Quantum Key Distribution Applications Research Articles

Flying-cat parity checks for quantum error correction

Long range, multiqubit parity checks have applications in both quantum error correction and measurement-based entanglement generation. Such parity checks could be performed using qubit-state-dependent phase shifts on propagating pulses of light described by coherent states |α〉 of the electromagnetic field. We consider “flying-cat” parity checks based on an entangling operation that is quantum nondemolition for Schrödinger's cat states |α〉±|−α〉. This operation encodes parity information in the phase of maximally distinguishable coherent states |±α〉, which can be read out using a phase-sensitive measurement of the electromagnetic field. In contrast to many implementations, where single-qubit errors and measurement errors can be treated as independent, photon loss during flying-cat parity checks introduces errors on physical qubits at a rate that is anticorrelated with the probability for measurement errors. We analyze this trade-off for three-qubit parity checks, which are a requirement for universal fault-tolerant quantum computing with the subsystem surface code. We further show how a six-qubit entangled “tetrahedron” state can be prepared using these three-qubit parity checks. The tetrahedron state can be used as a resource for controlled quantum teleportation of a two-qubit state or as a source of shared randomness with potential applications in three-party quantum key distribution. Finally, we provide conditions for performing high-quality flying-cat parity checks in a state-of-the-art circuit QED architecture, accounting for qubit decoherence, internal cavity losses, and finite-duration pulses, in addition to transmission losses. Published by the American Physical Society 2024

In‐field comparison between G.652 and G.655 optical fibres for polarisation‐based quantum key distribution

AbstractIntegration of Quantum Key Distribution (QKD) in existing telecommunication infrastructure is crucial for the widespread adoption of this quantum technology that offers the distillation of unconditionally secure keys between users. The authors report a field trial between the Points of Presence placed in Treviso and in Venezia—Mestre, Italy, exploiting the QuKy commercial polarisation‐based QKD platforms developed by ThinkQuantum S.r.l. and two different standards of single‐mode optical fibres, that is, G.652 and G.655 as a quantum channel. In this field trial, several configurations were tested, including the co‐existence of classical and quantum signals over the same fibre, providing a direct comparison between the performances of the G.652 and G.655 fibre standards for QKD applications.

Chip-Based Electronic System for Quantum Key Distribution.

Quantum Key Distribution (QKD) has garnered significant attention due to its unconditional security based on the fundamental principles of quantum mechanics. While QKD has been demonstrated by various groups and commercial QKD products are available, the development of a fully chip-based QKD system, aimed at reducing costs, size, and power consumption, remains a significant technological challenge. Most researchers focus on the optical aspects, leaving the integration of the electronic components largely unexplored. In this paper, we present the design of a fully integrated electrical control chip for QKD applications. The chip, fabricated using 28 nm CMOS technology, comprises five main modules: an ARM processor for digital signal processing, delay cells for timing synchronization, ADC for sampling analog signals from monitors, OPAMP for signal amplification, and DAC for generating the required voltage for phase or intensity modulators. According to the simulations, the minimum delay is 11ps, the open-loop gain of the operational amplifier is 86.2 dB, the sampling rate of the ADC reaches 50 MHz, and the DAC achieves a high rate of 100 MHz. To the best of our knowledge, this marks the first design and evaluation of a fully integrated driver chip for QKD, holding the potential to significantly enhance QKD system performance. Thus, we believe our work could inspire future investigations toward the development of more efficient and reliable QKD systems.

Reference‐Frame‐Independent Mode‐Pairing Quantum Key Distribution with Advantage Distillation

AbstractThe coordination between distance and the secure key rate is one of the main challenges in the practical application of quantum key distribution (QKD). Mode‐pairing quantum key distribution is one of the schemes that can surpass the secret key capacity for repeaterless QKD. However, the protocol utilizes phase to encode the information, which leads to the problem of active stabilization in the interferometer. In this paper, a reference‐frame‐independent mode‐pairing quantum key distribution (RFI‐MP‐QKD) is proposed as an effective scheme to solve this problem. Moreover, the performance of the RFI‐MP‐QKD protocol is improved by applying the Advantage Distillation (AD) method in data post‐processing, which separates the highly correlated raw key bits from the weakly correlated information. The simulation results show that the secure key rate of RFI‐MP‐QKD has almost no degradation for reference frame deviation angles of . Compared to RFI‐MP‐QKD without AD method, the AD method decreases the quantum bit error rate from 0.04 to 0.012 and increases the maximum transmission distance from 406 to 450 km. The scheme proposed is expected to facilitate the practical implementation of RFI‐MP‐QKD, especially in cases of concerning reference frame alignment and high channel loss.

Experimental high-dimensional quantum key distribution with orbital angular momentum

The development and practical applications of quantum key distribution are limited by the channel capacity and high error rate during long-distance transmission. High-dimensional quantum key distribution protocols solve these problems effectively. In this paper, a high-dimensional quantum key distribution protocol based on polarization-orbit angular momentum is achieved, using a decoy state method to ensure the security of the protocol. The experimental results show that the high-dimensional quantum key distribution protocol improves the encoding efficiency and the upper bound of the quantum bit error rate.

Experimental demonstration of a reconfigurable free-space receiver implementing polarization routing and filtering for daytime quantum key distribution.

Free-space quantum key distribution (QKD) systems are often designed to implement polarization-encoding protocols. Alternatively, time-bin/phase-encoding protocols are considerably more challenging to perform over a channel experiencing atmospheric turbulence. However, over the last decade, new and improved optical platforms have revived the interest in them. In this paper, we present a free-space multi-protocol receiver designed to work with three different time-bin/phase-encoding protocols highlighting its interoperability with different systems and architectures for potential satellite-based communications. We also present a detailed analysis of different experimental configurations when implementing the coherent one-way (COW) protocol in a free-space channel, as well as a polarization filtering technique showing how time-bin/phase-encoding protocols could be used for QKD applications in daylight conditions. We demonstrate secret key rates of several kbps for channels with a total 30 dB attenuation even with moderately high QBERs of ≈3.5%. Moreover, a 2.6 dB improvement in the signal to noise ratio is achieved by filtering background light in the polarization degree of freedom, a technique that could be used in daylight QKD.

Phase-locking an interferometer with single-photon detections

We report on a novel phase-locking technique for fiber-based Mach–Zehnder interferometers based on discrete single-photon detections, and demonstrate this in a setup. Our interferometer decodes relative-phase-encoded optical pulse pairs for quantum key distribution applications and requires no locking laser in addition to the weak received signal. Our new simple locking scheme is shown to produce an Ornstein–Uhlenbeck dynamic and achieve optimal phase noise for a given count rate. In case of wavelength drifts that arise during the reception of Doppler-shifted satellite signals, the arm-length difference gets continuously readjusted to keep the interferometer phase stable.

Precise On-Chip Spectral and Temporal Control of Single-Photon-Level Optical Pulses

The constantly growing need for fast and secure optical communication systems resulted in the rapid development of classical and quantum photonic technologies. Continuous research on improving the capacities of communication channels and the efficiency of optical information processing systems were naturally followed by miniaturization. Photonic integration combined with spectral-temporal multiplexing has led to improvements in communications. In this paper, we present accurate control of temporal and spectral profiles of optical pulses generated at the telecom wavelengths using an indium phosphide-based photonic integrated circuit. The device enables simultaneous temporally and spectrally resolved measurements using single photon counting. We discuss its applications in multidimensional time-frequency quantum key distribution, where security relies on time-frequency uncertainty, and other applications.

Application of quantum key distribution in intelligent security operation and maintenance of power communication networks

With the continuous development of power communication network and the improvement of intelligent degree, network security and operation and maintenance become more and more important. Therefore, this paper proposes a solution for intelligent security operation and maintenance of power communication network based on Quantum key distribution (QKD). QKD is an encryption technology based on the principle of quantum mechanics, which can realize secure Key exchange and resist attacks of traditional encryption algorithms. We use QKD technology to generate and distribute security keys for protecting communication and data transmission in power communication networks. The results show that the efficiency and accuracy of communication data acquisition, transmission, analysis and processing of the Artificial Intelligence for IT Operations system of the power communication network using the power Internet of Things technology are far higher than that of the traditional operation and maintenance technology, which improves the operation and maintenance accuracy of the power communication network by about 25%. The intelligent operation and maintenance system of the power communication network designed by the author, it effectively improves the communication data processing capability and the reliability of operation and maintenance results, and can meet the current practical engineering application requirements. This study provides an innovative solution for the secure operation and maintenance of power communication networks, which is of great significance for protecting the security and stable operation of power communication networks.

Securing Internet of Things (IoT) Ecosystems: A Quantum Cryptography Approach

In order to strengthen the stability of IoT ecosystems, this study explores the interface between Internet of Things (IoT) security and quantum cryptography. Our study goals included assessing the applicability of quantum key distribution, correcting flaws, and investigating the influence of environmental variables on security. We reveal fascinating insights through experimental research that alter IoT security paradigms. A comparison of the typical key setup durations and data transmission rates across various IoT devices and communication protocols shows how effective quantum cryptography is in speeding up key distribution. This effectiveness fits the changing environment of secure communication technologies. Our vulnerability evaluation also shows that quantum key distribution is a potent defense against weaknesses like man-in-the-middle and replay assaults. It is made clear how contextual factors and security interact, emphasizing the importance of adaptable security measures in line with environmental circumstances. We present a comprehensive methodology that balances security with deployment circumstances by merging this contextual sensitivity with quantum cryptography concepts. Our research points to prospective directions for joint investigation between quantum physicists and IoT developers to close the gap between theoretical promise and actual use. Comprehensive insights will be obtained by scaling the use of quantum cryptography to larger IoT networks and other circumstances, but caution is needed to survive the upcoming quantum computing era. The study's conclusion ignites a paradigm change and pushes the quantum revolution into IoT security. Our research promotes more investigation, innovation, and application as we stand on the brink of a new era, protecting IoT ecosystems in a quantum-secure embrace.

Applications of the Lambert–Tsallis Wq function in QKD

The Lambert–Tsallis W q function has found applications in several areas of physics, such as quantum optics, astronomy, and semiconductor physics. In this work, we discuss some applications of W q in quantum key distribution (QKD). Three problems are considered: (i) the quantum access network, (ii) analysis of an on-chip SiO2 amplitude modulator used in continuous-variable QKD (CV-QKD), and (iii) parameter estimation of a stochastic quantum channel. In quantum access networks, QKD and classical data travel on the same optical fiber. In this case, an increase of the quantum bit error rate is caused mainly by the spontaneous Raman scattering (SRS) produced by classical data. The amount of SRS produced depends on the fiber length. In the present work, we use the Lambert–Tsallis W q function to calculate analytically the fiber length needed to produce a given level of SRS. Thus, our formula allows the calculation of the QKD channel’s length when the probability of a click on the receiver side without having any incident photons from the quantum transmitter is defined a priori. On the other hand, a crucial step in the security of CV-QKD is the correct channel transmissivity and excess noise estimations. These parameters can be overestimated or underestimated when real devices do not behave as predicted by their models. In this direction, using the W q function, we provide an equation for calculation of the fluctuation of the concentration of free carriers in an integrated amplitude modulator and another equation for calculation of the parameter that models a stochastic quantum channel. These equations are useful in building strategies to avoid quantum hacking.

Compact implementation of high-dimensional mutually partially unbiased bases protocol

Transverse spatial mode of light is crucial in high-dimensional quantum key distribution (QKD). However, applications in realistic scenarios suffer from mode-dependent loss and the complexity of system, making it impractical to achieve higher-dimensional, longer-distance and low-cost communications. A mutually partially unbiased bases (MPUBs) protocol has been proposed to fundamentally eliminate the effects induced by mode-dependent loss for long propagation distances and limited sizes of apertures. Here, we demonstrate the first implementation of the MPUBs protocol in dimensions of and 6. By performing a controlled unitary transformation, we can actively switch the measurement basis and enable a compact measurement system. In consequence, a higher encoding dimension is available under finite system resources, resulting in higher key rates and stronger noise resistance. Our work enhances the practicability of MPUBs protocol, and may promote the applications of high-dimensional QKD in quantum networks.

Experimental Twin-Field Quantum Key Distribution over 1000km Fiber Distance.

Quantum key distribution (QKD) aims to generate secure private keys shared by two remote parties. With its security being protected by principles of quantum mechanics, some technology challenges remain towards practical application of QKD. The major one is the distance limit, which is caused by the fact that a quantum signal cannot be amplified while the channel loss is exponential with the distance for photon transmission in optical fiber. Here using the 3-intensity sending-or-not-sending protocol with the actively-odd-parity-pairing method, we demonstrate a fiber-based twin-field QKD over 1002km. In our experiment, we developed a dual-band phase estimation and ultra-low noise superconducting nanowire single-photon detectors to suppress the system noise to around 0.02Hz. The secure key rate is 9.53×10^{-12} per pulse through 1002km fiber in the asymptotic regime, and 8.75×10^{-12} per pulse at 952km considering the finite size effect. Our work constitutes a critical step towards the future large-scale quantum network.

Supporting quantum technologies with an ultralow-loss silicon photonics platform

Photonic integrated circuits (PICs) are expected to play a significant role in the ongoing second quantum revolution, thanks to their stability and scalability. Still, major upgrades are needed for available PIC platforms to meet the demanding requirements of quantum devices. We present a review of our recent progress in upgrading an unconventional silicon photonics platform toward this goal, including ultralow propagation losses, low-fiber coupling losses, integration of superconducting elements, Faraday rotators, fast and efficient detectors, and phase modulators with low-loss and/or low-energy consumption. We show the relevance of our developments and our vision in the main applications of quantum key distribution, to achieve significantly higher key rates and large-scale deployment; and cryogenic quantum computers, to replace electrical connections to the cryostat with optical fibers.

Evaluation of Quantum Key Distribution for Secure Satellite-integrated IoT Networks

There has been a dramatic increase in the number of Internet of Things (IoT) devices and their applications. Furthermore, there is a growing impetus to integrate IoT networks on a global scale, using satellites to expand the range of IoT connectivity into geographically remote areas. Ensuring the security of satellite backhaul for IoT networks is thus of paramount importance. The steady advance of quantum computing in recent years threatens to nullify classical cryptographic approaches based on assumptions of computational hardness, motivating the need for post-quantum cryptography. Quantum computing algorithms have been developed that, once a quantum computer of sufficient scale is realised, will be able to break classical cryptosystems efficiently (at polynomial-time complexity). A promising method of securing information against this threat at the physical layer has emerged in the form of quantum key distribution (QKD). QKD exploits the fundamental physical properties of light to guarantee information-theoretic security. Research into the application and standardisation of QKD to secure satellite backhaul, however, is still in its infancy. This paper presents a brief overview of the theoretical basis for QKD, whilst also providing a survey of contemporary QKD protocols. It evaluates the ability of these protocols to secure satellite backhaul in the context of a typical satellite-IoT network architecture. Furthermore, it highlights the vulnerabilities, as well as the technical challenges associated with this endeavour. Finally, it proposes directions for future research and development into protocols and standardisation for the satellite-integrated IoT domain. Several challenges must be overcome before QKD can evolve into a global-scale solution for securing satellite-IoT. Secret key generation rate remains very low in practical demonstrations of trusted-relay QKD satellite architectures. Further research is needed to overcome or mitigate the fundamental rate-distance trade-off before satellite QKD can be considered practicable in an IoT application. Alternatives that do not rely on trusted nodes are contingent on nascent technologies such as quantum repeaters and quantum memory. Whilst in theory QKD provides perfect information-theoretic security, it remains vulnerable to attacks that exploit imperfections in real-world equipment. Further effort is needed to develop QKD protocols that can safeguard against the aforementioned challenges.

Experimental free-space continuous-variable quantum key distribution with thermal source.

Passive-state-preparation (PSP) continuous-variable quantum key distribution (CVQKD) protocol explores the intrinsic field fluctuations of a thermal source. Compared with traditional Gaussian-modulated coherent-state CVQKD, it does not need active modulations and has promising applications in chip integration and portable free-space quantum key distribution. In this Letter, we propose and experimentally realize a PSP CVQKD scheme with transmitted local oscillator (LO) through fluctuating transmittance free-space channel using an off-the-shelf amplified spontaneous emission source for the first time. By proposing thermal-state polarization multiplexing transmitted LO, synchronized channel transmittance monitoring and fine-grained phase compensation techniques, secure keys within -15 dB transmittance of simulated free-space channel with turbulence are generated, with a final average secure key rate of 1.015 Mbps asymptotically. Equivalent atmospheric turbulence model analysis shows that the free-space PSP CVQKD scheme provides a promising outlook for high-speed and chip-based CVQKD for kilometer-level atmospheric channel networks.

Multi-rate and multi-protocol continuous-variable quantum key distribution.

Quantum key distribution (QKD) is an effective solution to ensure the secure transmission of information. However, for the large-scale application of QKD, the interoperability and flexibility of the transmitter and receiver are urgent issues to be solved. Here, for the first time, to the best of our knowledge, we experimentally verify the feasibility of one continuous-variable (CV) QKD system to achieve multiple protocols and rates. The flexibility of the system comes from the modulator realizing multiple protocols and a broadband coherent detector realizing multiple symbol rates. The results show that this system can switch between different rates and protocols to generate the secure key, and reveal its similarity to classical optical communication. Therefore, It can be adjusted according to user needs and provides a system-level solution for building a flexible quantum network.

2.5 GHz Gated InGaAs/InP Single-Photon Avalanche Diode with 44 ps Time Jitter

Gated single-photon avalanche diodes (SPADs) are practical solutions for quantum key distribution (QKD) applications. However, the gating frequency is limited by time jitter and afterpulse probability when SPADs operate at a frequency higher than 2 GHz. We find that filter distortion and variation of signal amplitude are the dominant mechanisms for time jitter in high-frequency scenarios, and design a specific low-pass filter (LPF) and a zero crossing discriminator to reduce the time jitter effectively. Specifically, the LPF extracts avalanche signals and forms accurate zero crossing points by its impulse response, and the zero crossing discriminator correctly discriminates avalanche signals to achieve low time jitter. When the SPAD is operated at a gating frequency of 2.5 GHz with a detection efficiency of 21%, the time jitter and afterpulse probability are 44 ps and 1.4%, respectively. Our results pave the way to practical QKD systems at higher clock rates.

Recent Progress in Quantum Key Distribution Network Deployments and Standards

Quantum key distribution (QKD) provides in principle unconditional security of key sharing based on the laws of physics only. In the last decade, several experimental and commercial QKD networks have been built and operated worldwide. Demonstrational applications of QKD in financial institutions, government networks, and critical infrastructures such as the power grid have been initially explored. However, large-scale deployment and full-scale commercialization of QKD networks still faces some technological and standardisation challenges. In this paper, recent developments and in-field deployments of QKD networks are reviewed and advancements in QKD standardisation are also discussed.

Time-division technique for quantum optical receivers utilizing single-photon detector array technology and spatial-multiplexing.

Free-space quantum key distribution (QKD) has been gaining popularity in recent years due to its advantages in creating networking options for the quantum internet. One of the main challenges to be addressed in QKD is the achievable secret key rate, which must meet current and future demand. Some of the existing solutions include the use of higher bandwidth electronics, untrusted relay architectures such as Twin-Field QKD, or high dimensional QKD. In this work, we proposed the use of a combination of spatial-multiplexing and time-division techniques, together with the use of 2D single-photon avalanche diode arrays to increase the final throughput. The main challenge in a free-space scenario is the effects introduced by turbulence. This paper demonstrates how appropriate time-division of the spatial-modes can reduce the quantum bit error rate due to optical crosstalk from 36% to 0%. With this technique, we believe the future need for superconducting nanowires single photon detectors, in some free-space QKD applications, can be relaxed, obtaining more cost-effective receiver systems.