Performance of integrated quantum and classical cryptographic model for password authentication

Single-photon detection concept is the most crucial and often difficult factor to determine the performance of quantum key distribution (QKD) systems. One solution to facilitate understanding this concept is to create a virtual environment for modeling, analyzing, and investigating the performance of single photon detectors. In this paper, a simulator for superconducting single photon detectors with time domain visualizer and configurable parameters is presented. The widely used ID281SNSPD in the QKD area was theoretically modeled in terms of pulse analysis, the impact of biasing current and the temperature on the dark counts rate and single photon-detection efficiency and influence of the number of photons per pulse on the single photon-detection efficiency. The simulated results were in good agreement with the theoretical results and the simulator demonstrated its adaptability.

Quantum cryptography, and especially quantum key distribution (QKD), is steadily progressing to become a viable tool for cryptographic services. In recent years we have witnessed a dramatic increase in the secure bit rate of QKD, as well as its extension to ever longer fibre- and air-based links and the emergence of metro-scale trusted networks. In the foreseeable future even global-scale communications may be possible using quantum repeaters or Earth–satellite links. A handful of start-ups and some bigger companies are already active in the field. The launch of an initiative to form industrial standards for QKD, under the auspices of the European Telecommunication Standards Institute, described in the paper by Laenger and Lenhart in this Focus Issue, can be taken as a sign of the growing commercial interest.

Detection-time-bin-shift (DTBS) is a scheme that projects the measurement bases or measured photon values into detection time-bins and then time division multiplexes a single photon detector in a quantum key distribution (QKD) system. This scheme can simplify the structure of a QKD system, reduce its cost and overcome the security problems caused by the dead-time introduced self-correlation and the unbalanced characteristics of detectors. In this paper, we present several DTBS schemes for QKD systems based on attenuated laser pulses and entangled photon sources. We study the security issues of these DTBS schemes, especially the time-bin-shift intercept-resend attack and its countermeasures. A fiber-based DTBS QKD system has been developed and its results are presented in this paper.

We suggest a combined modulation quantum key distribution (QKD) system which encodes each single-photon signal with both phase modulation and polarization modulation. With the aid of dual-velocity protocol of hybrid QKD system, we construct a scheme to realize this combined modulation QKD which generates two-bit key with one signal for increasing the efficiency of QKD. We also develop a combined modulation Michelson QKD system, then solve the stability problem of the combined modulation QKD system in principle.

A Mach–Zehnder modulator (MZM) is necessary for implementing a decoy-state protocol in a practical quantum key distribution (QKD) system. However, an MZM bias control method optimized for QKD systems has been missing to date. In this study, we propose an MZM bias control method using N (≥2) diagnostic pulses. The proposed method can be efficiently applied to a QKD system without any additional hardware such as light sources or detectors. Furthermore, it does not reduce the key rate significantly because it uses time slots allocated to existing decoy pulses. We conducted an experimental demonstration of the proposed method in a field-deployed 1 × 3 QKD network and a laboratory test. It is shown that our method can maintain the MZM extinction ratio stably over 20 dB (bit error rate ≤1%), even in an actual network environment for a significant period. Consequently, we achieved successful QKD performances.

One of the most important obstacles for the social deployment of Quantum Key Distribution (QKD) systems is the lack of standards for security certification. Proper selection of application will circumvent other obstacles, such as cost, key generation distance, and rate. Nevertheless, security certification of real systems, or “implementation security certification” is indispensable. Such certification should be clear and tractable for non‐experts and should be performed with standardized characterization methods. Otherwise, the potential users cannot adopt the QKD system with confidence, because the highest level of security is what QKD appeals. This report discusses the process of the implementation security certification and the requirements for device characterization. It also presents several case studies on the requirements and experimental characterization of QKD devices for the security certification.

Polarization encoded Quantum Key Distribution (QKD) is attracting great attention as it generates unlimitedly and unconditionally secure keys for different use cases. Despite its theoretical excellence based on quantum physics, commercial optical devices supporting QKD systems lack precision, which highly limits the final Key Exchange Rate (KER) of the system. Beside optical component imperfections, eavesdropping and unpredicted environmental events occurred in the quantum channel increase quantum Bit Error Rate (qBER), which leads to further KER reduction. In this paper, we propose DARIUS, a digital twin for polarization encoded QKD systems that bridges the gap between perfect theoretical QKD systems and real implementations to: i) address optical components' non-ideal behavior; ii) discern eavesdropping from high qBER; and iii) dynamically compensate for environmental events. Taking advantage of DARIUS, even moderate eavesdropping rates can be distinguished from qBER. Moreover, significant improvement in proactive environmental event compensation is achieved, as DARIUS can derive proper optical component tuning.

Quantum cryptography has become a subject of widespread interest. In particular, quantum key distribution, which provides a secure key agreement by using quantum systems, is believed to be the most important application of quantum cryptography. Quantum key distribution has the potential to achieve the “unconditionally” secure infrastructure. We also have many cryptographic tools that are based on “modern cryptography” at the present time. They are being used in an effort to guarantee secure communication over open networks such as the Internet. Unfortunately, their ultimate efficacy is in doubt. Quantum key distribution systems are believed to be close to practical and commercial use. In this paper, we discuss what we should do to apply quantum cryptography to our communications. We also discuss how quantum key distribution can be combined with or used to replace cryptographic tools based on modern cryptography.

A Single-Photon Avalanche Diode (SPAD) is a specialized optoelectronic device designed for the detection of single photons. It can discern the presence or absence of photons as well as their reception frequency. A significant challenge associated with SPADs is the generation of unintended pulses, which manifest in two primary categories: Dark Count Rate (DCR) and Afterpulse Probability (PAP). Dark counting refers to the occurrence of output pulses in the absence of photon detection, while afterpulse are undesirable outputs resulting from the trapping of carrier’s post-avalanche event within the detector. These trapped carriers are subsequently accelerated by the activation field applied to the detector, causing another avalanche breakdown known as an afterpulse. Both the afterpulse effect and dark counting are considered detrimental phenomena in SPADs, which are commonly utilized in Quantum Key Distribution (QKD) systems. Quantum Key Distribution is a cryptographic method for securely distributing secret keys, leveraging principles of quantum mechanics. Moreover, this method ensures that an eavesdropper cannot intercept or regenerate photon states. In this study, a novel approach is proposed to detect errors induced by these undesirable pulses in a quantum key distribution system. The proposed method, designed to mitigate the effects of unwanted pulses in QKD using the BB84 protocol, employs two quantum channels for photon transmission and a classical channel for coordination between the communicating parties and filtering the secret key (sifting). This method preserves the secure key rate when compared to the traditional BB84 protocol. The reduction of undesirable pulses in the proposed method is contingent upon factors such as the probability of afterpulse occurrence, dark count events, and the efficiency of the single-photon detector, with the reduction being computed in direct proportion to these parameters.

As a practical method, the passive scheme is useful to monitor the photon statistics of an untrusted source in a "Plug & Play" quantum key distribution (QKD) system. In a passive scheme, three kinds of monitor mode can be adopted: average photon number (APN) monitor, photon number analyzer (PNA) and photon number distribution (PND) monitor. In this paper, the security analysis is rigorously given for APN monitor, while for PNA, the analysis including statistical fluctuation and random noise, is addressed with a confidence level. The results show that the PNA can achieve better performance than the APN monitor and can asymptotically approach the theoretical limit of the PND monitor. Also, the passive scheme with the PNA works efficiently when the signal-to-noise ratio ($R^{SN}$) is not too low and so is highly applicable to solve the untrusted source problem in the QKD system.

As an important support for quantum communication, quantum key distribution (QKD) networks have achieved a relatively mature level of development, and they face higher requirements for multi-user end-to-end networking capabilities. Thus, QKD networks need an effective management plane to control and coordinate with the QKD resources. As a promising technology, software defined networking (SDN) can separate the control and management of QKD networks from the actual forwarding of the quantum keys. This paper systematically introduces QKD networks enabled by SDN, by elaborating on its overall architecture, related interfaces, and protocols. Then, three-use cases are provided as important paradigms with their corresponding schemes and simulation performances.

Usually, a shared reference frame is indispensable for practical quantum key distribution (QKD) systems. As a result, most QKD systems need active alignment of reference frame due to the unknown and slowly variances of reference frame introduced by environment. Quite interestingly, reference-free-independent (RFI) QKD can generate secret-key bits without alignment of reference frame. However, RFI QKD may be still vulnerable to detector side channel attacks. Here, we propose a new RFI QKD protocol, in which all detector side channels are removed. Furthermore, our protocol can still tolerate unknown and slow variance of reference frame without active alignment. And a numerical simulation shows that long security distance is probable in this protocol.

We assess the overall performance of our quantum key distribution (QKD) system implementing the measurement-device-independent (MDI) protocol using components with varying capabilities such as different single-photon detectors and qubit preparation hardware. We experimentally show that superconducting nanowire single-photon detectors allow QKD over a channel featuring 60 dB loss, and QKD with more than 600 bits of secret key per second (not considering finite key effects) over a 16 dB loss channel. This corresponds to 300 and 80 km of standard telecommunication fiber, respectively. We also demonstrate that the integration of our QKD system into FPGA-based hardware (instead of state-of-the-art arbitrary waveform generators) does not impact on its performance. Our investigation allows us to acquire an improved understanding of the trade-offs between complexity, cost and system performance, which is required for future customization of MDI-QKD. Given that our system can be operated outside the laboratory over deployed fiber, we conclude that MDI-QKD is a promising approach to information-theoretic secure key distribution.

We report here a new side channel attack on a practical continuous-variable (CV) quantum key distribution (QKD) system. Inspired by blinding attack in discrete-variable QKD, we formalize an attack strategy by inserting an external light into a CV QKD system implemented Gaussian-modulated coherent state protocol and show that our attack can compromise its practical security. In this attack, we concern imperfections of a balanced homodyne detector used in CV QKD. According to our analysis, if one inserts an external light into Bob’s signal port, due to the imperfect subtraction from the homodyne detector, the leakage of the external light contributes a displacement on the homodyne signal which causes detector electronics saturation. In consequence, Bob’s quadrature measurement is not linear with the quadrature sent by Alice. By considering such vulnerability, a potential Eve can launch a full intercept-resend attack meanwhile she inserts an external light into Bob’s signal port. By selecting proper properties of the external light, Eve actively controls the induced displacement value from the inserted light which results saturation of homodyne detection. In consequence, Eve can bias the excess noise due to the intercept-resend attack and the external light, such that Alice and Bob believe their excess noise estimation is below the null key threshold and they can still share a secret key. Our attack shows that the detector loopholes also exist in CV QKD, and it seems influence all the CV QKD systems using homodyne detection, since all the practical detectors have finite detection range.

Considering the air-water interface and ocean water’s optical attenuation, the performance of quantum key distribution (QKD) based on air-water channel is studied. The effects of photons’ various incident angles to air-water interface on quantum bit error rate (QBER) and the maximum secure transmission distance are analyzed. Taking the optical attenuation of ocean water into account, the performance bounds of QKD in different types of ocean water are discussed. The simulation results show that the maximum secure transmission distance of QKD gradually reduces as the incident angle from air to ocean water increases. In the clearest ocean water with the lowest attenuation, the maximum secure transmission distance of photons far exceeds the the working depth of underwater vehicles. In intermediate and murky ocean waters with higher attenuation, the secure transmission distance shortens, but the underwater vehicle can deploy other accessorial methods for QKD with perfect security. So the implementation of OKD between the satellite and the underwater vehicle is feasible.