CV-QKD System Research Articles

General multi-attack detection for continuous-variable quantum key distribution with local local oscillator

Abstract Continuous-variable quantum key distribution with a local local oscillator(LLO CVQKD) has been extensively researched due to its simplicity and security. For the practical security of the LLO CVQKD system, there are two main attack modes referred to as reference pulse attack and polarization attack presently. However, there is currently no general defense strategy against such attacks and the security of the system needs further investigation. Here, we employ a deep learning framework called generative adversarial networks (GANs) to detect both attacks. We first analyze the data in different cases, derive a feature vector as input to a GAN model, and then show the training and testing process of the GAN model for attack classification. The proposed model has two parts, a discriminator and a generator, both of which employ a convolutional neural network (CNN) to improve accuracy. Simulation results show that the proposed scheme can detect and classify attacks without reducing the secret key rate and the maximum transmission distance. It only establishes a detection model by monitoring features of the pulse without adding additional devices.

Underwater quantum key distribution with continuous-variable via photon additions

Quantum key distribution with continuous-variable (CVQKD) provides an alternative way for the security improvement of underwater optical communication, but facing into the awkward situation of short distance. For this reason, here we exploit photon addition operation at the emitter into enhancing the performance of underwater CVQKD. Based on numerical simulation, it is shown that applying photon-addition non-Gaussian operation between entangled resource and the sender can further enhance both the secure key rate and the transmission distance, compared to apply photon subtraction operation between entangled resource and the receiver. In addition, the influence of seawater quality factors in underwater CVQKD system is also considered, so as to provide technical support for underwater safety communication.

Rate-Adaptive Polar-Coding-Based Reconciliation for Continuous-Variable Quantum Key Distribution at Low Signal-to-Noise Ratio

Information reconciliation significantly impacts the performance of the practical continuous-variable quantum key distribution (CV QKD) system. Fixed-rate error-correction codes limit the potential applications of the CV QKD because they lead to reduced reconciliation efficiency when the signal-to-noise ratio of the quantum channel changes, further deteriorating the system's performance. Therefore, we propose a rate-adaptive polar-coding-based reconciliation scheme for practical CV QKD systems with the time-variant quantum channel. Experimental results verify that the proposed scheme can successfully extract secret keys within the range of signal-to-noise ratios from $\ensuremath{-}0.5$ to $\ensuremath{-}4.5$ dB, and the minimum frame-error rate can be less than ${10}^{\ensuremath{-}3}$. Moreover, the proposed scheme can promote the application of the CV QKD system in a realistic environment.

Modulation leakage-free continuous-variable quantum key distribution

Distributing cryptographic keys over public channels in a way that can provide information-theoretic security is the holy grail for secure communication. This can be achieved by exploiting quantum mechanical principles in so-called quantum key distribution (QKD). Continuous-variable (CV) QKD based on coherent states, in particular, is an attractive scheme for secure communication since it requires only standard telecommunication technology that can operate at room temperature. However, a recently discovered side-channel created in the process of state preparation leads to a leakage of information about the transmitted quantum state, opening a security loophole for eavesdroppers to compromise the security of the CVQKD system. Here, we present a CVQKD system without this modulation leakage vulnerability. Our implementation is based on a baseband modulation approach and uses an in-phase and quadrature (IQ) modulator for state preparation and radio frequency heterodyne detection together with carefully designed digital signal processing for state measurement. We consider practical aspects in the implementation and demonstrate the generation of a composable secret key secure against collective attacks. This work is a step towards protecting CVQKD systems against practical imperfections of physical devices and operational limitations without performance degradation.

Practical continuous-variable quantum key distribution with composable security

A quantum key distribution (QKD) system must fulfill the requirement of universal composability to ensure that any cryptographic application (using the QKD system) is also secure. Furthermore, the theoretical proof responsible for security analysis and key generation should cater to the number N of the distributed quantum states being finite in practice. Continuous-variable (CV) QKD based on coherent states, despite being a suitable candidate for integration in the telecom infrastructure, has so far been unable to demonstrate composability as existing proofs require a rather large N for successful key generation. Here we report a Gaussian-modulated coherent state CVQKD system that is able to overcome these challenges and can generate composable keys secure against collective attacks with N ≈ 2 × 108 coherent states. With this advance, possible due to improvements to the security proof and a fast, yet low-noise and highly stable system operation, CVQKD implementations take a significant step towards their discrete-variable counterparts in practicality, performance, and security.

Polarization Attack on Continuous-Variable Quantum Key Distribution with a Local Local Oscillator.

The estimation of phase noise of continuous-variable quantum key distribution protocol with a local local oscillator (LLO CVQKD), as a major process in quantifying the secret key rate, is closely relevant to the intensity of the phase reference. However, the transmission of the phase reference through the insecure quantum channel is prone to be exploited by the eavesdropper (Eve) to mount attacks. Here, we introduce a polarization attack scheme against the phase reference. Presently, in a practical LLO CVQKD system, only part of the phase reference pulses are measured to compensate for the polarization drift of the quantum signal pulses in a compensation cycle due to the limited polarization measurement rate, while the other part of the phase reference pulses are not measured. We show that Eve can control the phase noise by manipulating the polarization direction of the unmeasured phase reference to hide her attack on the quantum signal. Simulations show that Eve can obtain partial or total key rates information shared between Alice and Bob as the transmission distance increases. Improving the polarization measurement rate to 100% or monitoring the phase reference intensity in real-time is of great importance to protect the LLO CVQKD from polarization attack.

An efficient hybrid hash based privacy amplification algorithm for quantum key distribution

Privacy amplification (PA) is an essential part in a quantum key distribution (QKD) system, distilling a highly secure key from a partially secure string by public negotiation between two parties. The optimization objectives of privacy amplification for QKD are large block size, high throughput and low cost. For the global optimization of these objectives, a novel privacy amplification algorithm is proposed in this paper by combining multilinear-modular-hashing and modular arithmetic hashing. This paper proves the security of this hybrid hashing PA algorithm within the framework of both information theory and composition security theory. A scheme based on this algorithm is implemented and evaluated on a CPU platform. The results on a typical CV-QKD system indicate that the throughput of this scheme ( $$261\,\mathrm{Mbps}@2.6\times 10^8$$ input block size) is twice higher than the best existing scheme ( $$140\,\mathrm{Mbps}@1\times 10^8$$ input block size). Moreover, this scheme is implemented on a mobile CPU platform instead of a desktop CPU or a server CPU, which means that this algorithm has a better performance with a much lower cost and power consumption.

Phase-reference-intensity attack on continuous-variable quantum key distribution with a local local oscillator

In practical continuous-variable quantum key distribution system using local local oscillator (LLO CV-QKD), the phase noise related to coherent detection and phase-reference pulse intensity can be locally calibrated at the receiver side. Therefore, it can be regarded as trusted noise in order to obtain a higher secret key rate and transmission distance. However, security loopholes will be opened for the eavesdropper to launch attacks without appropriate countermeasures. To this end, we propose two practical phase-reference pulse intensity attack strategies, where Eve can reduce the trusted phase noise by manipulating the intensity of the phase-reference pulse, thereby hiding her attack on the signal pulse. The performance of the LLO CV-QKD system under these attacks has been analyzed. We show that precisely monitoring the intensity of the phase-reference pulse in real time is an essential countermeasure to prevent the proposed attacks. Moreover, an intensity-monitoring scheme for the phase-reference pulse is proposed to strengthen the security of the practical LLO CV-QKD system and make the trusted phase noise model more robust.

Experimental demonstration of confidential communication with quantum security monitoring

Security issues and attack management of optical communication have come increasingly important. Quantum techniques are explored to secure or protect classical communication. In this paper, we present a method for in-service optical physical layer security monitoring that has vacuum-noise level sensitivity without classical security loopholes. This quantum-based method of eavesdropping detection, similar to that used in conventional pilot tone systems, is achieved by sending quantum signals, here comprised of continuous variable quantum states, i.e. weak coherent states modulated at the quantum level. An experimental demonstration of attack detection using the technique was presented for an ideal fibre tapping attack that taps 1% of the ongoing light in a 10 dB channel, and also an ideal correlated jamming attack in the same channel that maintains the light power with excess noise increased by 0.5 shot noise unit. The quantum monitoring system monitors suspicious changes in the quantum signal with the help of advanced data processing algorithms. In addition, unlike the CV-QKD system which is very sensitive to channel excess noise and receiver system noise, the quantum monitoring is potentially more compatible with current optical infrastructure, as it lowers the system requirements and potentially allows much higher classical data rate communication with links length up to 100 s km.

Phase noise model for continuous-variable quantum key distribution using a local local oscillator

The value of residual phase noise, after phase compensation, is one of the key limitations of performance improvement for continuous-variable quantum key distribution using a local local oscillator (LLO CV-QKD) system, since it is the major excess noise. However, due to the nonideality of the realistic devices implemented in practice, for example, imperfect lasers, detectors, and unbalanced interferometers, the value of residual phase noise in the current system is still relatively large. Here, we develop a phase noise model to improve the phase noise tolerance of the LLO CV-QKD schemes. In our model, part of the phase-reference measurement noise associated with detection efficiency and electronic noise of Bob's detector as well as a real-time monitored phase-reference intensity at Bob's side is considered trusted because it can be locally calibrated by Bob. We show that using our phase noise model can significantly improve the secure key rate and transmission distance of the LLO CV-QKD system. We further conduct an experiment to substantiate the superiority of the phase noise model. Based on experimental data of a LLO CV-QKD system in the 25-km optical fiber channel, we demonstrate that the secure key rate under our phase noise model is approximately 40% higher than that under the conventional phase noise model.

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.

Impact of imperfect homodyne detection on measurements of vacuum states shot noise

The secret key rate of a continuous variables quantum key distribution system (CV-QKD) depends highly on the loss and excess noise of the quantum channel. A proper modelling of the quantum channel is essential to detect and prevent an eavesdropping attack. In CV-QKD, homodyne detection is widely used to measure the quadratures of the modulated optical signal. Nevertheless, the homodyne detector (HD) can be a source of noise for the CV-QKD system. In this paper, a HD is modelled considering intrinsic imperfections. Moreover, an improved method is proposed to obtain mean, variance and auto-covariance functions for estimation of the HD performance, considering ideal and unbalanced HD operation modes with ideal and non-ideal local oscillator pulse intensity functions being compared. The imbalance of the HD leads to non-zero mean output and a quadratic relation of voltage variance. Despite that, the rebalance of the system was attained with the cost of lowering the system voltage output intensity.

Discretely modulated continuous-variable quantum key distribution with an untrusted entanglement source

The discretely modulated continuous-variable quantum key distribution (DM CVQKD) has been proven to be secure, and a basic assumption for the current theoretical security proof of the DM CVQKD is that the signal source cannot be compromised. However, this assumption is quite unpractical in real quantum communication systems. In this paper, we investigate the DM CVQKD with a special configuration in which the entanglement source is placed at the untrusted channel. With this configuration, the source is no longer protected by the sender but is exposed to the vulnerable environment. In particular, we consider the configuration for two typical DM CVQKD protocols, which are the four-state protocol and the eight-state protocol. Security analysis based on linear bosonic channel shows that the DM CVQKD with an untrusted entanglement source is able to defend itself against the most powerful quantum collective attack in a certain distance range and it can still generate positive secret key rate when considering finite-size effect and composable security, thereby providing a theoretical proof for applying the DM CVQKD system to a realistic environment.

Multimode CV‐QKD with non‐Gaussian operations

Non-Gaussian operations have been studied intensively in recent years due to their ability to increase the secret key rate for certain CV-QKD protocols. However, most previous studies on such protocols are carried out in a single-mode setting, even though in reality any quantum state contains multi-mode components in frequency space. In this work we investigate the use of non-Gaussian operations in a multi-mode CV-QKD system. Our main finding is that, contrary to single-mode CV-QKD systems, in generic multi-mode CV-QKD systems it is possible to use non-Gaussian operations to increase the optimized secret key rate. More specifically, we find that at losses of order 30dB, which represents a distance of order 160km and the effective maximum distance for CV-QKD, the key rate for multi-mode non-Gaussian operations can be orders of magnitude higher than single-mode operations. Our results are important for real-world CV-QKD systems especially those dependent on quantum error correction - a process that requires non-Gaussian effects.

Practical security of continuous-variable quantum key distribution with reduced optical attenuation

In a practical CVQKD system, the optical attenuator can adjust the Gaussian-modulated coherent states and the local oscillator signal to an optimal value for guaranteeing the security of the system and optimizing the performance of the system. However, the performance of the optical attenuator may deteriorate due to the intentional and unintentional damage of the device. In this paper, we investigate the practical security of a CVQKD system with reduced optical attenuation. We find that the secret key rate of the system may be overestimated based on the investigation of parameter estimation under the effects of reduced optical attenuation. This opens a security loophole for Eve to successfully perform an intercept-resend attack in a practical CVQKD system. To close this loophole, we add an optical fuse at Alice's output port and design a scheme to monitor the level of optical attenuation in real time, which can make the secret key rate of the system evaluated precisely. The analysis shows that these countermeasures can effectively resist this potential attack.

Security Analysis of Practical Continuous-Variable Quantum Key Distribution Using a Heralded Noiseless Amplifier

The noiseless linear amplifier(NLA) can probabilistically amplify the amplitude of the coherent state while retaining the initial level of noise. It is widely proposed and discussed to enhance various CVQKD protocols. In the previous work, when analyzing the security of the NLA-based CVQKQ protocol, they usually included the impact of NLA in a set of equivalent parameters. In this article, we develop this equivalent method and propose a new one to analyze the protocol with NLA which simplifies the computational complexity. In particular, we have further considered the effect of NLA on entanglement parameter of the EPR state. Then by using our proposed method, we first obtain the secret key rate of the NLA-enhanced CVQKD system with imperfect detection. The results show that the NLA can also improve the transmission distance of CVQKD protocols with imperfect detection by equivalent $20\log _{10}g$ dB of losses.

Reference pulse attack on continuous variable quantum key distribution with local local oscillator under trusted phase noise

We show that partially trusting the phase noise associated with estimation uncertainty in a LLO CVQKD system allows one to exchange higher secure key rates than in the case of untrusted phase noise. However, this opens a security loophole through the manipulation of the reference pulse amplitude. We label this as "reference pulse attack" which is applicable to all LLO-CVQKD systems if the phase noise is trusted. We show that, at the optimal reference pulse intensity level, Eve achieves unity attack efficiency at 23.8km and 32.0km while using lossless and 0.14dB/km loss channels, respectively, for her attack. However, in order to maintain the performance enhancement from partially trusting the phase noise, countermeasures have been proposed. As a result, the LLO-CVQKD system with partially trusted phase noise owns a superior key rate at 20km by an order 9.5, and extended transmission distance by 45%, than that of the phase noise untrusted system.

Toward the Integration of CV Quantum Key Distribution in Deployed Optical Networks

In this letter, we report on the advances toward the integration of our developed continuous variables (CV) quantum key distribution (QKD) system in existing optical infrastructure and wavelength division multiplexed (WDM) networks. First, we investigate the tolerance of the CV-QKD system to spontaneous Raman scattering (SRS) noise, the latter being the most dominant impairment in a WDM co-existence environment for QKD. In particular, we investigate by experiment the impact of a WDM 10 × 10 Gb/s ON-OFF-keying signal in terms of induced SRS noise in the QKD channel. The spontaneous SRS noise influence is assessed for different transmission scenarios, i.e., for various optical launch powers of the WDM signal, and for different transmission links of 20, 40, 60, and 80 km. Based on the experimental data and on the measured system's parameters, we estimate the key rates and reach capabilities of the proposed CV-QKD system. The scheme supports a key rate of 90 kbit/s over 20 km, for an ideal QKD system multiplexed with 2-mW optical power. A step further, we demonstrate for first time the use of the aforementioned CV-QKD system to encrypt a 10GE client service over deployable optical transport network legacy equipment over 20 km. Our results prove the feasibility of the integration of our proposed scheme with legacy telecom equipment in existing WDM optical networks.

Experimental study on all-fiber-based unidimensional continuous-variable quantum key distribution

We experimentally demonstrated an all-fiber-based unidimensional continuous-variable quantum key distribution (CV QKD) protocol and analyzed its security under collective attack in realistic conditions. A pulsed balanced homodyne detector, which could not be accessed by eavesdroppers, with phase-insensitive efficiency and electronic noise, was considered. Furthermore, a modulation method and an improved relative phase-locking technique with one amplitude modulator and one phase modulator were designed. The relative phase could be locked precisely with a standard deviation of 0.5\ifmmode^\circ\else\textdegree\fi{} and a mean of almost zero. Secret key bit rates of 5.4 kbps and 700 bps were achieved for transmission fiber lengths of 30 and 50 km, respectively. The protocol, which simplified the CV QKD system and reduced the cost, displayed a performance comparable to that of a symmetrical counterpart under realistic conditions. It is expected that the developed protocol can facilitate the practical application of the CV QKD.

Detection of a PNS attack by means of the quasiprobability function of the weak coherent states with phase diffusion in a CV-QKD system

On this article, we present a technique to detect photon number splitting (PNS) attacks in the quantum channel of a continuous variable-quantum key distribution system using weak coherent states with phase diffusion by means of the S-parameterized quasiprobability function of a quantum state in steady state. Such quasiprobability function is obtained with an optoelectronic Costas loop. We compare the performances of PNS attack systems considering in first place just the attenuation and then the attenuation with the vacuum noise added due to a spy system. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1349–1352, 2015