Practical Security Analysis of Self-Referenced CV-QKD System in the Presence of Polarization Aberration

How to remove local oscillator (LO) side channel attacks has been a notoriously hard problem in continuous-variable quantum key distribution (CV-QKD). In the self-referenced CV-QKD schemes, the LO signal is locally generated at the receiver by an independent laser so that it is not co-transmitted with the quantum signal. This simple solution removes all LO side channels. However it also introduces some other practical vulnerabilities. Especially the polarization states of the quantum signal and LO signal may not be identical across the detector because of the presence of the polarization aberrations. Thus, the detection efficiency which is arguably the most critical experiment parameter of the practical implementation will be impaired. In this paper, we analyze the impact of polarization aberrations on the detection efficiency for CV-QKD and propose a self-referenced CV-QKD scheme in the presence of polarization aberrations by using an off-axis optical system. In the proposed scheme, the polarization states of the quantum signal would change with the off-axis optical system, thus impairing the heterodyne efficiency. Our security analysis shows a gap between the theory and practice of CV-QKD.

     

Refined hyperentanglement purification of two-photon systems for high-capacity quantum communication with cavity-assisted interaction

Hyperentanglement, defined as the entanglement in multiple degrees of freedom (DOFs) of a photonic quantum system, has attracted much attention recently as it can improve the channel capacity of quantum communication largely. Here we present a refined hyperentanglement purification protocol (hyper-EPP) for two-photon systems in mixed hyperentangled states in both the spatial-mode and polarization DOFs, assisted by cavity quantum electrodynamics. By means of the spatial (polarization) quantum state transfer process, the quantum states that are discarded in the previous hyper-EPPs can be preserved. That is, the spatial (polarization) state of a four-photon system with high fidelity can be transformed into another four-photon system with low fidelity, not disturbing its polarization (spatial) state, which makes this hyper-EPP take the advantage of possessing a higher efficiency.     

Robust Quantum Secure Communication with Spatial Quantum States of Single Photons

The polarization quantum states of photon systems are fragile to the channel noise. However, recent experiments showed that the spatial quantum states of photon systems are robust. Recently, Ren et al. proposed a robust quantum secure direct communication (QSDC) protocol with spatial entanglement (Ren et al., Eur. Phys. J. D 67:30, 2013). Here we proposed a robust QSDC protocol and a robust three-party quantum secret sharing protocol with the four nonorthogonal spatial quantum states of a sequence of single photons, respectively. Both these two quantum secure communication protocols have the advantage of having a robust character and not increasing the difficulty of their implementations in experiment, compared with almost all the existing quantum secure communication protocols which are based on the polarization quantum states of photon systems. Moreover, they are more feasible than the QSDC protocol by Ren et al. as they do not require Bell-state measurements.     

Polarization fluctuations of single photon states in a slant channel with non-Kolmogorov turbulence

Polarization properties of single photon states propagating through the non-Kolmogorov turbulence in a slant channel are studied based on the degree of polarization of quantum field. The degree of polarization of single photon states for linearly polarized quantum beam propagation in a slant turbulent channel are developed. Our results show that the effects of the outer scale fluctuations of atmospheric turbulence and the wavelength difference of the beams on the polarization can be ignored and the smaller inner scale of turbulence and larger zenith angle of communication channel will lead to larger fluctuations of the polarization of single-photon states. The effects of the inner scale of turbulence on polarization of single photon states are more significant for bigger refractive-index power and bigger zenith angles.     

Efficient Multipartite Polarization Entanglement Distribution Over Arbitrary Noise Channel

We present an efficient faithful multipartite polarization entanglement distribution protocol over an arbitrary noisy channel. The spatial degree of freedom is used to carry the entanglement during the transmission. We describe the principle by distributing n-qubit Greenberge-Horne-Zeilinger state and n-qubit W state. Our scheme can be used to distribute arbitrary n-qubit entangled states to n distant locations. The remote parties can obtain maximally entangled states deterministically on the polarization of photons. Only passive linear optics are employed in our setup, which makes our scheme more feasible and efficient for practical application in long distance quantum communication.     

Measurement-device-independent quantum key distribution with hyper-encoding

Measurement device-independent quantum key distribution(MDI-QKD) protocols are immune to all possible attacks on the photon detectors during quantum communication, but their key generation rates are low compared with those of other QKD schemes.Increasing each individual photon's channel capacity is an efficient way to increase the key generation rate, and high-dimensional(HD) encoding is a powerful tool for increasing the channel capacity of photons. In this paper, we propose an HD MDI-QKD protocol with qudits hyper-encoded in spatial mode and polarization degrees of freedom(DOFs). In the proposed protocol, keys can be generated using the spatial mode and polarization DOFs simultaneously. The proposed protocol is unconditionally secure,even for weak coherent pulses with decoy states. The proposed MDI-QKD protocol may be useful for future quantum secure communication applications.     

Three-party Quantum Secure Direct Communication with Single Photons in both Polarization and Spatial-mode Degrees of Freedom

We present an efficient three-party quantum secure direct communication (QSDC) protocol with single photos in both polarization and spatial-mode degrees of freedom. The three legal parties’ messages can be encoded on the polarization and the spatial-mode states of single photons independently with desired unitary operations. A party can obtain the other two parties’ messages simultaneously through a quantum channel. Because no extra public information is transmitted in the classical channels, the drawback of information leakage or classical correlation does not exist in the proposed scheme. Moreover, the comprehensive security analysis shows that the presented QSDC network protocol can defend the outsider eavesdropper’s several sorts of attacks. Compared with the single photons with only one degree of freedom, our protocol based on the single photons in two degrees of freedom has higher capacity. Since the preparation and the measurement of single photon quantum states in both the polarization and the spatial-mode degrees of freedom are available with current quantum techniques, the proposed protocol is practical.     

Efficient error correction for N-particle polarized entangled states distribution over the collective-noise channel exploiting time entanglement

We propose an efficient error correction protocol for N-particle polarization entanglement states distribution over the collective-noise channel.The time entanglement is exploited in our protocol for its stability in noisy environment. The protocol is deterministic and the successful probability is 100 %. Moreover, the setup is just composed of linear optical elements, which are feasible with existing experimental technology. Therefore, the quantum resources can be saved and the protocol can be easily realized in experiments. With all the advantages above, our protocol could be useful in remote quantum communication.     

Quantum key distribution on biphotons-ququarts with test states

The operational inclusion of the subclass of entangled states in a quantum key distribution protocol based on biphoton-ququarts is analyzed. Four Bell states are proposed to be used as test states to estimate the error level, leaving the subclass of 12 factorized polarization states of biphotons as information states. The elementary analysis of two strategies for an attack on a quantum communication channel, as well as of the key generation rate, has been performed.

     

Phase Estimation and Compensation for Continuous-Variable Quantum Key Distribution

Phase estimation and compensation is one of the enabling functionalities in continous-variable quantum key distribution (CVQKD). Recently, a novel CVQKD scheme has been independently proposed to combat the local oscillator (LO) side channel attacks. Furthermore, we have carried out a proof-of-principle experimental study on the feasibility of the CVQKD without sending a LO. However, this scheme contains a serious weakness: The phase noise caused by the two different lasers between the sender and the receiver would severely destroy the quantum signal and finally reduce the secure distance. In this paper, we investigate the optical phase noise and explore the optimal approach to estimate and compensate such kind of noise with appropriate data overhead. Numerical simulations show that our scheme can successfully reconstruct the phase drifts even at low signal-to-noise ratio conditions. We also suggest that a higher accuracy of phase estimation could be achieved by using the frequency division multiplexing scheme. This opens an opportunity to employ advanced pilot-aided phase estimation techniques in quantum communication system.

     

降雨背景下诱骗态协议最优平均光子数的变色龙自适应策略

为了应对降雨给采用诱骗态协议的量子通信系统带来的突发性干扰,根据降雨分布模型和退极化信道的特性,本文提出了基于变色龙算法的每脉冲最优平均光子数自适应策略;建立了降雨强度、链路距离与最优平均光子数之间的自适应关系;并对采用变色龙算法前后,系统的性能参数进行了比较.仿真结果表明,当降雨强度J为30 mm/24 h、链路距离L为30 km时,通过采用变色龙算法,系统的安全密钥生成率由2×10~(-4)提高到3.5×10~(-4);当J为60 mm/24 h,L为20 km时,系统的信道生存函数值由0.52提高到0.63;当要求生存函数不低于0.5时,系统能够应对的最大雨强由62 mm/24 h提高到74 mm/24 h.因此,根据降雨强度和链路距离,通过变色龙算法自适应地调整系统发送端信号脉冲所含的平均光子数,可以提高量子通信系统在降雨背景下的有效性和可靠性.     

Quantum key distribution on biphotons-ququarts with test states

The operational inclusion of the subclass of entangled states in a quantum key distribution protocol based on biphoton-ququarts is analyzed. Four Bell states are proposed to be used as test states to estimate the error level, leaving the subclass of 12 factorized polarization states of biphotons as information states. The elementary analysis of two strategies for an attack on a quantum communication channel, as well as of the key generation rate, has been performed.     

Efficient Polarization Entanglement Distribution for W State Assisted by Frequency Degree of Freedom

We present an efficient faithful polarization entanglement distribution protocol for W state over an arbitrary noise channel,which use the frequency degree of freedom to carry the entanglement during the transmission.We describe the transmission of three-photon W state as an example,and then generalize this scheme to n-qubit W state situation.The remote parties can obtain maximally entangled W states on the polarization of photons,and the success probability is 100% in principle.As there was few entanglement purification for W state,our scheme is an efficient and practical method to share W state entanglement between distant parties,which will be useful in quantum communication.We also show that our scheme can be used to distribute arbitrary multi-particle entangled state.     

Multi-party Measurement-Device-Independent Quantum Key Distribution Based on Cluster States

We propose a novel multi-party measurement-device-independent quantum key distribution (MDI-QKD) protocol based on cluster states. A four-photon analyzer which can distinguish all the 16 cluster states serves as the measurement device for four-party MDI-QKD. Any two out of four participants can build secure keys after the analyzers obtains successful outputs and the two participants perform post-processing. We derive a security analysis for the protocol, and analyze the key rates under different values of polarization misalignment. The results show that four-party MDI-QKD is feasible over 280 km in the optical fiber channel when the key rate is about 10^??6 with the polarization misalignment parameter 0.015. Moreover, our work takes an important step toward a quantum communication network.     

Quantum key distribution via fiber optic, fidelity and error corrections

We propose an interesting scheme on photon states generation using a fiber optic Mach Zehnder interferometer incorporating a fiber optic ring resonator without any optical pumping parts including in the system, which is available for long-distance link. In principle, the state of a quantum bit, it is known, unknown, or entangled to other systems. The desired quantum states are generated and transmitted in the link via a fiber optic. The transmission quality in terms of quantum fidelity is analyzed, where a high fidelity to the noiseless quantum channel is achieved by adding an ancillary photon after the signal photon within the correlation time of the fiber noise and by performing the quantum parity checking method. The error correction is also analyzed. For simplicity, feature and robustness against path-length mismatches among the nodes make our scheme suitable for multi-user quantum communication networks.     

The influence of non-Kolmogorov turbulence on the entanglement of spatial two-qubit states in a slant channel

The effects of atmospheric turbulence on the entanglement of spatial two-qubit states that are prepared using the signal and idler photons produced by parametric down-conversion are studied. Utilizing the non-Kolmogorov model for atmospheric turbulence and Rytov approximation method, we quantify the effects of atmospheric turbulence on the entanglement of the two-qubit state in terms of Wootters's concurrence. Our results show that the effects of the zenith angle of communication channel and the outer scale of turbulence on the concurrence of a spatial two-qubit state can be ignored and the smaller inner scale of turbulence, the smaller refractive-index power α, the shorter wavelength of beams and the longer propagation distance will lead to the larger fluctuations of the concurrence of a spatial two-qubit state.     

基于光量子态避错及容错传输的量子通信

量子通信以量子态为信息载体在远距离的通信各方之间传递信息,因此量子态的传输和远距离共享是量子通信的首要步骤.信道噪声不仅会影响通信效率还可能被窃听者利用从而威胁通信安全,对抗信道噪声是实现安全高效量子通信亟需解决的问题.本文介绍基于光量子态的两类对抗信道噪声的实用方法——量子态的避错传输和容错的量子通信,包括对抗噪声的基本原理和两种方法的代表性方案,并从资源消耗和可操作性的角度分析了方案的实用价值.     

光子两自由度超并行量子计算与超纠缠态操控

光子系统在量子信息处理和传输过程中有非常重要的应用. 譬如, 利用光子与原子(或人工原子)之间的相互作用, 可以完成信息的安全传输、存储和快速的并行计算处理等任务. 光子系统具有多个自由度, 如极化、空间模式、轨道角动量、时间-能量、频率等自由度. 光子系统的多个自由度可以同时应用于量子信息处理过程. 超并行量子计算利用光子系统多个自由度的光量子态同时进行量子并行计算, 使量子计算具有更强的并行性, 且需要的量子资源少, 更能抵抗光子数损耗等噪声的影响. 多个自由度同时存在纠缠的光子系统量子态称为超纠缠态, 它能够提高量子通信的容量与安全性, 辅助完成一些重要的量子通信任务. 在本综述中, 我们简要介绍了光子系统两自由度量子态在量子信息中的一些新应用, 包括超并行量子计算、超纠缠态分析、超纠缠浓缩和纯化三个部分.     

On the transmission rate of classical information through quantum communication channels

The coding of quantum communication channels in real time is considered as applied to the situation when information is coded into continuous quantum degrees of freedom (into the shape of the amplitude of quantum states with an arbitrary number of photons). It is shown that the nonlocalizability of states in quantum field theory requires that the identity of particles should be taken into account. This, together with the finiteness of the limit speed of propagation, leads to the fact that the formulas for the transmission rate of nonrelativistic communication channels have an asymptotic character; i.e., these formulas are formally valid only when the separation between messages is infinite (when the identity of particles can be neglected) and, hence, when the transmission rate in [bit/message s] is infinitely small. A real-time information capacity of a sequential relativistic quantum communication channel is obtained that takes into account the identity of particles for pure signal states with an arbitrary number of photons. An explicit analytic expression is obtained for the transmission rate of a quantum channel of finite bandwidth for one-photon input states.     

Direct measurement of the concurrence for two-photon polarization entangled pure states by parity-check measurements

We present a protocol for directly measuring the concurrence of a two-photon polarization entangled pure or mixed state without prior quantum state tomography. By parity-check measurements and simple operations on two copies of the two-photon polarization entangled pure state, the concurrence is encoded in the total probability of picking up the odd parity states from the signal states. This protocol makes use of highly efficient homodyne detection, and it could be feasible in the near future with the help of the weak cross-Kerr nonlinearity. Moreover, our protocol can be used in a distributed fashion to directly determine the entanglement of remote states, which may find its important applications in quantum communication.