Practical SARG04 quantum key distribution

We have presented a method to estimate parameters of the decoy state method based on one decoy state protocol for SARG04. This method has given lower bound of the fraction of single-photon counts (y ₁), the fraction of two-photon counts (y ₂), the upper bound QBER of single-photon pulses (e ₁), the upper bound QBER of two-photon pulses (e ₂), and the lower bound of key generation rate for both BB84 and SARG04. The numerical simulation has shown that the fiber based QKD and free space QKD systems using the proposed method for BB84 are able to achieve both a higher secret key rate and greater secure distance than that of SARG04. Also, it is shown that bidirectional ground to satellite and inter-satellite communications are possible with our protocol.     

Robustness of quantum cryptography: SARG04 key-distribution protocol

The SARG04 protocol is aimed at the improvement of the robustness of the main protocols in quantum cryptography (primarily, BB84 and B92) against the PNS attack in the case of the application of weak coherent pulses instead of the one-particle signals. An optimal attack of eavesdropper on the transmitted key is explicitly constructed for arbitrary angles between the basis states. A critical error up to which the key distribution is possible and the length of the final key are determined.     

High Efficient Quantum Key Distribution by Random Using Classified Signal Coherent States

The decoy-state method is a useful method in resisting the attacks on quantum key distribution. However, how to choose the intensities of decoy states and the ratio of the decoy states and the signal state is still an open question. We present a simple formula to analyse the problem. We also give a simple method to derive the bounds of the necessary counting rates and quantum bit error rates for BB84 and SARG04; the latter was previously proposed by Scarani et al. [Phys. Rev. Lett. 92 （2004）057901] We then propose a multi-signal-state method which employs different coherent states either as the decoy state or as the signal state to carry out quantum key distribution. We find our protocol more efficient and feasible.     

Practical decoy-state BB84 quantum key distribution with quantum memory

We generalize BB84 quantum key distribution(QKD) to the scenario where the receiver adopts a heralded quantum memory(QM). With the heralded QM, the valid dark count rate of the receiver's single photon detectors can be mitigated obviously, which will lower the quantum bit error rate, and thus improve the performance of decoy-state BB84 QKD systems in long distance range. Simulation results show that, with practical experimental system parameters, decoy-state BB84 QKD with QM can exhibit performance comparable to that of without QM in short distance range, and exhibit performance better than that without QM in long distance range.     

A fundamental threat to quantum cryptography: gravitational attacks

An attack on the “Bennett-Brassard 84” (BB84) quantum key-exchange protocol in which Eve exploits the action of gravitation to infer information about the quantum-mechanical state of the qubit exchanged between Alice and Bob, is described. It is demonstrated that the known laws of physics do not allow to describe the attack. Without making assumptions that are not based on broad consensus, the laws of quantum gravity, unknown up to now, would be needed even for an approximate treatment. Therefore, it is currently not possible to predict with any confidence if information gained in this attack will allow to break BB84. Contrary to previous belief, a proof of the perfect security of BB84 cannot be based on the assumption that the known laws of physics are strictly correct, yet. A speculative parameterization that characterizes the time-evolution operator of quantum gravity for the gravitational attack is presented. It allows to evaluate the results of gravitational attacks on BB84 quantitatively. It is proposed to perform state-of-the-art gravitational attacks, both for a complete security assurance of BB84 and as an unconventional search for experimental effects of quantum gravity.     

Passive decoy state SARG04 quantum-key-distribution with practical photon-number resolving detectors

SARG04 protocol has its advantages in defending photon number splitting attack, benefited from two-photon pulses part. In this paper, we present a passive decoy state SARG04 scheme combining with practical photon number resolving (PNR) detectors. Two kinds of practical detectors, transition-edge sensor and time-multiplexing detector, are taken into consideration. Theoretical analysis shows that both of them are compatible with the passive decoy state SARG04. Compared with the original SARG04, two detectors can boost the key generation rate and maximal secure distance obviously. Meanwhile, the result shows that quantum efficiency and dark count of the detector influence the maximal distance slightly, which indicates the prospect of implementation in real quantum key distribution system with imperfect practical PNS detectors.     

Upper bound on key generation rate of quantum key distribution with two-way or two-step quantum channels

We present an asymptotic security proof of deterministic quantum key distribution (DQKD) with a two-way quantum channel. The security proof of DQKD with a two-way quantum channel is different from that of BB84, because Eve can attack the travel qubits twice, both in line Bob to Alice and in line Alice to Bob. With the no-signaling principle and the property of mutual information, we obtain an upper bound of the final key generation of entanglement-based DQKD and hence single-photon four-state DQKD. Our results can be applied to the protocol of QKD with two-step quantum channels.     

Security of biased BB84 quantum key distribution with finite resource

In the original BB84 quantum key distribution protocol, the states are prepared and measured randomly, which lose the unmatched detection results. To improve the sifting efficiency, biased bases selection BB84 protocol is proposed. Meanwhile, a practical quantum key distribution protocol can only transmit a finite number of signals, resulting in keys of finite length. The previous techniques for finite-key analysis focus mainly on the statistical fluctuations of the error rates and yields of the qubits. However, the prior choice probabilities of the two bases also have fluctuations by taking into account the finite-size effect. In this paper, we discuss the security of biased decoy state BB84 protocol with finite resources by considering all of the statistical fluctuations. The results can be directly used in the experimental realizations.     

New protocols for non-orthogonal quantum key distribution

Combining the passive decoy-state idea with the active decoy-state idea, a non-orthogonal (SARG04) decoy-state protocol with one vacuum and two weak decoy states is introduced based on a heralded pair coherent state photon source for quantum key distribution. Two special cases of this protocol are deduced, i.e., a one-vacuum-and-one-weak-decoy-state protocol and a one-weak-decoy-state protocol. In these protocols, the sender prepares decoy states actively, which avoids the crude estimation of parameters in the SARG04 passive decoy-state method. With the passive decoy-state idea, the detection events on Bob’s side that are non-triggered on Alice’s side are not discarded, but used to estimate the fractions of single-photon and two-photon pulses, which offsets the limitation of the detector’s low efficiency and overcomes the shortcoming that the performance of the active decoy-state protocol critically depends on the efficiency of detector. The simulation results show that the combination of the active and passive decoy-state ideas increases the key generation rate. With a one-vacuum-and-two-weak-decoy-state protocol, one can achieve a key generation rate that is close to the theoretical limit of an infinite decoy-state protocol. The performance of the other two protocols is a little less than with the former, but the implementation is easier. Under the same condition of implementation, higher key rates can be obtained with our protocols than with existing methods.     

A fiber quantum communication system based on an autocompensation optical circuit

An experimental setup for studying the distribution of a quantum key in a fiber optic communication line designed at the Institute of Semiconductor Physics, Russian Academy of Sciences, Siberian Branch is presented. The structure of a unified control unit for studying the distributed quantum key setup using the BB84 protocol with phase encoding is described. Experimental results on quantum key generation are given.     

On the privacy-preserving cascade method for correcting errors in primary keys in quantum cryptography

A general method is proposed for privacy protection upon correction of errors in primary keys in quantum cryptography through an open communication channel. By the example of the cascade procedure for correcting errors, a method is described for removing information sent through an open communication channel when cleaning a key. The critical percentage is found for the quantum cryptography protocol BB84 to which the cascade error correction method with deletion guarantees the security of the final key. The method proposed for removing information sent through the open communication channel is quite general and can be used for many cleaning protocols for primary keys in quantum cryptography.     

基于BB84协议的量子密钥分发安全门限研究

本文分析了实际量子密钥分发过程中,量子态可能受到的各种影响因素;建立了相应的信道模型;推得了非理想信道BB84协议判别窃听的安全门限公式.通过计算与仿真,证明该公式用于估计BB84协议的安全通信门限更加准确,判断量子密钥分发过程中是否存在窃听更为有效,同时具有提高通信安全性和密钥分发效率的优点. 关键词： 信道模型 BB84协议 窃听判别 安全门限     

基于相干叠加态的非正交编码诱骗态量子密钥分发

非正交编码协议和诱骗态方法可以有效地抵御光子数分离攻击. 由于相干叠加态中单光子成分高达90%, 常作为单光子量子比特的替代出现, 用于量子信息过程处理和计算. 本文结合非正交编码协议和诱骗态方法提出一种新的量子密钥分发方案, 光源采用相干叠加态, 推导了单光子的密钥生成速率、计数率下限和误码率的上限, 利用Matlab 模拟了无限多诱骗态情况下和有限多诱骗态情况下密钥生成速率和传输距离的关系, 得出该方案可以提升密钥生成速率并且提高安全传输距离, 验证了该方案可以进一步提高量子密钥分发系统的性能.     

Integration of quantum cryptography into fiber-optic telecommunication systems

The prototype of a new quantum cryptosystem is described. Such a cryptosystem is naturally called time-shift quantum cryptography. It realizes all basic quantum-cryptography protocols [BB84, B92, BB84(4+2)] in a common fiber-optic system. This scheme does not involve Mach-Zehnder interferometers, which enables one to naturally realize the multiplex mode of secure key distribution and to naturally integrate this quantum-cryptography scheme into traditional fiber-optic telecommunication systems. The proposed time coding method in quantum cryptography makes it possible to significantly simplify experimental schemes and to exclude the finest fiber-optic part, the interferometer. In essence, the fundamental difference of the time coding method from the phase coding method is that the part using phase relations in superposition between “parts” of a quantum state is eliminated from the phase-coding method, and only the part using the time division principle is retained. The time division principle is common for both methods and is minimally necessary, in contrast to the phasecoding method, which can be excluded altogether. The proposed scheme is briefly compared with the two most developed phase-coding schemes (without self-compensation and with passive self-compensation).     

A full quantum network scheme

In this paper, we study the thermodynamic features of a two-dimensional charged black hole. Weinhold curvature and Ruppeiner curvature are explored as information geometry, respectively. Moreover, based on the Legendre invariant proposed by Hernando Quevedo, the geometro-thermodynamics behavior of this black hole is investigated.     

Simple proof of security of the BB84 quantum key distribution protocol

We prove that the 1984 protocol of Bennett and Brassard (BB84) for quantum key distribution is secure. We first give a key distribution protocol based on entanglement purification, which can be proven secure using methods from Lo and Chau's proof of security for a similar protocol. We then show that the security of this protocol implies the security of BB84. The entanglement purification based protocol uses Calderbank-Shor-Steane codes, and properties of these codes are used to remove the use of quantum computation from the Lo-Chau protocol.     

用于BB84相位编码量子密钥分发系统的不间断式主动相位补偿方案

提出一种新的不间断的主动相位补偿方案,在进行量子密钥分发的同时统计不匹配基量子比特在干涉仪不同输出端口上的随机计数分布,给出了由不匹配基量子比特统计数值计算相位漂移参数的计算公式,并由统计数值计算得到相位漂移参数.结果表明:该方案允许系统并行处理量子密钥分发与相位补偿,也充分利用了在原BB84协议中会被丢弃的不匹配基量...     

量子BB84协议在联合旋转噪音信道上的安全性分析

多数在理想条件下设计的量子密码协议没有考虑实际通信中噪音的影响,可能造成机密信息不能被准确传输,或可能存在窃听隐藏在噪音中的风险,因此分析噪音条件下量子密码协议的安全性具有重要的意义.为了分析量子BB84协议在联合旋转噪音信道上的安全性,本文采用粒子偏转模型,对量子信道中的联合噪音进行建模,定量地区分量子信道中噪音和窃听干扰;并且采用冯·诺依曼熵理论建立窃听者能窃取的信息量与量子比特误码率、噪音水平三者之间的函数关系,定量地分析噪音条件下量子信道的安全性;最后根据联合噪音模型及窃听者能窃取的信息量与量子比特误码率、噪音水平三者之间的关系,定量地分析了量子BB84协议在联合噪音条件下的安全性并计算噪音临界点.通过分析可知,在已有噪音水平条件下,窃听者最多能够从通信双方窃取25%的密钥,但是Eve的窃听行为会被检测出来,这样Alice和Bob会放弃当前协商的密钥,重新进行密钥协商,直至确认没有Eve的窃听为止.这个结果说明量子BB84协议在联合旋转噪音信道下的通信是安全的.     

Some conservative estimates in quantum cryptography

Relationship is established between the security of the BB84 quantum key distribution protocol and the forward and converse coding theorems for quantum communication channels. The upper bound Q _c ≈ 11% on the bit error rate compatible with secure key distribution is determined by solving the transcendental equation $H(Q_c ) = \bar C(\rho )/2$ , where ρ is the density matrix of the input ensemble, $\bar C(\rho )$ is the classical capacity of a noiseless quantum channel, and H(Q) is the capacity of a classical binary symmetric channel with error rate Q.     

Quantum scheme for an optimal attack on quantum key distribution protocol BB84

A quantum scheme for an optimal attack on protocol BB84, the most studied protocol of quantum cryptography, is constructed. The physical implementation of this scheme on the basis of linear fiber-optical elements and the two-particle “controlled NOT” quantum transformation is proposed.