Composable security of unidimensional continuous-variable quantum key distribution

We investigate the composable security of unidimensional continuous-variable quantum key distribution (UCVQKD) protocol in generally phase-sensitive channel; the UCVQKD protocol is based on the Gaussian modulation of a single quadrature of the coherent state of light, aiming to provide a simple implementation of key distribution compared to the symmetrically modulated Gaussian coherent-state protocols. This protocol neglects the necessity in one of the quadrature modulations in coherent states and hence reduces the system complexity. To clarify the influence of finite-size effect and the cost of performance degeneration, we establish the relationship of the balanced parameters of the unmodulated quadrature and estimate the precise secure region. Subsequently, we illustrate the composable security of the UCVQKD protocol against collective attacks and achieve the tightest bound of the UCVQKD protocol. Numerical simulations show the asymptotic secret key rate of the UCVQKD protocol, together with the symmetrically modulated Gaussian coherent-state protocols.     

Photon-monitoring attack on continuous-variable quantum key distribution with source in middle

Motivated by a fact that the non-Gaussian operation may increase entanglement of an entangled system, we suggest a photon-monitoring attack strategy in the entanglement-based (EB) continuous-variable quantum key distribution (CVQKD) using the photon subtraction operations, where the entangled source originates from the center instead of one of the legal participants. It shows that an eavesdropper, Eve, can steal large information from participants after intercepting the partial beams with the photon-monitoring attach strategy. The structure of the proposed CVQKD protocol is useful in simply analyzing how quantum loss in imperfect channels can decrease the performance of the CVQKD protocol. The proposed attack strategy can be implemented under current technology, where a newly developed and versatile no-Gaussian operation can be well employed with the entangled source in middle in order to access to mass information in the EB CVQKD protocol, as well as in the prepare-and-measure (PM) CVQKD protocol.     

Recurrent neural network approach to quantum signal: coherent state restoration for continuous-variable quantum key distribution

In continuous-variable quantum key distribution (CV-QKD), weak signal carrying information transmits from Alice to Bob; during this process it is easily influenced by unknown noise which reduces signal-to-noise ratio, and strongly impacts reliability and stability of the communication. Recurrent quantum neural network (RQNN) is an artificial neural network model which can perform stochastic filtering without any prior knowledge of the signal and noise. In this paper, a modified RQNN algorithm with expectation maximization algorithm is proposed to process the signal in CV-QKD, which follows the basic rule of quantum mechanics. After RQNN, noise power decreases about 15 dBm, coherent signal recognition rate of RQNN is 96%, quantum bit error rate (QBER) drops to 4%, which is 6.9% lower than original QBER, and channel capacity is notably enlarged.     

Quantum key distribution using continuous-variable non-Gaussian states

In this work, we present a quantum key distribution protocol using continuous-variable non-Gaussian states, homodyne detection and post-selection. The employed signal states are the photon added then subtracted coherent states (PASCS) in which one photon is added and subsequently one photon is subtracted from the field. We analyze the performance of our protocol, compared with a coherent state-based protocol, for two different attacks that could be carried out by the eavesdropper (Eve). We calculate the secret key rate transmission in a lossy line for a superior channel (beam-splitter) attack, and we show that we may increase the secret key generation rate by using the non-Gaussian PASCS rather than coherent states. We also consider the simultaneous quadrature measurement (intercept-resend) attack, and we show that the efficiency of Eve’s attack is substantially reduced if PASCS are used as signal states.     

Practical aspects of terahertz wireless quantum key distribution in indoor environments

We discuss the feasibility of continuous-variable wireless quantum key distribution (WQKD) with thermal Gaussian states in terahertz (THz) band. We rigorously analyze the secret key rate of practical WQKD system using direct reconciliation and homodyne detection against collective Gaussian attacks. The results show that in the case of low-gain antenna, the free space loss is the dominant limiting factor of secure transmission distance of THz WQKD system. When the antenna diameter increases from 1 to 5 cm, we can get the maximum security distance and its corresponding optimal frequency in the range of 0.1–1 THz. We also obtain that a security distance of 1.95 m can be achieved when using a 5-cm diameter antenna and controlling the excess noise below 0.25 simultaneously. Specifically, we study the finite size effects and show that the transmission distance can reach 78 cm with a 5-cm diameter antenna at the frequency of 300 GHz. This work takes an important step toward short distance wireless QKD system.     

An enhanced proposal on decoy-state measurement device-independent quantum key distribution

By employing pulses involving three-intensity, we propose a scheme for the measurement device-independent quantum key distribution with heralded single-photon sources. We make a comparative study of this scheme with the standard three-intensity decoy-state scheme using weak coherent sources or heralded single-photon sources. The advantage of this scheme is illustrated through numerical simulations: It can approach very closely the asymptotic case of using an infinite number of decoy-states and exhibits excellent behavior in both the secure transmission distance and the final key generation rate.     

Four-state quantum key distribution exploiting maximum mutual information measurement strategy

We propose a four-state quantum key distribution (QKD) scheme using generalized measurement of nonorthogonal states, the maximum mutual information measurement strategy. Then, we analyze the eavesdropping process in intercept–resend and photon number splitting attack scenes. Our analysis shows that in the intercept–resend and photon number splitting attack eavesdropping scenes, our scheme is more secure than BB84 protocol and has higher key generation rate which may be applied to high-density QKD.     

Upconversion-based receivers for quantum hacking-resistant quantum key distribution

We propose a novel upconversion (sum frequency generation)-based quantum-optical system design that can be employed as a receiver (Bob) in practical quantum key distribution systems. The pump governing the upconversion process is produced and utilized inside the physical receiver, making its access or control unrealistic for an external adversary (Eve). This pump facilitates several properties which permit Bob to define and control the modes that can participate in the quantum measurement. Furthermore, by manipulating and monitoring the characteristics of the pump pulses, Bob can detect a wide range of quantum hacking attacks launched by Eve.     

New scheme for measurement-device-independent quantum key distribution

We propose a new scheme for measurement-device-independent quantum key distribution (MDI-QKD) with a two-mode state source. In this scheme, the trigger state is split into different paths and detected at both senders; thus, four types of detection events can be obtained. Based on these events, the signal state is divided into four non-empty sets that can be used for parameter estimation and key extraction. Additionally, we carry out a performance analysis on the scheme with two-intensity (vacuum state and signal state) heralded single-photon sources. We also numerically study the statistical fluctuation in the actual system. Our simulations show that the error rate and the secure transmission distance of our two-intensity scheme are better than those of existing three- and four-intensity MDI-QKD schemes with different light sources. Considering statistical fluctuations, the maximum secure distance of our scheme can reach 344 km when the data length is 10¹³ and remains as long as 250 km when the data length is 10¹⁰. Moreover, our scheme improves the system performance and reduces the challenges of implementing the system.     

Differential phase encoded measurement-device-independent quantum key distribution

Quantum Information Processing - By employing Pauli measurements, we present some nonlinear steering criteria applicable for arbitrary two-qubit quantum systems and optimized ones for symmetric...     

Fundamental finite key limits for one-way information reconciliation in quantum key distribution

The security of quantum key distribution protocols is guaranteed by the laws of quantum mechanics. However, a precise analysis of the security properties requires tools from both classical cryptography and information theory. Here, we employ recent results in non-asymptotic classical information theory to show that one-way information reconciliation imposes fundamental limitations on the amount of secret key that can be extracted in the finite key regime. In particular, we find that an often used approximation for the information leakage during information reconciliation is not generally valid. We propose an improved approximation that takes into account finite key effects and numerically test it against codes for two probability distributions, that we call binary–binary and binary–Gaussian, that typically appear in quantum key distribution protocols.     

Fault-tolerant measurement-device-independent quantum key distribution in a decoherence-free subspace

Two modified measurement-device-independent quantum key distribution protocols based on the decoherence-free subspace are presented in this study. The proposed protocols are tolerant of the fault with collective-rotation noise and collective-dephasing noise. Exploiting the logical qubits comprised by two pairs of entanglement photons in decoherence-free subspace states, the mutually unbiased bases are formed by introducing the spatial degrees of freedom which reduces the experiment difficulty. There are only Bell-state preparation and collective Bell-state measurement needed in our protocols. Moreover, a brief discussion on the security of the proposal in the communication process is given.     

Nonlinear dynamics of continuous-variable quantum games with bounded rationality

We investigate the dynamics of a quantum duopoly game where the players use bounded rationality to adjust own decision. The stability conditions of the equilibrium points are analyzed. Furthermore, we present the numerical simulations to show the nonlinear behaviors: bifurcations, strange attractors, stability region.     

量子密钥分发光源时序校准系统设计

针对量子密钥分发终端设备中八路量子态光信号在设备出口处存在固有时间偏差的问题,设计了一套以TDC-GPX为核心的量子密钥分发光源时序校准系统。系统对量子态光信号进行光电转换、信号调理,使用高精度时间间隔测量芯片TDC-GPX分时对调理后的脉冲电信号进行采集,并通过FPGA进行数据处理,调整八路光信号的发光时间,使其满足在时间上的不可分辨性。测试结果表明,系统测量精度小于80 ps,用于实际量子密钥分发待校准设备中的校准性能良好,满足校准要求。     

Analysis of counterfactual quantum key distribution using error-correcting theory

Counterfactual quantum key distribution is an interesting direction in quantum cryptography and has been realized by some researchers. However, it has been pointed that its insecure in information theory when it is used over a high lossy channel. In this paper, we retry its security from a error-correcting theory point of view. The analysis indicates that the security flaw comes from the reason that the error rate in the users’ raw key pair is as high as that under the Eve’s attack when the loss rate exceeds 50 %.     

Key-leakage evaluation of authentication in quantum key distribution with finite resources

Partial information leakages of generation key undoubtedly influence the security of practical Quantum Key Distribution (QKD) system. In this paper, based on finite-key analysis and deep investigation on privacy amplification, we present a method for characterizing information leakages gained by adversary in each authentication round and therefore take the theory derived by Cederlöf and Larsson (IEEE Trans Inf Theory 54:1735–1741, 2008) into practical case. As the authentication key is fed from one round of generation keys to the next except the first round, by considering its security weakness due to information leakages and finite size effect, we further propose a universal formula for calculating the lifetime of initial authentication key used in QKD with finite resources. Numerical simulations indicate that our bound for estimating information leakages strictly characterizes the stability of practical QKD against information-leakage-based attacks, and our calculation formula in terms of lifetime can precisely evaluate the usage time of initial authentication key. Our work provides a practical solution for evaluating authentication security of QKD.     

Improved statistical fluctuation analysis for measurement-device-independent quantum key distribution

Measurement-device-independent quantum key distribution (MDI-QKD) is a promising protocol for realizing long-distance secret keys sharing. However, its key rate is relatively low when the finite-size effect is taken into account. In this paper, we consider statistical fluctuation analysis for the three-intensity decoy-state MDI-QKD system based on the recent work (Zhang et al. in Phys Rev A 95:012333, 2017) and further compare its performance with that of applying the Gaussian approximation technique and the Chernoff bound method. The numerical simulations demonstrate that the new method has apparent enhancement both in key generation rate and transmission distance than using Chernoff bound method. Meanwhile, the present work still shows much higher security than Gaussian approximation analysis.     

Round-robin differential-phase-shift quantum key distribution with heralded pair-coherent sources

Round-robin differential-phase-shift (RRDPS) quantum key distribution (QKD) scheme provides an effective way to overcome the signal disturbance from the transmission process. However, most RRDPS-QKD schemes use weak coherent pulses (WCPs) as the replacement of the perfect single-photon source. Considering the heralded pair-coherent source (HPCS) can efficiently remove the shortcomings of WCPs, we propose a RRDPS-QKD scheme with HPCS in this paper. Both infinite-intensity decoy-state method and practical three-intensity decoy-state method are adopted to discuss the tight bound of the key rate of the proposed scheme. The results show that HPCS is a better candidate for the replacement of the perfect single-photon source, and both the key rate and the transmission distance are greatly increased in comparison with those results with WCPs when the length of the pulse trains is small. Simultaneously, the performance of the proposed scheme using three-intensity decoy states is close to that result using infinite-intensity decoy states when the length of pulse trains is small.     

Trojan horse attack free fault-tolerant quantum key distribution protocols

This work proposes two quantum key distribution (QKD) protocols—each of which is robust under one kind of collective noises—collective-dephasing noise and collective-rotation noise. Due to the use of a new coding function which produces error-robust codewords allowing one-time transmission of quanta, the proposed QKD schemes are fault-tolerant and congenitally free from Trojan horse attacks without having to use any extra hardware. Moreover, by adopting two Bell state measurements instead of a 4-GHZ state joint measurement for decoding, the proposed protocols are practical in combating collective noises.     

Bipartite coherent-state quantum key distribution with strong reference pulse

We propose an optical scheme for quantum key distribution in which bits are encoded in relative phases of four bipartite weak coherent states ${|\alpha, \alpha\rangle, |-\alpha, -\alpha\rangle, |-\alpha, \alpha\rangle}$ and ${|\alpha, -\alpha \rangle}$ , with respect to a strong reference pulse. We discuss security of the scheme against eavesdropping strategies like, photon number splitting, photon beam splitting and intercept-resend attacks. It is found that present scheme is more sensitive against these eavesdropping strategies than the two-dimensional non-orthogonal state based protocol and BB84 protocol. Our scheme is very simple, requires only passive optical elements like beam splitters, phase shifters and photon detectors, hence is at the reach of presently available technology.