The simultaneous quantum and classical communication (SQCC) system implements continuous variable quantum key distribution (CVQKD) and classical communication using the same basic communication facilities. However, the SQCC system has a very low tolerance for phase noise, which affects its transmission distance and secret key rate under the local local oscillator (LO) design. In order to reduce the phase noise in the SQCC system, this paper proposes a method to automatically compensate the signal phase based on the long-short-term memory network (LSTM) model. Firstly, the LSTM model is trained to predict the phase value of the reference pulse during operation. Then the predicted value of the LSTM model to compensate for the phase drift of the quantum signal, thus reducing the phase noise. The results demonstrate that the automatic phase compensation method based on LSTM exhibits excellent prediction performance and compensation accuracy, which can significantly improve the transmission distance and secret key rate of the SQCC system without requiring any additional quantum resources and extra experimental hardware.

The simultaneous classical and continuous variable quantum communication can allow classical communication and continuous variable quantum key distribution with a local local oscillator (LLO CVQKD) to be implemented simultaneously using the same communication architecture. Such scheme enables LLO CVQKD to be integrated with classical communication at the minimum cost, but the existence of reference pulses will bring security loopholes to the simultaneous communication system, allowing eavesdroppers to launch attacks. In this paper, we analyze and simulate the performance of the system by introducing the trusted noise model under the phase reference pulse intensity attack, where Eve can hide her attack by manipulating the intensity of the reference pulse using a phase insensitive amplifier. A reconstructed noise model is proposed to solve this problem. Moreover, we show that the reconstructed noise model can strengthen the security of the practical simultaneous communication system.

Measurement-device-independent quantum digital signature (MDI-QDS) guarantees the authenticity and integrity of a message with security against all detector side-channel attacks. However, the experimental implementation of MDI-QDS in actual systems is extremely complicated since: (i) high-visibility interference requires several additional active control units for mode matching; (ii) a key assumption in MDI-QDS is that the laser source located at each user should be trusted. In this work, we propose a plug-and-play MDI-QDS scheme with a single untrusted source. On the one hand, it completely eliminates the requirement that each user must have a trusted laser source, and only needs to set an untrusted source at the Relay. On the other hand, the mode matching of MDI-QDS can be realized automatically with a plug-and-play structure. We then discuss the performance of the scheme by using the two-intensity decoy-state method with full parameter optimization. The numerical simulations show that the plug-and-play MDI-QDS with a single untrusted source provides comparable performances to the original MDI-QDS in a low-loss regime but with a simpler experimental setup. Consequently, the scheme can be flexibly implemented with common optical components in an optical fiber network since users can share both detectors and a complicated laser source.

The entropy source of a quantum random number generator (QRNG) is theoretically unpredictable, but in practice, imperfections in local oscillators and measuring devices introduce classical noise that inevitably contaminates the unpredictability of the quantum entropy source. This makes it a challenge for traditional methods to accurately estimate the min-entropy of QRNG, which in turn poses potential threats to the practical security of such systems. To address this issue, a novel min-entropy predictor based on deep neural networks is proposed to solve the min-entropy estimation problem for continuous-variable QRNG (CV-QRNG). First, the process of random number generation is systematically analyzed in CV-QRNG and the deviation between theoretical and practical randomness is discussed. Next, deep neural networks are used to construct a comprehensive min-entropy estimation strategy. Finally, extensive entropy estimation tests are conducted on various types of real-world quantum random number data. Experimental results show that the proposed predictor provides higher accuracy and reliability in min-entropy estimation for CV-QRNG, while also improving execution efficiency. In conclusion, the proposed predictor offers a simple and effective method for min-entropy estimation in CV-QRNG.

Reconfigurable origami-inspired folding has recently gained significant attention due to its potential to achieve multi-shape changes through crease topological morphing, resulting in multi-functionality from a single original structure. However, it is one of the major challenges to determine all possible mountain-valley crease (MV) assignment that ensure rigid foldability and obtain duplicate configurations in large 2D origami tessellations or 3D cellular structures.In this study, we developed a straightforward algorithm that combines graph theory and motion compatibility conditions to assess the reconfigurability of origami structures with 4-crease vertices, as well as their 2D and even 3D cellular tessellations. Our analysis has revealed three types of oligo-modal origami tessellations characterised by a consistent number of MV assignments independent of tessellation size, and pluri-modal cellular origami structures that exhibit negative, zero, and positive Poisson's ratios which were verified through experiment. This work could serve as a stepping stone towards developing multifunctional metamaterials and devices based on reconfigurable origami through mountain-valley crease assignment.

The quantum random number generator (QRNG) is theoretically capable of generating unpredictable random numbers based on the inherent uncertainty of quantum mechanics, which is of paramount importance for information security. However, the security of practical QRNG is susceptible to the influence of unknown classical noise introduced by imperfect measurement devices, leading to potential security threats. In this paper, we propose a deep learning-based prediction model for analyzing the bidirectional security of QRNG where the quantum source is contaminated by classical noise. Firstly, we train a deep learning model capable of evaluating the randomness of mixed entropy source composed of quantum source and classical source, which exhibits excellent performance and effectively avoids the limitations of Statistical tests in evaluating the randomness of the mixture of quantum and classical source results. Secondly, systematically analyzes the impact of non-ideal measurement devices on the practical security of the continuous variable QRNG, which provides an explicit basis for compensating the discrepancy between theory and experiment. Finally, we perform correlation detection on QRNG output sequences with a deep learning model and focus on both the forward and backward security of random numbers. Through bidirectional security detection, random number sequences that may be biased or manipulated can be more accurately and comprehensively evaluated, further preventing potential correlations from opening security holes for eavesdroppers.

Continuous-variable quantum key distribution with a local local oscillator (LLO CVQKD) has been extensively researched due to its simplicity and security. For practical security of an LLO CVQKD system, there are two main attack modes referred to as reference pulse attack and polarization attack presently. However, there is currently no general defense strategy against such attacks, and the security of the system needs further investigation. Here, we employ a deep learning framework called generative adversarial networks (GANs) to detect both attacks. We first analyze the data in different cases, derive a feature vector as input to a GAN model, and then show the training and testing process of the GAN model for attack classification. The proposed model has two parts, a discriminator and a generator, both of which employ a convolutional neural network (CNN) to improve accuracy. Simulation results show that the proposed scheme can detect and classify attacks without reducing the secret key rate and the maximum transmission distance. It only establishes a detection model by monitoring features of the pulse without adding additional devices.

In a practical continuous-variable quantum-key-distribution (CVQKD) system, it is vital to accurately evaluate and then compensate for the phase drifts of the signals, so that the involved system can achieve better performance and stability. In this paper, based on the long short-term memory network (LSTM) model, an automatic phase compensation approach of the CVQKD system is proposed. The LSTM model is first trained to predict the phase drift value of the quantum signal relative to the local oscillator over time. Then, the predicted phase drift value can be used by Alice to reconstruct her data. Finally, Alice and Bob can obtain the raw key, so that the CVQKD system can achieve enhancements in terms of performance and stability. The experimental results indicate that the proposed LSTM-based automatic phase compensation algorithm can accurately predict the phase drift value and perform phase compensation instead of real-time phase drift measurement, which improves the performance of the CVQKD system without requiring any additional quantum resources and extra experimental hardware.

We propose a new scheme to enhance the performance of the Gussian-modulated coherent-state continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) system via quantum scissors (QS) operation at Bob’s side. As an non-deterministic amplifying setup, we firstly introduce the QS-enhanced CV-MDI-QKD protocol and then investigate the success probability of the QS operation in accordance with the equivalent one-way scheme. Afterwards, we investigate the effect of the QS operation on the proposed scheme and analyze the performance of the QS-enhanced CV-MDI-QKD system under the extreme asymmetric circumstance. Simulation results show that the QS operation can indeed improve the performance of the CV-MDI-QKD system considerably. QS-enhanced CV-MDI-QKD protocol outperforms the original CV-MDI-QKD protocol in both the maximum transmission distance and the secret key rate. Moreover, the better the performance of QS operation, the more significant the improvement of performance of the system.

In this article, we propose a low-complexity quantum principal component analysis (qPCA) algorithm. Similar to the state-of-the-art qPCA, it achieves dimension reduction by extracting principal components of the data matrix, rather than all components of the data matrix, to quantum registers, so that the samples of measurement required can be reduced considerably. Both our qPCA and Lin’s qPCA are based on quantum singular-value thresholding (QSVT). The key of Lin’s qPCA is to combine QSVT, and modified QSVT is to obtain the superposition of the principal components. The key of our algorithm, however, is to modify QSVT by replacing the rotation-controlled operation of QSVT with the controlled-not operation to obtain the superposition of the principal components. As a result, this small trick makes the circuit much simpler. Particularly, the proposed qPCA requires three phase estimations, while the state-of-the-art qPCA requires five phase estimations. Since the runtime of qPCA mainly comes from phase estimations, the proposed qPCA achieves a runtime of roughly 3/5 of that of the state of the art. We simulate the proposed qPCA on the IBM quantum computing platform, and the simulation result verifies that the proposed qPCA yields the expected quantum state.

When developing a practical continuous-variable quantum key distribution（CVQKD）, the detector is necessary at the receiver’s side. We investigate the practical security of the CVQKD system with an unbalanced heterodyne detector. The results show that unbalanced heterodyne detector introduces extra excess noise into the system and decreases the lower bound of the secret key rate without awareness of the legitimate communicators, which leaves loopholes for Eve to attack the system. In addition, we find that the secret key rate decreases more severely with the increase in the degree of imbalance and the excess noise induced by the imbalance is proportional to the intensity of the local oscillator（LO） under the same degree of imbalance. Finally, a countermeasure is proposed to resist these kinds of effects.

Self-referenced continuous-variable quantum key distribution (SR-CVQKD) protocols, which operate with a real local local oscillator (LLO) at the receiver side, provide the major advantage that they can defend against all attacks associated with the local oscillator. We propose a method to enhance the performance of the SR-CVQKD by adding an optical parametric amplifier into the receiver’s apparatus. Considering reverse reconciliation, the secret key rate against collective eavesdropping attacks is calculated. For the plug-and-play dual-phase-modulated coherent-states (DPMCS) SR-CVQKD protocol, we demonstrate that an ideal phase-sensitive amplifier (PSA) can enhance the efficiency of a homodyne detector, and the compensation ability of a realistic phase-insensitive amplifier (PIA) for heterodyne detection varies according to different values of N, which indicates the noise variance of the PIA. Moreover, these arrangements of optical amplifiers also have applications in one-way SR-CVQKD systems.

In the continuous variable measurement-device-independent quantum key distribution (CV-MDI-QKD) protocol, both Alice and Bob send quantum states to an untrusted third party, Charlie, for detection through the quantum channel. In this paper, we mainly study the performance of the CV-MDI-QKD system using the noiseless linear amplifier (NLA). The NLA is added to the output of the detector at Charlie's side. The research results show that NLA can increase the communication distance and secret key rate of the CV-MDI-QKD protocol. Moreover, we find that the more powerful the improvement of the performance with the longer gain of NLA and the optimum gain is given under different conditions.

In the Gaussian-modulated coherent states (GMCS) quantum key distribution (QKD), basis choice is a necessary procedure if homodyne detector is used. However, finite bandwidth of the digital-to-analog convertor (DAC) at the receiver's side may lead to the imperfection of the basis choice, namely, maybe Bob can not exactly modulate 0 or pi 2<i on the phase modulator to implement the homodyne detection.We investigate the GMCS QKD scheme with imperfect basis choice under realistic conditions of quantum channel and detector.We show that the imperfection causes a misestimate of the transmittance and the excess noise, the secret key rate and the security of the system are dramatically decreased consequently. To achieve a reliable quantum key distribution, a security bound to the noise induced by the imperfect basis choice is derived for reverse reconciliation and homodyne detector. Besides, we propose and demonstrate a countermeasure against the noise due to imperfect basis choice.

In a practical continuous-variable quantum-key-distribution (CVQKD) system, a strong local oscillator (LO) signal in a quantum channel may be manipulated by eavesdropper Eve to steal information about secret keys through general attacks (e.g., entangling cloner attack or intercept-resend attack) without being detected, which is an effective quantum hacking strategy, i.e., LO attacks. To guarantee the practical security of the system, a general monitoring scheme has been previously demonstrated to resist this quantum hacking attack, where the channel transmittance was regarded as a fixed value related to transmission distance. However, practical communication environments are complex, which may result in the time-varying transmittance. This deviation may affect the effectiveness of this monitoring scheme. In this paper, we investigate the monitoring of CVQKD systems running in complex communication environments. We first model the LO attacks on practical CVQKD systems in complex environments, where the channel transmittance is assumed to obey a fixed distribution. Then, the low bound of intensity disturbance of the LO signal for Eve successfully concealing herself is obtained based on this model, where we consider all noise that can be used by Eve in complex communication environments. Simultaneously, we obtain an optimal monitoring condition to resist the LO attacks. Our numerical analysis confirms that the proposed monitoring scheme can effectively resist the LO attacks on practical CVQKD systems in complex environments. Subsequently, the feasibility of this way is again demonstrated by using a quantitative example. It is important that this monitoring scheme can also be extended to resist other quantum hacking attacks.

For the practical Faraday mirror ( FM) in a continuous-variable quantum key distribution (CVQKD) system there exists a rotation angle deviation that can lead to a loss of photon number when a light message passes through the polarization beam splitter. Here we employ the entanglement-based model to research the influence of the FM's imperfection. Based on parameter estimation, the imperfection can introduce a loophole for Eve to obtain information. The secret key rate of the CVQKD will be impaired. Finally, a novel method is proposed to ensure system performance by compensating optical power, and we further study the relationship between the secret key rate and the modulation variance.

Quantum secure direct communication (QSDC) allows secret messages to be directly communicated over a quantum channel; its further development could lead to many practical applications. In this paper, we propose a novel continuous-variable (CV)-based QSDC which could be compatible with fully developed optical telecommunication technologies to minimize the implementation costs. Through a security analysis we show that the proposed scheme can effectively resist an eavesdropper’s attacks on the light intensity. Furthermore, our work should inspire the improvement, development and application of CV-based QSDC.

Local oscillator (LO) acted as a reference to define the phase of quantum signals is necessary for coherent detection in continuous-variable quantum key distribution (CVQKD), and LO fluctuation caused by atmospheric channel brings the added excess noise into CVQKD. Here, we investigate the influence of atmospheric effects on the excess noise in free space CVQKD, and analyse the excess noise tolerance of systems with diverse modulations. And we obtain the critical condition that the excess noise related to LO remains constant under atmospheric effects. The results show that under general attacks, Gaussian-modulation which performs better in the short range propagation of high key rate has better excess noise tolerance than discrete-modulation that is propitious to distribute low key rate over distant propagation. Moreover, in discrete-modulation under collective attack, higher-order-state has higher secret key rate, better excess noise tolerance and longer propagation distance than lower-order-state.

Atmospheric effects are the chief threats to the quantum properties of propagating quantum signals and may degrade the performance of quantum key distribution seriously. As one of the most important parts of continuous-variable quantum key distribution (CVQKD), a parameter estimation method has not been specially proposed in an atmospheric channel, and usually the security analysis is based on the assumption that the relevant parameters, especially the excess noise, have been previously obtained. Here we propose a parameter estimation method for Gaussian modulated coherent state continuous-variable quantum key distribution over the atmospheric link, and we investigate the impact of the atmospheric channel on the estimated values of the parameters. Based on this method, we study theoretically the effect of link fluctuations on the achievable secret key rate of CVQKD under different practical transmitted conditions. The results show that this method is unified with the physical model and can effectively resist entanglement-distillation attack. The proposed method fills in the blank of parameter estimation for implementation of practical atmospheric CVQKD.