Quantum Key Distribution with the BB84 protocol has been shown to be unconditionally secure even using weak coherent pulses instead of single-photon signals. The distances that can be covered by these methods are limited due to the loss in the quantum channel (e.g. loss in the optical fiber) and in the single-photon counters of the receivers. One can argue that the loss in the detectors cannot be changed by an eavesdropper in order to increase the covered distance. Here we show that the security analysis of this scenario is not as easy as is commonly assumed, since already two-photon processes allow eavesdropping strategies that outperform the known photon-number splitting attack. For this reason there is, so far, no satisfactory security analysis available in the framework of individual attacks.
It is well-known that in certain scenarios weakly entangled states can generate stronger nonlocal effects than their maximally entangled counterparts. In this paper, we consider violations of the CHSH Inequality when one party has inefficient detectors, a scenario known as an asymmetric Bell experiment. %We show that violations can occur if and only if the detection efficiency is above $50\%$. For any fixed detection efficiency, we derive a simple upper bound on the entanglement needed to violate the inequality by more than some specified amount $κ\geq 0$. When $κ=0$, the amount of entanglemen…
We present an algorithm of quantum engineering of large-amplitude>5 high-fidelity>0.99 even/odd Schrodinger cat states (SCSs) using a single mode squeezed vacuum (SMSV) state as resource. Set of k beam splitters (BSs) with arbitrary transmittance and reflectance coefficients sequentially following each other acts as a hub that redirects a multiphoton state into the measuring modes simultaneously measured by photon number resolving (PNR) detectors. We show that the multiphoton state splitting guarantees significant increase of the success probability of the cat state generator compared to its i…
We propose an efficient way to generate optical analogs of both even and odd Schrodinger cat states (SCSs) with high fidelity, large amplitude and reasonable generation rate. The resource consumed are a single-mode squeezed vacuum state (SMSV) and possibly a single photon or nothing. We report the generation of even (odd) SCS with amplitude 4.2, fidelity higher than 0.99 and reasonable generation rate by subtraction of 30(31) photons from SMSV by ideal photon number detection. In the case of inefficient detectors, maintaining SCS size and its fidelity at the same level as in the case of ideal …
Fast gated avalanche photodiodes (APDs) are the most commonly used single photon detectors for high bit rate quantum key distribution (QKD). Their robustness against external attacks is crucial to the overall security of a QKD system or even an entire QKD network. Here, we investigate the behavior of a gigahertz-gated, self-differencing InGaAs APD under strong illumination, a tactic Eve often uses to bring detectors under her control. Our experiment and modelling reveal that the negative feedback by the photocurrent safeguards the detector from being blinded through reducing its avalanche prob…
The practical homodyne detector model of continuous-variable quantum key distribution models the inherent imperfections of the practical homodyne detector, namely the limited detection efficiency and the electronic noise, into trusted loss. However, the conventional practical homodyne detector model is valid only when both the imperfections of the practical homodyne detector are calibrated. In this paper, we show a modified practical homodyne detector model that can model the imperfections separately. The phase-sensitive amplifier is further applied to compensate the imperfections of the pract…
This is a brief comment on the Letter by Balygin and his coworkers [Laser Phys. Lett. 15, 095203 (2018)]. We point out an error that invalidates the Letter's conclusions.
One of the most pressing issues in quantum key distribution (QKD) is the problem of detector side- channel attacks. To overcome this problem, researchers proposed an elegant "time-reversal" QKD protocol called measurement-device-independent QKD (MDI-QKD), which is based on time-reversed entanglement swapping. However, MDI-QKD is more challenging to implement than standard point- to-point QKD. Recently, an intermediary QKD protocol called detector-device-independent QKD (DDI-QKD) has been proposed to overcome the drawbacks of MDI-QKD, with the hope that it would eventually lead to a more effici…
Security of high speed quantum key distribution with finite detector dead time
分析了有限检测器死时间τ下高速量子密钥分发系统的安全性。当传输速率变得高于各个检测器的最大计数速率（1/τ）时，安全问题影响用于筛选比特的算法。分析计算和数值模拟的Bennett-Bullard BB84协议进行。我们研究罗杰斯等人。的协议（在“Detector dead-time effects and paralyzability in high-speed量子密钥分发，”New J.Phys.9（2007）319中介绍），其中存在具有执行拦截-重发攻击的能力的主动窃听者Eve。它表明，罗杰斯等人。的协议不再安全更具体地说，Eve可以在接收器端引起依赖于基底的检测效率。然后介绍了在存在死区时间和主动窃听者的情况下安全的修改的密钥筛选方案。我们分析和比较这些安全的筛选方案，这个活跃的Eve场景，并计算和模拟他们的密钥生成率。结果表明，被动基选择的最大密钥生成速率为1/（2τ），主动基选择的最大密钥生成速率为1/τ。有限检测器死时间的安全性分析也扩展到诱饵状态BB84协议。
Quantum random number generators (QRNG) represent an advanced solution for randomness generation, essential in every cryptographic applications. In this context, integrated arrays of single photon detectors have promising applications as QRNGs based on the spatial detection of photons. For the employment of QRNGs in Cryptography, it is necessary to have efficient methods to evaluate the so called quantum min-entropy that corresponds to the amount of the true extractable quantum randomness from the QRNG. Here we present an efficient method that allow to estimate the quantum min-entropy for a mu…
Quantum key distribution (QKD) protocols with threshold detectors are driving high-performance QKD demonstrations. The corresponding security proofs usually assume that all physical detectors have the same detection efficiency. However, the efficiencies of the detectors used in practice might show a mismatch depending on the manufacturing and setup of these detectors. A mismatch can also be induced as the different spatial-temporal modes of an incoming signal might couple differently to a detector. Here we develop a method that allows to provide security proofs without the usual assumption. Ou…
The first device-independent quantum private query protocol (MRT17) which is proposed by Maitra \emph{et al.} [Phys. Rev. A 95, 042344 (2017)] to enhance the security through the certification of the states and measurements. However, the MRT17 protocol works under an assumption of perfect detectors, which increases difficulty in the implementations. Therefore, it is crucial to investigate what would affect the security of this protocol if the detectors were imperfect. Meanwhile, Maitra \emph{et al.} also pointed out that this problem remains open. In this paper, we analyze the security of MRT1…
Finite-key security analysis of decoy-state QKD with source and detector imperfections
University of Vigo（维戈大学）；Vigo Quantum Communication Center, University of Vigo（维戈量子通信中心，维戈大学）；Escuela de Ingeniería de Telecomunicación, Department of Signal Theory and Communications, University of Vigo（电信工程学院，信号理论与通信系，维戈大学）；atlanTTic Research Center, University of Vigo（atlanTTic研究中心，维戈大学）
Quantum key distribution (QKD) has been proved to be information-theoretically secure in theory. Unfortunately, the imperfect devices in practice compromise its security. Thus, to improve the security property of practical QKD systems, a commonly used method is to patch the loopholes in the existing QKD systems. However, in this work, we show an adversary's capability of exploiting the imperfection of the patch itself to bypass the patch. Specifically, we experimentally demonstrate that, in the detector under test, the patch of photocurrent monitor against the detector blinding attack can be d…
We propose a near-optimum receiver for the discrimination of binary phase-shift-keyed coherent states employing photon-number-resolving detectors. The receiver exploits a discrimination strategy based on both the so-called homodyne-like and the direct detection, thus resulting in a hybrid scheme. We analyse the performance and the robustness of the proposed scheme under realistic conditions, namely, in the presence of inefficient detection and dark counts. We show that the present hybrid setup is near-optimum and beats both the standard-quantum-limit and the performance of the Kennedy receiver…
Detector-device-independent quantum key distribution (ddiQKD) held the promise of being robust to detector side-channels, a major security loophole in QKD implementations. In contrast to what has been claimed, however, we demonstrate that the security of ddiQKD is not based on post-selected entanglement, and we introduce various eavesdropping strategies that show that ddiQKD is in fact insecure against detector side-channel attacks as well as against other attacks that exploit device's imperfections of the receiver. Our attacks are valid even when the QKD apparatuses are built by the legitimat…
Quantum key distribution protocols are known to be vulnerable against a side channel attack that exploits the time difference in detector responses used to obtain key bits. The recommended solution against this timing side channel attack is to use a large time bin width instead of high resolution timing information. Common notion is that using a large bin width reduces the resolution of detector responses, hence supposedly minimizes the information leakage to an eavesdropper. We challenge this conventional wisdom, and demonstrate that increasing the bin width does not monotonically reduce the …
We analyse a nonlinear interferometer, also known as an SU(1,1) interferometer, in the presence of internal losses and inefficient detectors. To overcome these limitations, we consider the effect of seeding one of the interferometer input modes with either a number state or a coherent state. We derive analytical expressions for the interference visibility, contrast, phase sensitivity, and signal-to-noise ratio, and show a significant enhancement in all these quantities as a function of the seeding photon number. For example, we predict that, even in the presence of substantial losses and highl…
In the last decade, efforts have been made to reconcile theoretical security with realistic imperfect implementations of quantum key distribution (QKD). Implementable countermeasures are proposed to patch the discovered loopholes. However, certain countermeasures are not as robust as would be expected. In this paper, we present a concrete example of ID Quantique's random-detector-efficiency countermeasure against detector blinding attacks. As a third-party tester, we have found that the first industrial implementation of this countermeasure is effective against the original blinding attack, bu…
We experimentally demonstrate optical control of negative-feedback avalanche diode (NFAD) detectors using bright light. We deterministically generate fake single-photon detections with a better timing precision than normal operation. This could potentially open a security loophole in quantum cryptography systems. We then show how monitoring the photocurrent through the avalanche photodiode can be used to reveal the detector is being blinded.
The squash operation, or the squashing model, is a useful mathematical tool for proving the security of quantum key distribution systems using practical (i.e., non-ideal) detectors. At the present, however, this method can only be applied to a limited class of detectors, such as the threshold detector of the Bennett-Brassard 1984 type. In this paper we generalize this method to include multi-partite measurements, such that it can be applied to a wider class of detectors. We demonstrate the effectiveness of this generalization by applying it to the device-independent security proof of the Ekert…
Long-range photonic device-independent quantum key distribution using SPDC sources and linear optics
University of Warsaw（华沙大学）；Polish Academy of Sciences（波兰科学院）；Gdańsk University of Technology（格但斯克理工大学）；ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology（ICFO-巴塞罗那光子科学研究所，巴塞罗那科学技术研究院）；LIP6, CNRS, Sorbonne Université（LIP6实验室，法国国家科学研究中心，索邦大学）
Trustworthiness of detectors in quantum key distribution with untrusted detectors
与测量设备无关的量子密钥分发（MDI-QKD）协议是一种有效的解决检测器侧信道攻击的方案。MDI-QKD的主要优点之一是可以证明安全性，而无需对测量设备如何工作进行任何假设。与传统的量子密钥分发（QKD）协议（如诱饵态BB84协议）相比，代价是相对较低的安全密钥速率。最近，一个新的QKD协议，旨在弥补MDI-QKD的强安全性和传统QKD的高效率，已经被提出。在该协议中，合法接收方采用可信线性光学网络对从不安全量子信道接收到的光子信息进行编码，然后使用不可信检测器进行贝尔态测量（Bell state measurement，BSM）。在大多数这些研究中，一个关键的假设是，位于接收者实验室内的不可信BSM不能向外部发送任何不需要的信息。在这里，我们表明，如果BSM是完全不可信的，一个简单的计划将允许BSM发送信息到外部。结合特洛伊木马攻击，该方案可以让Eve在不被发现的情况下获得量子密钥的信息。为了防止上述攻击，需要针对特洛伊木马攻击的对策或对“不可信”BSM设备的某种可信度。
