Sagnac-loop integrated quantum key distribution with weak measurement enhanced fiber-optic sensing for disturbance magnitude and location

A multi-parameter collaborative optimization method for continuous-variable quantum key distribution security level incorporating physical prior knowledge

We present an experimentally feasible implementation of a secure multiparty computation application enabled by quantum oblivious transfer (QOT) on an entanglement-based physical layer. The QOT protocol uses polarization-encoded entangled states to share oblivious keys between two parties with quantum key distribution (QKD) providing authentication. Our system integrates the post-processing for QOT and QKD, both sharing a single physical layer, ensuring efficient key generation and authentication, respectively. Authentication involves hashing messages into a cryptographic context, verifying tags, and replenishing keys. This process uses a parallel QKD pipeline specifically for authentication, not for secure key generation. Oblivious keys are generated over a distance up to 25.8 km with a channel loss of 8.47 dB. In a back-to-back setup, a QOT rate of [Formula: see text] OTs/second is achieved, corresponding to 1 minute and 53 seconds per OT, primarily limited by the entanglement source. Using pre-distributed oblivious keys improved the rate to 0.11 OTs/second, or 9.1 seconds per OT. The considered QOT protocol is statistically correct, computationally secure for an honest receiver, and statistically secure for an honest sender, assuming a computationally hiding, statistically binding commitment. An experimentally feasible use case is demonstrated for privacy-preserving fingerprint matching against no-fly lists for border control. The fingerprint is secret-shared across two sites, ensuring security, while the matching is performed using the MASCOT protocol, supported by QOT. The application required 128 1-out-of-2 OTs, each with message length of 128 bits, with the highest security achieved in 20 minutes and 39 seconds. This work demonstrates the feasibility of QOT in secure quantum communication applications.

Abstract We investigate the use of twin optical fingerprints derived from correlated physical unclonable functions (PUFs), as a hardware-based platform for cryptographic key generation and distribution. Each fingerprint is associated with a random, yet reproducible speckle pattern, generated when coherent light is scattered by a disordered optical structure. We consider a pair of correlated optical PUFs, and study the conditions under which two honest parties can establish a common secret key, despite fabrication-induced variability and environmental noise. An explicit information-theoretic key-agreement protocol is developed, incorporating secure sketches, error reconciliation, and privacy amplification. We quantify information leakage due to public helper data, and derive lower bounds on the length of the final secret key. The analysis identifies the noise regimes in which secure key agreement is feasible, and examines the performance of both practical and near-capacity reconciliation schemes. Finally, we discuss how twin optical PUFs could be integrated into quantum key distribution (QKD) networks, as a mechanism for establishing an initial pre-shared secret key between two honest users, without relying on computational assumptions or trusted third parties.

Security of device-independent quantum key distribution under sequential attack

Noise-robust O-band quantum key distribution at 1295.56 nm for coexistence in WDM networks

Structural optimization of SOI buried rectangular and plasmonic waveguide for quantum key distribution application

Fully integrated photonic continuous-variable quantum key distribution is a promising route toward compact and scalable secure optical links, yet the achievable transmission distance has been limited by the performance of integrated receiver chips and excess noise suppression methods. Here, we demonstrate a Gaussian-modulated continuous-variable quantum key distribution system with an integrated silicon photonic receiver, operating over 60 km. Enabled by a high-clearance, broadband silicon photonic receiver, in conjunction with a robust digital signal processing framework that utilizes a time-domain superposition algorithm and secure dynamic single-tap equalization, the system effectively achieves an asymptotic secret key rate of 1.68 Mbps, and a finite-size secret key rate of 0.80 Mbps at a data block length of 1.55×109. This work extends chip-based continuous-variable quantum key distribution to metropolitan-scale distances, confirming the viability of integrated receivers for large-scale quantum networks.

Continuous-variable (CV) quantum key distribution (QKD) allows for quantum secure communication with the benefit of being close to classical coherent communication. In recent years, CV QKD protocols using a discrete number of displaced coherent states have been studied intensively as the modulation can be directly implemented with real devices with finite resolution. Until now, experiments only calculated key rates in the asymptotic regime. Here, we present a CV QKD system using discrete modulation that is especially designed for atmospheric channels. We use polarization encoding to exploit the nonbirefringent nature of the turbulent atmosphere. This allows to expand CV QKD networks beyond the existing fiber backbone. In a laboratory demonstration with a static 3-decibel loss channel, we implemented a recently developed security proof allowing to calculate composable finite-size key rates against independently and identically distributed collective attacks. We applied the full QKD protocol including a quantum random number generator, error correction, and privacy amplification to extract secret keys.

Polarization instability is a critical challenge for polarization-entangled satellite quantum key distribution (QKD), where atmospheric effects and platform motion continuously distort photon polarization. To maintain entanglement fidelity, these transformations must be accurately identified and compensated prior to detection. In this work, a compact and fast polarization-compensation approach based on liquid-crystal (LC) variable retarders is presented, using a co-propagating classical reference signal (beacon) for real-time polarization tracking. An LC-based polarimeter is implemented, and its performance is evaluated using both direct and Fourier-based Stokes parameter reconstruction. Experimental results indicate that accurate polarization estimation can be achieved with a limited number of measurements, enabling a favorable trade-off between speed and precision. The impact of liquid-crystal switching dynamics is also analyzed, highlighting the importance of selecting appropriate operating conditions for real-time applications. In addition, the effect of polarimetric inaccuracies on QKD performance is assessed through simulations of an entanglement-based protocol. The results show that only a moderate increase in quantum-bit error rate is introduced, while remaining compatible with secure key distribution. These findings demonstrate that LC-based polarization control represents an efficient and practical solution for real-time compensation in satellite QKD systems.

Security boundaries of two-way continuous-variable quantum key distribution under multiple source imperfections

This study theoretically investigates the optimal filter configuration for separating a quantum key distribution (QKD) channel from classical C-band communication channels in a discrete-variable (DV) QKD-over-dense wavelength division multiplexing (DWDM) communication system with an O-band QKD channel. Three filter configurations are analyzed and compared in terms of quantum bit error rate (QBER), secret key rate (SKR), and fiber link distance: two cascaded 1310-nm bandpass filters, a 1310-nm uniform fiber Bragg grating (FBG) with a 1310-nm circulator, and a 1310-nm apodized FBG with a 1310-nm circulator. The findings reveal that the optical filter configuration incorporating a Gaussian apodized FBG exhibits the best performance with an isolation of ~ 93 dB between 1310nm and 1550nm wavelengths. This optimal performance can be attributed to the Gaussian apodization profile, which effectively suppresses spectral side-lobes to enhance channel isolation while maintaining a lower insertion loss compared to the cascaded bandpass filter configuration. For 50 classical WDM channels, the QKD fiber link distance is 35.4km with a Gaussian apodized FBG, while a configuration incorporating two cascaded 1310-nm bandpass filters yields only 18.3km. Finally, the impact of adopting a raised-cosine apodized FBG to further increase filter channel isolation is examined. Despite a substantial increase in channel isolation by using a raised-cosine apodized FBG filter, QKD fiber link distance yields negligible improvement for this system with a QKD channel in O-band, although this filter may be highly useful for QKD channels in C- or L-bands.

QuCloud : Enhancing cloud storage security by combining quantum key distribution, post-quantum cryptography, and custom proxy re-encryption

Continuous-variable quantum key distribution (CV-QKD) suffers from significant secret key rate (SKR) limitations in long-haul transmission. Existing hardware-oriented capacity boosting approaches are hindered by prohibitive costs and intricate system architectures. To address these challenges, this paper proposes a time-domain unitary precoding scheme to enhance the SKR by making full use of the non-flat noise frequency spectrum inherent to practical CV-QKD systems. This paper derives an analytical expression for the optimal precoder when only excess or electronic noise is colored and employs the Riemann Conjugate Gradient (RCG) algorithm for iterative optimization when both types of noise are colored. Numerical simulations demonstrate that, for a 40 km transmission link with both colored excess noise and electronic noise, the optimized scheme attains a 1.07 to 5.6 fold SKR improvement compared to conventional CV-QKD systems.

Four-state continuous variable quantum key distribution with source leakage

Quantum key distribution using hBN single-photon emitters at a 40 MHz clock rate

Quantum key distribution (QKD) is a promising method for sharing an information-theoretically secure secret key using the properties of quantum mechanics. Although QKD is the most practical field in quantum information science, its widespread adoption is constrained by high implementation costs. Despite the cost-effectiveness of propagating QKD with classical communication using wavelength division multiplexing (WDM), the noise generated by relatively strong classical signals degrades QKD performance. Even though several mitigation strategies exist, they often involve complexity or additional hardware. In this paper, we apply advantage distillation (AD) to enhance the performance of a practical decoy-state BB84 QKD system operating in a WDM environment. We perform finite-key analysis in both co-propagation and counter-propagation scenarios with three classical channels, and furthermore we examine the effects of varying the number of classical channels and the optical power per channel. A comparison of simulation results obtained with and without AD demonstrates that AD is robust to noise originating from classical channels. These results show that AD increases the secure key rate and transmission distance in the WDM-based QKD environment under both co-propagation and counter-propagation without requiring hardware changes. These findings suggest advantage distillation is a promising strategy for the widespread adoption of WDM-based QKD, including in challenging counter-propagation cases.

Most quantum key distribution (QKD) silicon photonic chips rely on carrier depletion modulators (CDM), which typically require driving voltages beyond 5 V due to their limited modulation efficiency. In this work, we employ a carrier injection modulator (CIM) for quantum bit encoding, which offers a fundamentally higher modulation efficiency and thus a path to significantly lower operating voltages. Through structural optimization, the CIM achieves a half-wave voltage of 1.1 V (DC) and 1.28 V (at 100 MHz), with a modulation depth exceeding 24 dB under both conditions. Successful demonstrations of intensity state preparation as well as polarization state modulation were also conducted. These results confirm the suitability of the CIM for compact and energy-efficient QKD transmitters operating at CMOS-compatible voltages.

Time-bin encoded quantum states of light are crucial for quantum technology applications. The integration of manipulation functionalities into chip-scale devices is essential for deploying scalable, high-performace, and cost-effective quantum networks. Here we develop a fully integrated, high-throughput quantum receiver based on the thin-film lithium niobate (TFLN) platform, capable of high-speed electro-optic manipulation of time-bin encoded quantum states. The device's novel architecture enables active switching of time-bin quantum states with an electro-optic bandwidth exceeding 30 GHz, while supporting real-time arbitrary projective measurements with a bandwidth of over 1 GHz. We showcase its versatility and performance through several applications, including the certification of entanglement with Bell's inequality violation by 38 standard deviations and with >95% visibility. We then apply it to a fiber-based quantum communication scenario, where we experimentally demonstrate an entanglement-based quantum key distribution (QKD) protocol, achieving stable finite-size secure key rates exceeding 25 kbit/s over 12 h of continuous operation. By leveraging a high-speed active switching scheme, the system overcomes the need for temporal post-selection, eliminating a fundamental loophole that compromises the security of time-bin entanglement-based QKD protocols and relaxes the temporal resolution requirements of single-photon detectors. Moreover, it enables active selection of the projection basis, increasing the flexibility for communication parties. This approach establishes a versatile and scalable architecture for time-bin encoded quantum communication, enabling practical protocols on industry-grade photonic technology.

Free-space optical communication links are vulnerable to beam wandering and turbulence-induced aberrations, which degrade optical wavefront quality and fiber coupling efficiency. These systems are usually designed to completely illuminate the receiver aperture to reduce the scintillation generated by the wandering of the laser beam, generating antenna losses. This work demonstrates the feasibility of a fully refractive adaptive optics receiver designed to dynamically correct beam wandering and wavefront aberrations. We present the results obtained from a laboratory test replicating the conditions present in a 270-meter ground-to-ground link used to perform quantum key distribution in the center of Rome. The beam wandering is artificially generated using a fast-steering mirror, while a Deformable Mirror introduces high-order aberrations. Corrections for beam wandering and residual tip-tilt errors are achieved with fast steering prisms, while high-order aberrations are corrected using a multi-actuator lens. This design makes the adaptive optics system fully refractive, which has the potential to enhance compactness compared to conventional systems based on deformable mirrors. Experimental results demonstrate an average improvement in the Strehl ratio of up to 87% and an average single-mode fiber coupling efficiency of up to 36%, preserving all the light sent from the transmitter.