherent electronic noise, can seriously decrease the transmission distance of continuous-variable measurement-device-independent quantum key distribution systems. Owing to the difficulties inherent in realizing a high-efficiency fiber homodyne detector, challenges still exist in continuous-variable measurement-device-independent quantum key distribution system implementation. We offer an alternative approach in an attempt to solve these difficulties and improve the potential for system implementation. Here, a novel practical detector modeling method is utilized, which is combined with a one-time shot-noise-unit calibration method for the purpose of system realization. The new modeling method benefits greatly from taking advantage of one-time shot-noise-unit calibration methods, such as measuring electronic noise and shot noise directly to a novel shot-noise unit, so as to eliminate the statistical fluctuations found in previous methods; this makes the implementation of such systems simpler, and the calibration progress more accurate. We provide a simulation of the secret key rate versus distance with different parameters. In addition, the minimal detection efficiency required at each distance, as well as the contrast between the two methods, are also shown, so as to provide a reference in terms of system realization.
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.