Measurement-Device-Independent Quantum Key Distribution Over a 404 km Optical Fiber

Time-bin entanglement at telecom wavelengths from a hybrid photonic integrated circuit

Mass-deployable implementations for quantum communication require compact, reliable, and low-cost hardware solutions for photon generation, control and analysis. We present a fiber-pigtailed hybrid photonic circuit comprising nonlinear waveguides for photon-pair generation and a polymer interposer reaching 68 dB of pump suppression and photon separation based on a polarizing beam splitter with > 25 dB polarization extinction ratio. The optical stability of the hybrid assembly enhances the quality of the entanglement, and the efficient background suppression and photon routing further reduce accidental coincidences. We thus achieve a96 - 8 + 3 % concurrence and a96 - 5 + 2 % fidelity to a Bell state. The generated telecom-wavelength, time-bin entangled photon pairs are ideally suited for distributing Bell pairs over fiber networks with low dispersion.

Experimental quantum e-commerce

E-commerce, a type of trading that occurs at a high frequency on the internet, requires guaranteeing the integrity, authentication, and nonrepudiation of messages through long distance. As current e-commerce schemes are vulnerable to computational attacks, quantum cryptography, ensuring information-theoretic security against adversary's repudiation and forgery, provides a solution to this problem. However, quantum solutions generally have much lower performance compared to classical ones. Besides, when considering imperfect devices, the performance of quantum schemes exhibits a notable decline. Here, we demonstrate the whole e-commerce process of involving the signing of a contract and payment among three parties by proposing a quantum e-commerce scheme, which shows resistance of attacks from imperfect devices. Results show that with a maximum attenuation of 25 dB among participants, our scheme can achieve a signature rate of 0.82 times per second for an agreement size of approximately 0.428 megabit. This proposed scheme presents a promising solution for providing information-theoretic security for e-commerce.

A 1.25-GHz multi-amplitude modulator driver in 0.18 μm SiGe BiCOMOS technology for high speed quantum key distribution

Quantum key distribution (QKD) research has yielded highly fruitful results and is currently undergoing an industrialization transformation. In QKD systems, electro-optic modulators are typically employed to prepare the required quantum states. While various QKD systems operating at GHz repetition frequency have demonstrated exceptional performance, they predominantly rely on instruments or printed circuit boards to fulfill the driving circuit function of the electro-optic modulator. Consequently, these systems tend to be complex with low integration levels. To address this challenge, we have introduced a modulator driver integrated circuit in 0.18 µm SiGe BiCMOS technology. The circuit can generate multiple-level driving signals with a clock frequency of 1.25 GHz and a rising edge of ∼50 ps. Each voltage amplitude can be independently adjusted, ensuring the precise preparation of quantum states. The measured signal-to-noise ratio was more than 17 dB, resulting in a low quantum bit error rate of 0.24% in our polarization-encoding system. This work will contribute to the advancement of QKD system integration and promote the industrialization process in this field.

Does the degree of polarization of vector beams remain unchanged on atmospheric propagation?

All roads lead to Rome. In this article we propose a novel theoretical framework to demonstrate vector beams whose degree of polarization does not change on atmospheric propagation. Inspired by the Fresnel equations, we derive the reflected and refracted field of vector beams propagating through a phase screen by employing the continuity of electromagnetic field. We generalize the conventional split-step beam propagation method by considering the vectorial properties in the vacuum diffraction and the refractive properties of a single phase screen. Based on this vectorial propagation model, we extensively calculate the change of degree of polarization (DOP) of vector beams under different beam parameters and turbulence parameters both in free-space and satellite-mediated links. Our result is that whatever in the free-space or satellite-mediated regime, the change of DOP mainly fluctuates around the order of 10-13 to 10-6, which is almost negligible.

Proof-of-Principle Demonstration of Fully Passive Quantum Key Distribution

Quantum key distribution (QKD) offers information-theoretic security based on the fundamental laws of physics. However, device imperfections, such as those in active modulators, may introduce side-channel leakage, thus compromising practical security. Attempts to remove active modulation, including passive decoy intensity preparation and polarization encoding, have faced theoretical constraints and inadequate security verification, thus hindering the achievement of a fully passive QKD scheme. Recent research [W. Wang et al., Phys. Rev. Lett. 130, 220801 (2023).PRLTAO0031-900710.1103/PhysRevLett.130.220801; 2V. Zapatero et al., Quantum Sci. Technol. 8, 025014 (2023).2058-956510.1088/2058-9565/acbc46] has systematically analyzed the security of a fully passive modulation protocol. Based on this, we utilize the gain-switching technique in combination with the postselection scheme and perform a proof-of-principle demonstration of a fully passive quantum key distribution with polarization encoding at channel losses of 7.2 dB, 11.6 dB, and 16.7 dB. Our work demonstrates the feasibility of active-modulation-free QKD in polarization-encoded systems.

Field test of quantum key distribution with high key creation efficiency

Quantum key distribution (QKD) promises unconditional security for communication. However, the random choices of the measurement basis in QKD usually result in low key creation efficiency. This drawback is overcome in the differential-phase-shift QKD, provided that each photon can be prepared in a large number of time slots with a proper waveform. In this work we develop a miniature room-temperature 1550-nm single-photon source to generate narrowband single photon in 50 time slots with a nearly optimal waveform for achieving unity key creation efficiency. By utilizing these single photons in the field test, we demonstrate the differential-phase-shift QKD with a key creation efficiency of 97%. Our work shows that the practical QKD can benefit from the narrowband single photons with controllable waveforms.

Measurement device-independent quantum key distribution with vector vortex modes under diverse weather conditions

Most quantum key distribution schemes exploiting orbital angular momentum-carrying optical beams are based on conventional set-ups, opening up the possibility of detector side-channel attacks. These optical beams also suffer from spatial aberrations due to atmospheric turbulence and unfavorable weather conditions. Consequently, we introduce a measurement device-independent quantum key distribution implemented with vector vortex modes. We study the transmission of vector vortex and scalar beams through a turbulent atmospheric link under diverse weather conditions such as rain or haze. We demonstrate that a maximum secure key transmission distance of 178 km can be achieved under clear conditions by utilizing the vector vortex beams, which have been mainly ignored in the literature. When raindrops have a diameter of 6 mm and fog particles have a radius of 0.5 [Formula: see text]m, the signals can reach 152 km and 160 km, respectively. Since these distances are comparable, this work sheds light into the feasibility of implementing measurement device-independent quantum key distribution using vector vortex modes under diverse weather conditions. Most significantly, this opens the door to practical secure quantum communications.

Asynchronous measurement-device-independent quantum key distribution with hybrid source

The linear constraint of secret key rate capacity is overcome by the twin-field quantum key distribution (QKD). However, the complex phase-locking and phase-tracking technique requirements throttle the real-life applications of the twin-field protocol. The asynchronous measurement-device-independent (AMDI) QKD, also called the mode-pairing QKD, protocol can relax the technical requirements and keep the similar performance of the twin-field protocol. Here, we propose an AMDI-QKD protocol with a nonclassical light source by changing the phase-randomized weak coherent state to a phase-randomized coherent-state superposition in the signal state time window. Simulation results show that our proposed hybrid source protocol significantly enhances the key rate of the AMDI-QKD protocol, while exhibiting robustness to imperfect modulation of nonclassical light sources.

Experimental Quantum Communication Overcomes the Rate-Loss Limit without Global Phase Tracking

Secure key rate (SKR) of point-point quantum key distribution (QKD) is fundamentally bounded by the rate-loss limit. Recent breakthrough of twin-field (TF) QKD can overcome this limit and enables long distance quantum communication, but its implementation necessitates complex global phase tracking and requires strong phase references that not only add to noise but also reduce the duty cycle for quantum transmission. Here, we resolve these shortcomings, and importantly achieve even higher SKRs than TF-QKD, via implementing an innovative but simpler measurement-device-independent QKD that realizes repeaterlike communication through asynchronous coincidence pairing. Over 413 and 508 km optical fibers, we achieve finite-size SKRs of 590.61 and 42.64  bit/s, which are respectively 1.80 and 4.08 times of their corresponding absolute rate limits. Significantly, the SKR at 306 km exceeds 5  kbit/s and meets the bitrate requirement for live one-time-pad encryption of voice communication. Our work will bring forward economical and efficient intercity quantum-secure networks.

Fully Passive Quantum Key Distribution

We propose a fully passive linear optical quantum key distribution (QKD) source that implements both random decoy-state and encoding choices with postselection only, thus eliminating all side channels in active modulators. Our source is general purpose and can be used in, e.g., BB84, the six-state protocol, and reference-frame-independent QKD. It can even potentially be combined with measurement-device-independent QKD to achieve robustness against side channels in both detectors and modulators. We also perform a proof-of-principle experimental source characterization to show its feasibility.

Forty thousand kilometers under quantum protection

Quantum key distribution (QKD) is a revolutionary cryptography response to the rapidly growing cyberattacks threat posed by quantum computing. Yet, the roadblock limiting the vast expanse of secure quantum communication is the exponential decay of the transmitted quantum signal with the distance. Today's quantum cryptography is trying to solve this problem by focusing on quantum repeaters. However, efficient and secure quantum repetition at sufficient distances is still far beyond modern technology. Here, we shift the paradigm and build the long-distance security of the QKD upon the quantum foundations of the Second Law of Thermodynamics and end-to-end physical oversight over the transmitted optical quantum states. Our approach enables us to realize quantum states' repetition by optical amplifiers keeping states' wave properties and phase coherence. The unprecedented secure distance range attainable through our approach opens the door for the development of scalable quantum-resistant communication networks of the future.

A Resource-Adaptive Routing Scheme with Wavelength Conflicts in Quantum Key Distribution Optical Networks

Quantum key distribution (QKD) has great potential in ensuring data security. Deploying QKD-related devices in existing optical fiber networks is a cost-effective way to practically implement QKD. However, QKD optical networks (QKDON) have a low quantum key generation rate and limited wavelength channels for data transmission. The simultaneous arrival of multiple QKD services may also lead to wavelength conflicts in QKDON. Therefore, we propose a resource-adaptive routing scheme (RAWC) with wavelength conflicts to achieve load balancing and efficient utilization of network resources. Focusing on the impact of link load and resource competition, this scheme dynamically adjusts the link weights and introduces the wavelength conflict degree. Simulation results indicate that the RAWC algorithm is an effective approach to solving the wavelength conflict problem. Compared with the benchmark algorithms, the RAWC algorithm can improve service request success rate (SR) by up to 30%.

Materials Innovations for Quantum Technology Acceleration: A Perspective

A broad perspective of quantum technology state of the art is provided and critical stumbling blocks for quantum technology development are identified. Innovations in demonstrating and understanding electron entanglement phenomena using bulk and low-dimensional materials and structures are summarized. Correlated photon-pair generation via processes such as nonlinear optics is discussed. Application of qubits to current and future high-impact quantum technology development is presented. Approaches for realizing unique qubit features for large-scale encrypted communication, sensing, computing, and other technologies are still evolving; thus, materials innovation is crucially important. A perspective on materials modeling approaches for quantum technology acceleration that incorporate physics-based AI/ML, integrated with quantum metrology is discussed.

Efficient room-temperature molecular single-photon sources for quantum key distribution

Quantum key distribution (QKD) allows the distribution of cryptographic keys between multiple users in an information-theoretic secure way, exploiting quantum physics. While current QKD systems are mainly based on attenuated laser pulses, deterministic single-photon sources could give concrete advantages in terms of secret key rate (SKR) and security owing to the negligible probability of multi-photon events. Here, we introduce and demonstrate a proof-of-concept QKD system exploiting a molecule-based single-photon source operating at room temperature and emitting at 785 nm. With an estimated maximum SKR of 0.5 Mbps, our solution paves the way for room-temperature single-photon sources for quantum communication protocols.

Airborne Quantum Key Distribution Performance Analysis under Supersonic Boundary Layer

Airborne quantum key distribution (QKD) that can synergize with terrestrial networks and quantum satellite nodes is expected to provide flexible and relay links for the large-scale integrated communication network. However, the photon transmission rate would be randomly reduced, owing to the random distributed boundary layer that surrounding to the surface of the aircraft when the flight speed larger than Mach 0.3. Here, we investigate the airborne QKD performance with the BL effects. Furthermore, we take experimental data of supersonic BL into the model and compare the airborne QKD performance under different conditions. Simulation results show that, owing to the complex small-scale turbulence structures in the supersonic boundary layer, the deflection angle and correspondingly drifted offset of the beam varied obviously and randomly, and the distribution probability of photons are redistributed. And the subsonic and supersonic boundary layer would decrease ~35.8% and ~62.5% of the secure key rate respectively. Our work provides a theoretical guidance towards a possible realization of high-speed airborne QKD.

Reference-frame-independent quantum key distribution with advantage distillation

Advantage distillation (AD) provides a means of separating highly correlated raw key bits from weakly correlated information in quantum key distribution (QKD). In this Letter, we apply the AD method to improve the performance of reference-frame-independent QKD (RFI-QKD). Simulation results show that, compared with RFI-QKD without AD, RFI-QKD with AD can tolerate higher system errors and obtain better performance on the secret key rate and transmission distance. Furthermore, we extend the AD method to RFI measurement-device-independent QKD (RFI-MDI-QKD) and demonstrate that the AD method can improve the performance of RFI-MDI-QKD more significantly.

Finite-Key Analysis for Quantum Key Distribution with Discrete-Phase Randomization

Quantum key distribution (QKD) allows two remote parties to share information-theoretic secret keys. Many QKD protocols assume the phase of encoding state can be continuous randomized from 0 to 2π, which, however, may be questionable in the experiment. This is particularly the case in the recently proposed twin-field (TF) QKD, which has received a lot of attention since it can increase the key rate significantly and even beat some theoretical rate-loss limits. As an intuitive solution, one may introduce discrete-phase randomization instead of continuous randomization. However, a security proof for a QKD protocol with discrete-phase randomization in the finite-key region is still missing. Here, we develop a technique based on conjugate measurement and quantum state distinguishment to analyze the security in this case. Our results show that TF-QKD with a reasonable number of discrete random phases, e.g., 8 phases from {0,π/4,π/2,…,7π/4}, can achieve satisfactory performance. On the other hand, we find the finite-size effects become more notable than before, which implies that more pulses should be emit in this case. More importantly, as a the first proof for TF-QKD with discrete-phase randomization in the finite-key region, our method is also applicable in other QKD protocols.

Experimental Mode-Pairing Measurement-Device-Independent Quantum Key Distribution without Global Phase Locking

In the past two decades, quantum key distribution networks based on telecom fibers have been implemented on metropolitan and intercity scales. One of the bottlenecks lies in the exponential decay of the key rate with respect to the transmission distance. Recently proposed schemes mainly focus on achieving longer distances by creating a long-arm single-photon interferometer over two communication parties. Despite their advantageous performance over long communication distances, the requirement of phase locking between two remote lasers is technically challenging. By adopting the recently proposed mode-pairing idea, we realize high-performance quantum key distribution without global phase locking. Using two independent off-the-shelf lasers, we show a quadratic key-rate improvement over the conventional measurement-device-independent schemes in the regime of metropolitan and intercity distances. For longer distances, we also boost the key rate performance by 3 orders of magnitude via 304 km commercial fiber and 407 km ultralow-loss fiber. We expect this ready-to-implement high-performance scheme to be widely used in future intercity quantum communication networks.

Cosmic coding and transfer for ultra high security near-field communications

By using true random number (TRN) generators, encoding with the highest security can be realized. However, a completely secure strategy to transfer these TRNs has not yet been devised. Quantum key distribution (QKD) has attempted to establish secure key distribution methodology of this kind; however, several quantum cracking strategies have been predicted and experimentally demonstrated. In this work, COSMOCAT was invented as a solution for next-generation ultrahigh security near-field communications. With COSMOCAT, TRNs are generated from naturally occurring and ubiquitous cosmic-ray muons and the generated cosmic keys are distributed by these muons with an unprecedented level of security. The successful results of this experiment indicate that our prototype and the new key-generation-and-distribution standard can be utilized for practical encoding and near-field data transfer at rates of 10-100 Mbps. It is anticipated that COSMOCAT will be one of key techniques for future high security, near-field communication management.

Encryption chain based on measurement result and its applications on semi-quantum key distribution protocol

This study proposes a new encoding method, also known as an encryption chain based on the measurement result. Then, using the encryption chain to propose a unitary-operation-based semi-quantum key distribution protocol (SQKD) protocol. In the existing SQKD protocols, semi-quantum environments adopt a round-trip transmission strategy. In round-trip transmission, the classical participant must resend the received photons to the quantum participant after implementing local operations. Therefore, round-trip transmissions are vulnerable to Trojan horse attacks. Hence, the classical participant must be equipped with a photon number splitter and an optical wavelength filter device against Trojan horse attacks. This is illogical for semi-quantum environments because the burden on the classical participant is significantly increased as it involves the prevention of Trojan horse attacks. The proposed SQKD protocol is congenitally immune to Trojan horse attacks and involves no extra hardware because it is designed based on a one-way transmission as opposed to a round-trip transmission. When compared to the existing SQKD protocols, the proposed SQKD protocol provides the best qubit efficiency, and classical participants only require two quantum capabilities, which enhance its practicability. Moreover, the proposed SQKD protocol is free from collective attacks, Trojan horse attacks, and intercept-resend attacks. Thus, the proposed scheme is more efficient and practical than the existing SQKD protocols.

More optimal relativistic quantum key distribution

A great challenge in the field of quantum cryptography is the design and implementation of optimal quantum key distribution (QKD) scheme. An optimal scheme in terms of security is the so-called relativistic quantum key distribution; it ensures the security of the system by using both quantum phenomena and relativity. However, the existing relativistic schemes have not demonstrated optimality in terms of efficiency and rate (including secret key rate). Here we report two point-to-point relativistic quantum key distribution schemes implemented with weak coherent pulses. Both schemes rely on high-dimensional quantum systems (phase and polarization encodings are utilized for establishing key bits). One of the proposed schemes is a system comprised of two sequentially connected interferometers, as the first (interferometer) controls the behavior of the second one. The other proposed scheme represents a setup of a classic relativistic QKD, but with slight modification. Both of the proposed schemes are characterized with high secret key rate. The latter scheme has the highest secret key rate of all the relativistic QKD protocols. However, the values for the secret key rate are relevant for distances of up to 150 km. The former scheme has lower secret key rate, but longer operating distances (the work could operate at distances of up to 320 km). Those values of rate are obtained without disturbing the security. Secret-key-rate comparison between distinct models is reported. The proposed relativistic models are compared to twin-field QKD protocols. Furthermore, the work proposes a metric for evaluating the optimality of a QKD. It is defined as a ratio between the secret key rate (at a given distance) and the amount of quantum resources (qubits) used in the QKD of concern. It is shown that one of the proposed schemes in this article is the most optimal relativistic key distribution and more optimal than the original twin-field. It is also verified that the proposed schemes excels the original twin-field in terms of secret key rate, but for short distances.

Post-matching quantum conference key agreement

Twin-field interference-based quantum conference key agreement protocols have been proposed and have achieved good performance in terms of the key rate and transmission distance in the finite-key regime. However, its performance significantly decreases when the strict constraint is broken regarding the optical pulse intensity and probability. Here, we propose a post-matching QCKA protocol to remove this constraint while obtaining a higher key rate. Numerical results in the symmetric case show that our protocol can obtain a transmission distance 25% more than the previous asymmetric QCKA protocol when the decoy state optical pulse intensity is 1% higher than the ideal value of the constraint, and can obtain a transmission distance 100% higher when the decoy state optical pulse intensity is 10% higher than the ideal value of the constraint.

Quantum interference of identical photons from remote GaAs quantum dots

Photonic quantum technology provides a viable route to quantum communication^1,2, quantum simulation³ and quantum information processing⁴. Recent progress has seen the realization of boson sampling using 20 single photons³ and quantum key distribution over hundreds of kilometres². Scaling the complexity requires architectures containing multiple photon sources, photon counters and a large number of indistinguishable single photons. Semiconductor quantum dots are bright and fast sources of coherent single photons^5-9. For applications, a roadblock is the poor quantum coherence on interfering single photons created by independent quantum dots^10,11. Here we demonstrate two-photon interference with near-unity visibility (93.0 ± 0.8)% using photons from two completely separate GaAs quantum dots. The experiment retains all the emission into the zero phonon line-only the weak phonon sideband is rejected; temporal post-selection is not employed. By exploiting quantum interference, we demonstrate a photonic controlled-not circuit and an entanglement with fidelity of (85.0 ± 1.0)% between photons of different origins. The two-photon interference visibility is high enough that the entanglement fidelity is well above the classical threshold. The high mutual coherence of the photons stems from high-quality materials, diode structure and relatively large quantum dot size. Our results establish a platform-GaAs quantum dots-for creating coherent single photons in a scalable way.

Afterpulse effect in measurement-device-independent quantum key distribution

There is no doubt that measurement-device-independent quantum key distribution (MDI-QKD) is a crucial protocol that is immune to all possible detector side channel attacks. In the preparation phase, a simulation model is usually employed to get a set of optimized parameters, which is utilized for getting a higher secure key rate in reality. With the implementation of high-speed QKD, the afterpulse effect which is an intrinsic characteristic of the single-photon avalanche photodiode is no longer ignorable, this will lead to a great deviation compared with the existing analytical model. Here we develop an afterpulse-compatible MDI-QKD model to get the optimized parameters. Our results indicate that by using our afterpulse-compatible model, we can get a much higher key rate than the prior afterpulse-omitted model. It is significant to take the afterpulse effect into consideration because of the improvement of the system working frequency.

Simple security proof of coherent-one-way quantum key distribution

Coherent-one-way quantum key distribution (COW-QKD), which requires a simple experimental setup and has the ability to withstand photon-number-splitting attacks, has been not only experimentally implemented but also commercially applied. However, recent studies have shown that the current COW-QKD system is insecure and can only distribute secret keys safely within 20 km of the optical fiber length. In this study, we propose a practical implementation of COW-QKD by adding a two-pulse vacuum state as a new decoy sequence. This proposal maintains the original experimental setup as well as the simplicity of its implementation. Utilizing detailed observations on the monitoring line to provide an analytical upper bound on the phase error rate, we provide a high-performance COW-QKD asymptotically secure against coherent attacks. This ensures the availability of COW-QKD within 100 km and establishes theoretical foundations for further applications.

Multi-User Measurement-Device-Independent Quantum Key Distribution Based on GHZ Entangled State

As a multi-particle entangled state, the Greenberger-Horne-Zeilinger (GHZ) state plays an important role in quantum theory and applications. In this study, we propose a flexible multi-user measurement-device-independent quantum key distribution (MDI-QKD) scheme based on a GHZ entangled state. Our scheme can distribute quantum keys among multiple users while being resistant to detection attacks. Our simulation results show that the secure distance between each user and the measurement device can reach more than 280 km while reducing the complexity of the quantum network. Additionally, we propose a method to expand our scheme to a multi-node with multi-user network, which can further enhance the communication distance between the users at different nodes.

Practical underwater quantum key distribution based on decoy-state BB84 protocol

Polarization encoding quantum key distribution has been proven to be a reliable method to build a secure communication system. It has already been used in an inter-city fiber channel and near-Earth atmosphere channel, leaving an underwater channel the last barrier to conquer. Here we demonstrate a decoy-state BB84 quantum key distribution system over a water channel with a compact system design for future experiments in the ocean. In the system, a multiple-intensity modulated laser module is designed to produce the light pulses of quantum states, including signal state, decoy state, and vacuum state. Classical communication and synchronization are realized by wireless optical transmission. Multiple filtering techniques and wavelength division multiplexing are further used to avoid cross talk of different lights. We test the performance of the system and obtain a final key rate of 245.6 bps with an average quantum bit error rate of 1.91% over a 2.4 m water channel, in which the channel attenuation is 16.35 dB. Numerical simulation shows that the system can tolerate up to 21.7 dB total channel loss and can still generate secure keys in 277.9 m Jerlov type I ocean channel.

Weak Randomness Analysis of Measurement-Device-Independent Quantum Key Distribution with Finite Resources

The ideal quantum key distribution (QKD) protocol requires perfect random numbers for bit encoding and basis selecting. Perfect randomness is of great significance to the practical QKD system. However, due to the imperfection of practical quantum devices, an eavesdropper (Eve) may acquire some random numbers, thus affecting the security of practical systems. In this paper, we analyze the effects of the weak randomness in the measurement-device-independent QKD (MDI-QKD) with finite resources. We analytically derive concise formulas for estimating the lower bound of the single-photon yield and the upper bound of the phase error rate in the case of the weak randomness. The simulation demonstrates that the final secret key rate of MDI-QKD with finite resources is sensitive to state preparation, even with a small proportion of weak randomness, the secure key rate has a noticeable fluctuation. Therefore, the weak randomness of the state preparation may bring additional security risks. In order to ensure the practical security of the QKD system, we are supposed to strengthen the protection of state preparation devices.

Experimental Side-Channel-Secure Quantum Key Distribution

Quantum key distribution can provide unconditionally secure key exchange for remote users in theory. In practice, however, in most quantum key distribution systems, quantum hackers might steal the secure keys by observing the side channels in the emitted photons, such as the photon frequency spectrum, emission time, propagation direction, spatial angular momentum, and so on. It is hard to prevent such kinds of attacks because side channels may exist in many dimensions of the emitted photons. Here we report an experimental realization of a side-channel-secure quantum key distribution protocol which is not only measurement-device independent, but also immune to all side-channel attacks to the photons emitted from Alice's and Bob's labs. We achieve a secure key rate of 1.73×10^{-6} per pulse through 50 km fiber spools.

Finite-Key Analysis of 1-Decoy Method Quantum Key Distribution with Intensity Fluctuation

The decoy state quantum key distribution (QKD) protocol is proven to be an effective strategy against the photon number splitting attack. It was shown that the 1-decoy state protocol, easier to implement in the practical QKD system, outperforms the 2-decoy state protocol for block sizes of up to 108 bits. How intensity fluctuations influence the performance of the 1-decoy state protocol with finite resources remains a pending issue. In this paper, we present a finite-key analysis of the 1-decoy state protocol with intensity fluctuations and obtain the secret key rate formula about intensity fluctuations. Our numerical simulation results show that the stronger the intensity fluctuations, the lower the secret key rate for a small data block size of a few bits. Our research can provide theoretical implications for the selection of data size in the QKD system with intensity fluctuations.

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  \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1.5\%$$\end{document}1.5% over 40 km of optical fiber. This considering the error correction and privacy amplification steps necessary to obtain a final secure key.

Four-intensity measurement-device-independent quantum key distribution protocol with modified coherent state sources

We propose a scheme of double-scanning 4-intensity MDI-QKD protocol with the modified coherent state (MCS) sources. The MCS sources can be characterized by two positive parameters, ξ and c. In all prior works, c was set to be the same for all sources. We show that the source parameter c can be different for the sources in the X basis and those in the Z basis. Numerical results show that removing such a constraint can greatly improve the key rates of the protocol with MCS sources. In the typical experiment conditions, comparing with the key rates of WCS sources, the key rates of MCS sources can be improved by several orders of magnitude, and the secure distance is improved by about 40 km. Our results show that MCS sources have the potential to improve the practicality of the MDI-QKD protocol.

Experimental demonstration of free-space two-photon interference

Quantum interference plays an essential role in understanding the concepts of quantum physics. Moreover, the interference of photons is indispensable for large-scale quantum information processing. With the development of quantum networks, interference of photons transmitted through long-distance fiber channels has been widely implemented. However, quantum interference of photons using free-space channels is still scarce, mainly due to atmospheric turbulence. Here, we report an experimental demonstration of Hong-Ou-Mandel interference with photons transmitted by free-space channels. Two typical photon sources, i.e., correlated photon pairs generated in spontaneous parametric down conversion (SPDC) process and weak coherent states, are employed. A visibility of 0.744 ± 0.013 is observed by interfering with two photons generated in the SPDC process, exceeding the classical limit of 0.5. Our results demonstrate that the quantum property of photons remains even after transmission through unstable free-space channels, indicating the feasibility and potential application of free-space-based quantum interference in quantum information processing.

Optical injection locking based local oscillator regeneration for continuous variable quantum key distribution

We develop an optical injection locking (OIL) based local oscillator (LO) regeneration for continuous variable quantum key distribution (CVQKD) by sending a weak polarization multiplexed pilot carrier from the transmitter. The OIL at the receiver has superior performance in terms of minimum input power and noise level at offset frequencies to the erbium-doped fiber amplifier (EDFA)-based scheme. The weak pilot carrier is recovered both in power and phase via the OIL while incurring little excess noise to the CVQKD system. The phase-locked LO enables heterodyne detection of a Gaussian modulated quantum signal with a simple data-aided phase recovery without pilot tone. The obtained parameters are compatible with a raw key rate of 0.83 Mbit/s in the asymptotic regime over a 22-km fiber transmission. The technique is expected to be used in more phase-sensitive quantum optical applications.

Prefixed-Threshold Real-Time Selection for Free-Space Sending-or-Not Twin-Field Quantum Key Distribution

As a variant of the twin-field quantum key distribution (TF-QKD), the sending-or-not twin-field quantum key distribution (SNS TF-QKD) is famous for its higher tolerance of misalignment error, in addition to the capacity of surpassing the rate–distance limit. Importantly, the free-space SNS TF-QKD will guarantee the security of the communications between mobile parties. In the paper, we first discuss the influence of atmospheric turbulence (AT) on the channel transmittance characterized by the probability distribution of the transmission coefficient (PDTC). Then, we present a method called prefixed-threshold real-time selection (P-RTS) to mitigate the interference of AT on the free-space SNS TF-QKD. The simulations of the free-space SNS TF-QKD with and without P-RTS are both given for comparison. The results showed that it is possible to share the secure key by using the free-space SNS TF-QKD. Simultaneously, the P-RTS method can make the free-space SNS TF-QKD achieve better and more stable performance at a short distance.

Nanomaterials for Quantum Information Science and Engineering

Quantum information science and engineering (QISE)-which entails the use of quantum mechanical states for information processing, communications, and sensing-and the area of nanoscience and nanotechnology have dominated condensed matter physics and materials science research in the 21st century. Solid-state devices for QISE have, to this point, predominantly been designed with bulk materials as their constituents. This review considers how nanomaterials (i.e., materials with intrinsic quantum confinement) may offer inherent advantages over conventional materials for QISE. The materials challenges for specific types of qubits, along with how emerging nanomaterials may overcome these challenges, are identified. Challenges for and progress toward nanomaterials-based quantum devices are condidered. The overall aim of the review is to help close the gap between the nanotechnology and quantum information communities and inspire research that will lead to next-generation quantum devices for scalable and practical quantum applications.

Measurement-device-independent quantum key distribution with insecure sources

Measurement-device-independent quantum key distribution (MDI-QKD) can remove all detection side channels but still makes additional assumptions on sources that can be compromised through uncharacterized side channels in practice. Here, we combine a recently proposed reference technique to prove the security of MDI-QKD against possible source imperfections and/or side channels. This requires some reference states and an upper bound on the parameter that describes the quality of the sources. With this formalism we investigate the asymptotic performance of single-photon sources, and the results show that the side channels have a great impact on the key rates.

Integrated Fabry-Perot filter with wideband noise suppression for satellite-based daytime quantum key distribution

Spectral filtering is essential in daytime quantum key distribution (QKD), which can suppress the strong background noise caused by scattered solar irradiation. An integrated Fabry-Perot filter is implemented based on a scheme that combines a Fabry-Perot etalon and a dense-wavelength-division-multiplex filter for narrow linewidth filtering and broad-spectrum noise suppression, respectively. This filter is integrated into a butterfly package with single-mode fibers for optical input and output, thereby enhancing high robustness and ease of use. The measurement results show that the filter has a linewidth of 25.6 pm, a noise suppression of over 44.7 dB ranging between 1380-1760 nm, an optical efficiency of 74.5% with variation less than 0.9% in 120 min, and a polarization fidelity after compensation exceeding 99.9%. The ability of fine-tuning the central wavelength with 9.5 pm/°C makes it very suitable for satellite-based applications under the Doppler effect. Further analysis is also given to demonstrate the prospects of applying this filter in future satellite-based daytime QKD applications.

Single-shot analysis of amplified correlated light

Correlated beams are important in classical and quantum communication as well as other technologies. However, classical amplifiers, which are essential for long transmission of correlated beams, degrade the correlation due to noise and due to the amplifier spectral response. We measure, with a novel high resolution single-shot measurement system, the impact of amplifiers on correlated beams. We develop a new method for analyzing the correlation between the signal and idler beams by choosing peaks in the pulses according to their power levels. We demonstrate how to tailor the correlation after the amplifier to obtain either higher or lower correlation. Our research may influence the future use of amplifiers in non-classical communication systems as well as the transmission of quantum information over long distances.

Vulnerability of Satellite Quantum Key Distribution to Disruption from Ground-Based Lasers

Satellite-mediated quantum key distribution (QKD) is set to become a critical technology for quantum-secure communication over long distances. While satellite QKD cannot be effectively eavesdropped, we show it can be disrupted (or ‘jammed’) with relatively simple and readily available equipment. We developed an atmospheric attenuation and satellite optical scattering model to estimate the rate of excess noise photons that can be injected into a satellite QKD channel by an off-axis laser, and calculated the effect this added noise has on the quantum bit error rate. We show that a ground-based laser on the order of 1 kW can significantly disrupt modern satellite QKD systems due to photons scattering off the satellite being detected by the QKD receiver on the ground. This class of laser can be purchased commercially, meaning such a method of disruption could be a serious threat to effectively securing high-value communications via satellite QKD in the future. We also discuss these results in relation to likely future developments in satellite-mediated QKD systems, and countermeasures that can be taken against this, and related methods, of disruption.

Long-distance twin-field quantum key distribution with entangled sources

Twin-field quantum key distribution (TFQKD), using single-photon-type interference, offers a way to exceed the rate-distance limit without quantum repeaters. However, it still suffers from photon losses and dark counts, which impose an ultimate limit on its transmission distance. In this Letter, we propose a scheme to implement TFQKD with an entangled coherent state source in the middle to increase its range, as well as comparing its performance under coherent attacks with that of TFQKD variants. Simulations show that our protocol has a theoretical distance advantage of 400 km. Moreover, the scheme has great robustness against the misalignment error and finite-size effects. Our work is a promising step toward long-distance secure communication and is greatly compatible with future global quantum networks.

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.

Free Space Measurement Device Independent Quantum Key Distribution with Modulating Retro-Reflectors under Correlated Turbulent Channel

Modulating retro-reflector (MRR), originally introduced to support laser communication, relieves most of the weight, power, and pointing requirements to the ground station. In this paper, a plug-and-play measurement device independent quantum key distribution (MDI-QKD) scheme with MRR is proposed not only to eliminate detector side channels and allow an untrusted satellite relay between two users, but also to simplify the requirements set-ups in practical flexible moving scenarios. The plug-and-play architecture compensates for the polarization drift during the transmission to provide superior performance in implementing the MDI-QKD on a free-space channel, and the MRR device is adopted to relax the requirements on both communication terminals. A double-pass correlated turbulent channel model is presented to investigate the complex and unstable channel characteristics caused by the atmospheric turbulence. Furthermore, the security of the modified MDI-QKD scheme is analyzed under some classical attacks and the simulation results indicate the feasibility under the situation that the system performance deteriorates with the increase of fading correlation coefficient and the turbulence intensity, which provides a meaningful step towards an MDI-QKD based on the moving platforms to join a dynamic quantum network with untrusted relays.

Secure quantum secret sharing without signal disturbance monitoring

Quantum secret sharing (QSS) is an essential primitive for the future quantum internet, which promises secure multiparty communication. However, developing a large-scale QSS network is a huge challenge due to the channel loss and the requirement of multiphoton interference or high-fidelity multipartite entanglement distribution. Here, we propose a three-user QSS protocol without monitoring signal disturbance, which is capable of ensuring the unconditional security. The final key rate of our protocol can be demonstrated to break the Pirandola-Laurenza-Ottaviani-Banchi bound of quantum channel and its simulated transmission distance can approach over 600 km using current techniques. Our results pave the way to realizing high-rate and large-scale QSS networks.

Synchronization using quantum photons for satellite-to-ground quantum key distribution

Time synchronization is crucial for quantum key distribution (QKD) systems. In order to compensate for the time drift caused by the Doppler effect and adapt to the unstable optical link in satellite-to-ground QKD, previous demonstrations generally adopted synchronization methods requiring additional hardware. In this paper, we present a novel synchronization method based on the detected quantum photons, thus simplifying additional hardware and reducing the complexity and cost. This method adopts target frequency scanning to realize fast frequency recovery, utilizes polynomial fitting to compensate for the Doppler effect, and takes use of the vacuum state in the decoy-state BB84 protocol to recover the time offset. This method can avoid the influence of synchronization light jitter, thus improving the synchronization precision and the secure keys as well. Successful satellite-to-ground QKD based on this new synchronization scheme has been conducted to demonstrate its feasibility and performance. The presented scheme provides an effective synchronization solution for quantum communication applications.

Improved analysis of measurement-device-independent quantum key distribution with non-phase-randomized coherent states

Measurement-device-independent quantum key distribution (MDI-QKD) can remove all detector side-channel attacks, which can be implemented with phase-randomized coherent states (PRCS) or non-phase-randomized coherent states (NPRCS). In this paper, we focus on the MDI-QKD protocol with NPRCS and provide an improved analysis. In contrast with the original MDI-QKD with NPRCS which modulates the same intensity of coherent states in the key and test bases, we propose to modulate different intensities of coherent states in the key and test bases. Simulation results show that the secret key rate and transmission distance of MDI-QKD with NPRCS can be significantly improved. Furthermore, it is noteworthy that the modulation of different intensities does not bring extra complexity for experimental researchers, which can be easily done by adding an intensity modulator.

Pathways for Entanglement-Based Quantum Communication in the Face of High Noise

Entanglement-based quantum communication offers an increased level of security in practical secret shared key distribution. One of the fundamental principles enabling this security-the fact that interfering with one photon will destroy entanglement and thus be detectable-is also the greatest obstacle. Random encounters of traveling photons, losses, and technical imperfections make noise an inevitable part of any quantum communication scheme, severely limiting distance, key rate, and environmental conditions in which quantum key distribution can be employed. Using photons entangled in their spatial degree of freedom, we show that the increased noise resistance of high-dimensional entanglement can indeed be harnessed for practical key distribution schemes. We perform quantum key distribution in eight entangled paths at various levels of environmental noise and show key rates that, even after error correction and privacy amplification, still exceed 1 bit per photon pair and furthermore certify a secure key at noise levels that would prohibit comparable qubit based schemes from working.

The Performance of Satellite-Based Links for Measurement-Device-Independent Quantum Key Distribution

Measurement-device-independent quantum key distribution (MDI-QKD) protocol has high practical value. Satellite-based links are useful to build long-distance quantum communication network. The model of satellite-based links for MDI-QKD was proposed but it lacks practicality. This work further analyzes the performance of it. First, MDI-QKD and satellite-based links model are introduced. Then considering the operation of the satellite the performance of their combination is studied under different weather conditions. The results may provide important references for combination of optical-fiber-based links on the ground and satellite-based links in space, which is helpful for large-scale quantum communication network.

Experimental measurement-device-independent quantum key distribution with the double-scanning method

The measurement-device-independent quantum key distribution (MDI-QKD) can be immune to all detector side-channel attacks. Moreover, it can be easily implemented combining with the matured decoy-state methods under current technology. It, thus, seems a very promising candidate in practical implementation of quantum communications. However, it suffers from a severe finite-data-size effect in most existing MDI-QKD protocols, resulting in relatively low key rates. Recently, Jiang et al. [Phys. Rev. A103, 012402 (2021).PLRAAN1050-294710.1103/PhysRevA.103.012402] proposed a double-scanning method to drastically increase the key rate of MDI-QKD. Based on Jiang et al.'s theoretical work, here we for the first time, to the best of our knowledge, implement the double-scanning method into MDI-QKD and carry out corresponding experimental demonstration. With a moderate number of pulses of 10¹⁰, we can achieve 150 km secure transmission distance, which is impossible with all former methods. Therefore, our present work paves the way toward practical implementation of MDI-QKD.