QKD System Research Articles

Polarization based discrete variables quantum key distribution via conjugated homodyne detection

Optical homodyne detection is widely adopted in continuous-variable quantum key distribution for high-rate field measurement quadratures. Besides that, those detection schemes have been being implemented for single-photon statistics characterization in the field of quantum tomography. In this work, we propose a discrete-variable quantum key distribution (DV-QKD) implementation that combines the use of phase modulators for high-speed state of polarization (SOP) generation, with a conjugate homodyne detection scheme which enables the deployment of high speed QKD systems. The channel discretization relies on the application of a detection threshold that allows to map the measured voltages as a click or no-click. Our scheme relies also on the use of a time-multiplexed pilot tone—quantum signal architecture which enables the use of a Bob locally generated local oscillator and opens the door to an effective polarization drift compensation scheme. Besides that, our results shows that for higher detection threshold values we obtain a very low quantum bit error rate (QBER) on the sifted key. Nevertheless, due to huge number of discarded qubits the obtained secure key length abruptly decreases. From our results, we observe that optimizing the detection threshold and considering a system operating at 500 MHz symbol generation clock, a secure key rate of approximately 46.9 Mbps, with a sifted QBER of 1.5% over 40 km of optical fiber. This considering the error correction and privacy amplification steps necessary to obtain a final secure key.

Simple Multiuser Twin-Field Quantum Key Distribution Network

Twin-field quantum key distribution (TFQKD) systems have shown great promise for implementing practical long-distance secure quantum communication due to its measurement-device-independent nature and its ability to offer fundamentally superior rate-loss scaling than point-to-point QKD systems. A surge of research and development effort in the last two years has produced many variants of protocols and experimental demonstrations. In terms of hardware topology, TFQKD systems interfering quantum signals from two remotely phase-locked laser sources are in essence giant Mach-Zehnder interferometers (MZIs) requiring active phase stabilization. Such configurations are inherently unsuitable for a TFQKD network, where more than one user-pair share the common quantum measurement station, because it is practically extremely difficult, if not impossible, to stabilize MZIs of largely disparate path lengths, a situation that is inevitable in a multi-user-pair TFQKD network. On the other hand, Sagnac interferometer based TFQKD systems exploiting the inherent phase stability of the Sagnac ring can implement asymmetric TFQKD, and are therefore eminently suitable for implementing a TFQKD network. In this work, we experimentally demonstrate a proof-of-principle multi-user-pair Sagnac TFQKD network where three user pairs sharing the same measurement station can perform pair-wise TFQKD through time multiplexing, with channel losses up to 58 dB, and channel loss asymmetry up to 15 dB. In some cases, the secure key rates still beat the rate-loss bounds for point-to-point repeaterless QKD systems, even in this network configuration. It is to our knowledge the first multi-user-pair TFQKD network demonstration, an important step in advancing quantum communication network technologies.

Security Analysis of Continuous-Variable Measurement-Device-Independent Quantum Key Distribution Systems in Complex Communication Environments.

Continuous-variable measure-device-independent quantum key distribution (CV-MDI QKD) is proposed to remove all imperfections originating from detection. However, there are still some inevitable imperfections in a practical CV-MDI QKD system. For example, there is a fluctuating channel transmittance in the complex communication environments. Here we investigate the security of the system under the effects of the fluctuating channel transmittance, where the transmittance is regarded as a fixed value related to communication distance in theory. We first discuss the parameter estimation in fluctuating channel transmittance based on these establishing of channel models, which has an obvious deviation compared with the estimated parameters in the ideal case. Then, we show the evaluated results when the channel transmittance respectively obeys the two-point distribution and the uniform distribution. In particular, the two distributions can be easily realized under the manipulation of eavesdroppers. Finally, we analyze the secret key rate of the system when the channel transmittance obeys the above distributions. The simulation analysis indicates that a slight fluctuation of the channel transmittance may seriously reduce the performance of the system, especially in the extreme asymmetric case. Furthermore, the communication between Alice, Bob and Charlie may be immediately interrupted. Therefore, eavesdroppers can manipulate the channel transmittance to complete a denial-of-service attack in a practical CV-MDI QKD system. To resist this attack, the Gaussian post-selection method can be exploited to calibrate the parameter estimation to reduce the deterioration of performance of the system.

Robust message authentication in the context of quantum key distribution

Universal hashing-based message authentication code (MAC) is used as the de facto method to achieve information-theoretically secure authentication in quantum key distribution. We present a critical look at the most widely used type, namely Wegman-Carter MAC based on polynomial hashing and analyse its robustness against physical attacks exploiting side information. In particular, we mount a classical DPA attack on the hash part of the Wegman-Carter MAC which leads to a possible intercept-and-resend attack on the BB84-like QKD protocols. We illustrate this case with polynomial-evaluation MACs as their variants are used in commercial QKD systems. We show that our attack methodology is much simpler compared to that of Belaid et al. at ASIACRYPT 2014. Finally, we present an algebraic countermeasure so that the resulting MAC is not susceptible to the identified attack.

Polarization control algorithm for QKD systems

The crucial task for polarization-encoding fiber QKD is to compensate polarization drift occurring in a quantum channel. To solve this problem, the receiver usually uses a polarization controller. For proper operation, this device must be efficiently managed in real-time. In this work, a gradient-descent-based algorithm is proposed to solve this problem. The algorithm was implemented and tested on a QRate commercial QKD fiber system, that utilizes BB84-protocol. Low and stable QBER has been obtained during a day of continuous operation.

Generation of frequency harmonics for QKD systems at subcarrier waves

This paper demonstrates the efficiency of using optical feedback without amplifiers to generate subcarrier waves in the microwave phase modulator output spectrum for quantum communications. It is proposed ideas for expanding the functionality of quantum communication systems and communication systems using DWDM standarts.

Field Test of Twin-Field Quantum Key Distribution through Sending-or-Not-Sending over 428km.

Quantum key distribution endows people with information-theoretical security in communications. Twin-field quantum key distribution (TF-QKD) has attracted considerable attention because of its outstanding key rates over long distances. Recently, several demonstrations of TF-QKD have been realized. Nevertheless, those experiments are implemented in the laboratory, and therefore a critical question remains about whether the TF-QKD is feasible in real-world circumstances. Here, by adopting the sending-or-not-sending twin-field QKD (SNS-TF-QKD) with the method of actively odd parity pairing (AOPP), we demonstrate a field-test QKD over 428km of deployed commercial fiber and two users are physically separated by about 300km in a straight line. To this end, we explicitly measure the relevant properties of the deployed fiber and develop a carefully designed system with high stability. The secure key rate we achieved breaks the absolute key rate limit of repeaterless QKD. The result provides a new distance record for the field test of both TF-QKD and all types of fiber-based QKD systems. Our work bridges the gap of QKD between laboratory demonstrations and practical applications and paves the way for an intercity QKD network with measurement-device-independent security.

Experimental vulnerability analysis of QKD based on attack ratings

Inspired by the methodology used for classical cryptographic hardware, we consider the use of attack ratings in the context of QKD security evaluation. To illustrate the relevance of this approach, we conduct an experimental vulnerability assessment of CV-QKD against saturation attacks, for two different attack strategies. The first strategy relies on inducing detector saturation by performing a large coherent displacement. This strategy is experimentally challenging and therefore translates into a high attack rating. We also propose and experimentally demonstrate a second attack strategy that simply consists in saturating the detector with an external laser. The low rating we obtain indicates that this attack constitutes a primary threat for practical CV-QKD systems. These results highlight the benefits of combining theoretical security considerations with vulnerability analysis based on attack ratings, in order to guide the design and engineering of practical QKD systems towards the highest possible security standards.

Countermeasure Against Quantum Hacking Using Detection Statistics

Detector blinding attacks have been proposed in the last few years, and they could potentially threaten the security of QKD systems. Even though no complete QKD system has been hacked yet, it is nevertheless important to consider countermeasures to avoid information leakage. In this paper, we present a new countermeasure against these kind of attacks based on the use of multi-pixel detectors. We show that with this method, we are able to estimate an upper bound on the information an eavesdropper could have on the key exchanged. Finally, we test a multi-pixel detector based on SNSPDs to show it can fulfill all the requirement for our countermeasure to be effective.

Quantum secret sharing using discretely modulated coherent states

Point-to-point quantum privacy communication over a standard telecommunication fiber link can be implemented by continuous-variable quantum key distribution (CV QKD). However, as communication networks develop, the two-party CV QKD system may hardly meet the requirements of secret key sharing of multiple users (at least three users). In this paper, we consider a protocol called quantum secret sharing (QSS) which allows a legitimate user, a so-called dealer, to share a secret key with multiple remote users through an insecure quantum channel. These users can correctly recover the dealer's secret key only when they work cooperatively. We carry out QSS with discretely modulated coherent states (DMCSs) because they are easy to prepare and resilient to losses. An asymptotic security proof for the proposed DMCS-based QSS protocol against both eavesdroppers and dishonest users is presented. Numerical simulation based on a linear bosonic channel shows that the maximal transmission distance of the DMCS-based QSS protocol reaches more than 100 km, and it can be further lengthened by exploiting a higher-dimensional discrete modulation strategy. Moreover, the composable security of the DMCS-based QSS protocol is also presented.

Performance of subcarrier-wave quantum key distribution in the presence of spontaneous Raman scattering noise generated by classical DWDM channels

In this paper, we study the performance of the subcarrier-wave quantum key distribution system (SCW QKD) in the presence of spontaneous Raman scattering (SpRS) noise generated by classical channels of the dense wavelength division multiplexing (DWDM) network within a single-mode optical fiber. We present the mathematical model for evaluation of the quantum bit error rate and secure key generation rate with the SpRS noise taken into account. We consider two regimes of the SCW QKD system: the continuous wave regime, which uses a continuous wave laser, and the pulsed regime. For these regimes, performance of the system is analyzed depending on receiver sensitivity of classical DWDM. It is found that the pulsed regime outperforms the continuous wave regime in both the secure key generation rate and maximum achievable distance.

Dynamic secret-key provisioning in quantum-secured passive optical networks (PONs).

Quantum cryptography (QC) is currently under investigation to build highly secure optical communication networks. QC requires distribution of quantum keys (also called "secret" keys) on separate wavelength channels than those used to transmit the encrypted data. Hence, we propose a quantum-secured passive optical network (QS-PON) that supports both i) the traditional wavelength channels for secured data transmission, and ii) a quantum key distribution network (QKDN) running on separate dedicated wavelengths. The QKDN generates and stores secret keys that are then assigned to users' demands served on traditional PON channels. To generate secret keys, quantum transmitters at the optical network units (ONUs) exchange qubits with a quantum receiver at the optical line terminal (OLT). Then, the generated secret keys are stored in quantum key pools (QKPs) installed at both OLT and the ONUs and assigned to users' demands. Point-to-multipoint QKD systems have been experimentally demonstrated over various forms of quantum access networks (QANs), showing that an efficient mechanism to generate and assign quantum keys based on traffic requests is a critical component of QANs. In this study, we present a new QS-PON architecture, and we propose a dynamic secret-key provisioning (DSKP) algorithm that effectively generates and assigns secret keys from users' demands. Our proposed DSKP algorithm features two phases, the lowest-first secret-key generation (LF-SKG) phase and the hierarchical-clustering secret-key consumption (HC-SKC) phase. In this study, we also provide an analytical model that describes how secret keys are generated and consumed in QKPs. In our illustrative numerical evaluation, we compare our algorithm for secret-key provisioning with a baseline IPACT-based solution in terms of service-rejection ratio, time-slot utilization, and guard- and relay-time saving. Results show that DSKP reduces service-rejection ratio and guard- and relay-time of about 16% and 39.54%, respectively.

ВЕРОЯТНОСТНЫЕ ХАРАКТЕРИСТИКИ ПОРОГОВОГО АЛГОРИТМА ОБНАРУЖЕНИЯ СИНХРОИМПУЛЬСОВ В СИСТЕМЕ КВАНТОВОГО РАСПРЕДЕЛЕНИЯ КЛЮЧА НА ОСНОВЕ ИНФОРМАЦИИ СО СМЕЖНОЙ ПАРЫ ВРЕМЕННЫХ СЕГМЕНТОВ

Quantum key distribution systems (QKD) provide increased security of transmitted information.For the stable operation of the QKD system, accurate synchronization of user stations isrequired with minimal time costs. An algorithm for detecting a sync signal with a threshold test isproposed. It is assumed that the sync pulse is simultaneously in two adjacent time segments. Theprobability of detecting a pair of time segments where a sync pulse is present is determined by theprobability of exceeding the threshold level by the total number of signal and noise pulses recordedin two adjacent segments. The purpose of the research is aimed at a comparative analysis of thethreshold level and probabilistic characteristics of synchronization equipment during thresholdtesting of each pair of time segments within a time frame, obtained when orienting on the Gaussand Poisson model for the number of photons and dark current pulses (DCP) received during thetime segment analysis. The probabilistic characteristics of the detection algorithm for sync signalsare studied in a quantum key distribution system based on a comparison of the number of photonsfrom an adjacent pair of time segments with a threshold level. The application of the approximationof the statistical properties of the processes at the output of the photodetector by the Poissonlaw and the normal distribution is analyzed. The influence of the Poisson and Gaussian models onthe choice of the threshold level and the calculation of the synchronization efficiency during thethreshold testing of each pair of time segments within the time frame are estimated, obtained byorientation on the Gauss and Poisson models for the number of photons and DCP received duringthe analysis of the time segment. It was established that the choice of the threshold level based onthe normal distribution gives an underestimated value. The approximation of the statistics of photonsand pulses of dark current by a normal law provides a threshold level lower than the requiredone. Moreover, the difference grows with stricter requirements for the probability of false positives.The obtained probabilistic properties of the sync signal detection algorithm based on theanalysis of the sum of counts from an adjacent pair of segments with a threshold level allow us toformulate recommendations for choosing an approximation of the signal statistics: for expresscalculations of probabilistic characteristics, it is advisable to use the Gaussian model; if a higheranalysis accuracy is required, it is recommended to use the Poisson model.

A Fully Symmetrical Quantum Key Distribution System Capable of Preparing and Measuring Quantum States* *Supported by the Fundamental Research Funds for the Central Universities (Grant No. 2019XD-A02), and the State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications (Grant No. IPO2019ZT06).

We propose a fully symmetrical QKD system that enables quantum states to be prepared and measured simultaneously without compromising system performance. Over a 25.6 km fiber channel, we demonstrate point-to-point QKD operations with asymmetric Mach–Zehnder interferometer modules. Two interference visibilities of above 99% indicate that the proposed system has excellent stability. Consequently, the scheme not only improves the feasibility of distributing secret keys, but also enables QKD closer to more practical applications.

Analysis of the chromatic dispersion effect on the subcarrier wave QKD system.

In this paper we investigate the chromatic dispersion impact on the quantum key distribution system based on multi-mode weak coherent phase-coded states. We provide an asymptotic secure key rate estimation, taking into account error detection probability due to chromatic dispersion. We demonstrate numerically and experimentally that the effect of chromatic dispersion in an optical fiber without any compensation hinders the secret key distribution at a distance more than 53 km. Finally, we propose a modification to the considered quantum communication system in order to mitigate the influence of chromatic dispersion on its performance.

Modified error correction method using one-time pad in qkd systems

This paper analyzes and adapts the known method of error correction of CV-QKD systems, proposed by Mario Milicevic, which modulates a code word with a correlated Gaussian sequence in amplitude and phase quadratures of coherent states, for use in DV-QKD systems, in which information is encoded by single-photon polarization states, by using a sieved key in the form of classical bits, from which the codeword is created, and also a one-time pad (Vernam cipher), which uses a fairly easy to implement classic Boolean “exclusive OR” operation (XOR). The main task of error correction is to correct errors in order to share the same secure key between the two parties (usually called Alice and Bob). Various factors can cause errors. The unreliability of the quantum channel is due to the fact that photons can cause noise when changing the polarization vector of the photon. In the proposed solution, the quantum component of the protocol occurs only in the first stage of transmission of the protected key, where the exchange of photons. The quantum key distribution system has inevitable errors in the sieved key, which must be corrected by the error correction algorithm to create a secure key. Unlike other known QKD systems, where at the stage of error correction the sifted keys are corrected and passed to the next stage of increasing confidentiality, in the modified method it is proposed to use sifted keys as keys in disposable Vernam ciphers, and the vector that will go to the next stage way and combine with the sifted key. The difference in the sieved keys between the two parties is corrected by means of low-density parity check matrices (LDPC), which are known to both of them in advance. Parity check matrices are created by an algorithm proposed by David MacKay and Radford Neal. Radford Neal open-source program code, is used to create final work program to checking proposed algorithm.

Quantum Key Distribution with On‐Chip Dissipative Kerr Soliton

AbstractQuantum key distribution (QKD) can distribute symmetric key bits between remote legitimate users with the guarantee of quantum mechanics principles. For practical applications, there is increasing attention to integrating the source, detectors, and modulators on a photonic chip. Here, the Kerr dissipative soliton is introduced in a microresonator as the light source for GHz QKD systems, and the proof‐of‐principle experiment demonstrates the parallel QKD can be achieved using coherent comb lines from the soliton. The performance of parallel QKD is also verified by using the off‐the‐shelf wavelength division modules and the on‐chip soliton source. Since the on‐chip soliton can provide hundreds of carriers covering the C and L bands, the results exhibit the feasibility to achieve ultra‐high rate secure key bits through massively parallel QKD, especially when incorporated with the thriving photonic integrated circuit technology.

Simple method for asymmetric twin-field quantum key distribution

Twin-field quantum key distribution (TF-QKD) can beat the linear bound of repeaterless QKD systems. After the proposal of the original protocol, multiple papers have extended the protocol to prove its security. However, these works are limited to the case where the two channels have equal amount of loss (i.e. are symmetric). In a practical network setting, it is very likely that the channels are asymmetric due to e.g. geographical locations. In this paper we extend the ‘simple TF-QKD’ protocol to the scenario with asymmetric channels. We show that by simply adjusting the two signal states of the two users (and not the decoy states) they can effectively compensate for channel asymmetry and consistently obtain an order of magnitude higher key rate than previous symmetric protocol. It also can provide 2–3 times higher key rate than the strategy of deliberately adding fibre to the shorter channel until channels have equal loss (and is more convenient as users only need to optimize their laser intensities and do not need to physically modify the channels). We also perform simulation for a practical case with three decoy states and finite data size, and show that our method works well and has a clear advantage over prior art methods with realistic parameters.

Discrete modulation continuous-variable quantum key distribution based on quantum catalysis

Compared with discrete variable quantum key distribution (DVQKD), continuous variable (CV) QKD has high security bit rate and other advantages, which, however, are slightly insufficient in secure transmission distance. In addition, the application of quantum catalysis has significantly improved the performance of Gaussian modulated (GM) CVQKD, especially in secure transmission distance. Recently, the application of quantum catalysis has significantly improved the performance of GM-CVQKD. However, whether it can be used to improve the performance of discrete modulated (DM) CVQKD protocol is still ambiguous. Therefore, a scheme of DM CVQKD protocol based on quantum catalysis is proposed in this paper to further improve the performance of the proposed protocol in terms of secure key rate, secure transmission distance and maximum tolerable noise. Our results show that under the same parameters, when the transmittance <i>T</i> introduced by quantum catalysis is optimized, the proposed scheme can effectively further improve the performance of QKD system compared with the original four-state modulation CVQKD scheme. In particular, when the tolerable excess noise is 0.002, the use of quantum catalysis can break the safe communication distance of 300 km with a key rate of 10<sup>–8</sup> bits/pulse. However, if this noise is too large, the improvement in the effect of quantum catalysis on protocol performance will be restrained. In addition, in order to highlight the advantages of the use of quantum catalysis, the ultimate limit PLOB (Pirandola-Laurenza-Ottaviani-Banchi) bound of point-to-point quantum communication is given in this paper. The simulation results indicate that although neither the original scheme nor the proposed scheme can break the bound, compared with the former, the latter can be close to the boundary in long-distance transmission. These results provide theoretical basis for achieving the ultimate goal of global quantum security communication.

Sending-or-Not-Sending Twin-Field Quantum Key Distribution with Light Source Monitoring.

Twin-field quantum key distribution (TF-QKD) is proposed to achieve a remote key distribution with a maximum secure transmission distance up to over 500 km. Although the security of TF-QKD in its detection part is guaranteed, there are some remaining problems in the source part. The sending-or-not-sending (SNS) protocol is proposed to solve the security problem in the phase post-selection process; however, the light source is still assumed to be an ideal coherent state. This assumption is not satisfied in real-life QKD systems, leading to practical secure issues. In this paper, we discuss the condition that the photon number distribution (PND) of the source is unknown for the SNS protocol, demonstrate that the security analysis is still valid under a source with unknown PND, and show that with light source monitoring, the performance of the SNS protocol can remain almost unchanged.