Polarization-entangled photon-based quantum key distribution (QKD) has seen notable advancements, requiring users to share a common frame of reference that might lead to a significant technical overhead of constant monitoring of their frame of reference. In this work, we characterize and demonstrate hybrid entangled photon pairs for their utilization in rotational alignment-free QKD applications. By employing vortex half-wave retarders that have an azimuthally varying fast axis with a topological charge of q=12, we generate high brightness rotationally invariant hybrid entangled photons and show their resilience against the misalignment of the frame of reference. We demonstrate high fidelity (>92%) entanglement and depict a low quantum bit error rate (QBER) of ∼5−6% (lower than the threshold value of 11%) at a significantly high angle of misalignment θ>45°), in contrast to the polarization-entangled photon pairs that suffer from the degradation of entanglement quality due to a rotational misalignment between the reference frame of communicating entities. The rotationally invariant hybrid photon pairs with robust entanglement properties offer a significant advantage for secure and reliable quantum communication, making them lucrative candidates for applications such as satellite-to-satellite and ground-to-satellite QKD networks. We also show the reliability and stability of the entangled photon source over a long period (measured over 6 h) by demonstrating the BBM92 protocol with a raw key rate of ∼2.8 kbps and a QBER of ∼5% in a single-pass configuration. The performance metrics of our quantum source, measured with conventional avalanche photodiodes (SPADs, quantum efficiency ∼60%, dark counts ∼40 cps), are among the best values reported so far with these detectors.
Polarization-entangled photon-based quantum key distribution (QKD) has seen notable advancements, requiring users to share a common frame of reference that might lead to a significant technical overhead of constant monitoring of their frame of reference. In this work, we characterize and demonstrate hybrid entangled photon pairs for their utilization in rotational alignment-free QKD applications. By employing vortex half-wave retarders that have an azimuthally varying fast axis with a topological charge of q=12, we generate high brightness rotationally invariant hybrid entangled photons and show their resilience against the misalignment of the frame of reference. We demonstrate high fidelity (>92%) entanglement and depict a low quantum bit error rate (QBER) of ∼5−6% (lower than the threshold value of 11%) at a significantly high angle of misalignment θ>45°), in contrast to the polarization-entangled photon pairs that suffer from the degradation of entanglement quality due to a rotational misalignment between the reference frame of communicating entities. The rotationally invariant hybrid photon pairs with robust entanglement properties offer a significant advantage for secure and reliable quantum communication, making them lucrative candidates for applications such as satellite-to-satellite and ground-to-satellite QKD networks. We also show the reliability and stability of the entangled photon source over a long period (measured over 6 h) by demonstrating the BBM92 protocol with a raw key rate of ∼2.8 kbps and a QBER of ∼5% in a single-pass configuration. The performance metrics of our quantum source, measured with conventional avalanche photodiodes (SPADs, quantum efficiency ∼60%, dark counts ∼40 cps), are among the best values reported so far with these detectors.
Entanglement-based quantum key distribution (QKD) protocols, such as E91 and BBM92, offer strong information-theoretic security and are naturally suited for satellite-to-satellite QKD (SatQKD) links. However, implementing these protocols over long-distance inter-satellite free-space optical (FSO) channels poses critical physical-layer challenges that are not addressed in the existing literature. In particular, photon losses due to beam divergence, pointing errors, and background noise can severely degrade the key generation rate and quantum bit error rate (QBER), especially under narrow receiver field-of-view (FoV) constraints. This paper presents a comprehensive performance analysis of entanglement-based inter-satellite QKD, focusing on photon-level modeling and the impact of practical impairments. We develop analytical expressions for signal detection probabilities, background photon influence, multi-pair emissions, and QBER, incorporating key parameters such as link distance, transmitter tracking jitter, receiver misalignment, and photon pair generation rate. Simulation results reveal the nonlinear sensitivity of system performance to tracking error and FoV limitations, and highlight optimal parameter regimes that jointly maximize secret key rate while maintaining QBER below acceptable thresholds. The proposed model provides actionable design insights for reliable and efficient deployment of entanglement-based SatQKD systems.
Quantum Key Distribution (QKD) is a cutting-edge field that leverages the principles of quantum mechanics to enable secure communication between parties involved. Single-photon quantum emitters offer remarkable on-demand photon emission, near-unitary indistinguishability, and low multiphoton generation, thereby enhancing the performance of QKD protocols. Standard approaches in which the polarization degree-of-freedom is exploited are limited by the precise alignment between the communicating parties. To overcome this obstacle, the Orbital Angular Momentum (OAM) of light represents a suitable candidate for encoding the information, as it allows the implementation of rotational-invariant photonic states that remove the need for a fixed physical reference frame between the communicating parties. Here, we demonstrate an alignment-free BB84 implementation using on-demand single photons from a quantum-dot emitter at 925 nm. Time-to-spatial demultiplexing and Q -plates encode rotationally invariant spin–orbit states that we transmit over a 2 m free-space link while rotating the receiver between 0 ∘ and 90 ∘ . The source exhibits g (2) (0)≈0.03; the measured QBER remains (4.04±0.02)% and is essentially independent of the relative angle. A simple security model tailored to sub-Poissonian emission, which explicitly incorporates g (2) (0), yields a positive asymptotic secret fraction and an effective QBER threshold of ∼9.6% under our conditions. These results validate rotationally invariant QKD with an on-demand single-photon source and outline its scalability to long-distance scenarios.
Quantum information and quantum optics offer powerful innovative tools for secure communication and cryptography, with Quantum Key Distribution (QKD) becoming a standard technology in the cryptographic world. By exploiting fundamental principles such as quantum superposition, entanglement, and the no-cloning theorem, QKD enables two users to establish a shared secret key with information-theoretic security. Quantum communication is a fundamental element for the realization of the quantum internet, a network of distant nodes connected by quantum channel. The realization of such networks requires methods to distribute states and correlations with high fidelity and low losses. This thesis explores the use of innovative photonic technologies for the realization of experiment aiming at improving quantum communication and quantum channels. Single photons based on QD are generated on-demand and show high purity, near-optimal indistinguishability and low multi-photons emission. By exploiting the features of these photons, a method to realize entangled states is implemented. Using the polarization degree of freedom, these entangled states are ideal candidates for free-space QKD protocols based on the Bell’s inequality. To address the problem of the reference frame alignment using the polarization, hybrid states are studied and implemented. The specific combination of polarization and orbital angular momentum (OAM) can produce photons that are not modified by rotation of the reference frame. We implemented an experiment to realize a QKD that is independent on the relative rotation of the two user. The final goal of my thesis was devoted to the in-field demonstration of quantum teleportation as a fundamental building block of distributed quantum networks, enabling the reliable transmission of quantum information between remote nodes. Exploiting the free-space infrastructure in the "Sapienza" University of Rome, a link of around ∼ 300 m, we implemented the quantum teleportation between two different QDs showing the possibility of using this technology into real-case scenario. The results obtained show not only that QDs are ideal sources for quantum communication, but also the possibility of using new states to improve current technologies, advancing the development of efficient and reliable quantum networks.
Global satellite infrastructure has transformed maritime data links, allowing ships to share real-time details—such as location, speed, and environmental readings—with orbiting platforms. Yet conventional encryption methods can be compromised, threatening the confidentiality of these exchanges. Quantum Key Distribution (QKD) inherently detects any interception during key setup, offering a robust security layer. In our Satellite Vehicular Communication (SVC) framework, we integrate two entanglement-based QKD protocols: BBM92, which uses satellite-generated entangled photon pairs, and Measurement-Device-Independent QKD, which removes trust assumptions from the measurement hardware. We evaluate performance over free-space optical paths up to 25 km under realistic noise and weather scenarios. Findings show that MDIQKD achieves higher raw key generation rates but experiences rising quantum bit error rates at extended ranges, whereas BBM92 delivers lower error rates and steadier secure key throughput at moderate distances.
Abstract Next-generation smart cities are increasingly reliant on high-speed, secure communication systems to support 6 G networks, IoT infrastructure, and autonomous systems. Classical cryptographic techniques are becoming vulnerable to quantum computing attacks, prompting the integration of quantum-secure communication mechanisms. In this context, Quantum Key Distribution (QKD) over Visible Light Communication (VLC) and Free Space Optical (FSO) networks offers a promising path toward ultra-secure, energy-efficient transmission. We propose an Entangled Photon Quantum Key Distribution (EP-QKD) framework that advances secure communication by integrating polarization-entangled photons, adaptive Quantum Bit Error Rate (QBER) optimization, and hybrid quantum-classical authentication anchored with blockchain. This advancement addresses critical limitations of traditional QKD systems—including high sensitivity to ambient interference, limited scalability, and high error rates—by employing entanglement-based noise resilience and real-time detection threshold control. Existing approaches to QoS and security—such as RSA-based encryption, BB84 and E91 protocols, and VLC-oriented modulation techniques—struggle with low secure key rates, high QBER under ambient light, and vulnerability to man-in-the-middle (MITM) attacks. EP-QKD overcomes these limitations by enhancing photon distribution fidelity, enabling decentralized authentication, and sustaining low error rates across dynamic urban optical environments. Experimental results demonstrate that EP-QKD achieves an average QBER of 1.2% and maintains QBER below 2.5% at 1000 lux, reducing errors by 40% over BB84 and 60% over E91. It attains secure key generation rates of over 1200 Mbps at 5 meters, outperforming BB84 and E91 by a significant margin. At 20 meters, EP-QKD maintains 300 Mbps, while BB84 drops to 40 Mbps. The MITM attack success rate is reduced to 0.01%, compared to 15% with BB84 + RSA. Additionally, EP-QKD achieves energy efficiency of 0.7 nJ/bit—nearly half of BB84’s requirement—making it suitable for energy-constrained IoT and mobile systems. Multi-user scalability testing confirms that EP-QKD can support up to 50 concurrent users with a key rate above 80 Mbps per user and QBER < 1.8%. A 3D analysis further shows that QBER remains below 5% even at 50 meters, demonstrating the framework’s robustness for long-distance secure optical communication. Overall, the proposed EP-QKD framework significantly enhances the performance, scalability, and resilience of quantum communication systems, making it a viable candidate for 6 G, IoT, and satellite-based secure infrastructure.
Free-space quantum key distribution has recently achieved several milestones, such as \nthe launch and results of the first quantum satellite, Micius. The emergence of quantum satellites has certainly made progress towards the realization of a global quantum cryptographic network. In this thesis, two challenges in the development of an optical quantum ground station \nfor a free-space quantum satellite link are studied. The first is the development of a high brightness, fiber pigtailed waveguide that is to be used as a polarization entangled photon source. The high pair production rate is required in order to meet the requirements for a satellite up-link configuration. The portability, robustness and ease of alignment were motivations for choosing a fiber pigtailed source. Certain challenges that are fundamental to the source design were characterized and several solutions to these challenges were investigated. The other main investigation in this thesis, is the development of a passive polarization \ncompensation using polarization maintaining fibers. The birefringence in standard single mode optical fibers causes random polarization rotations to the light passing through the fiber. Polarization maintaining fibers, though very high in birefringence, are used with entangled photons and techniques from reference frame independent quantum key distribution protocols are shown to compensate for random polarization rotations while preserving the entanglement. In addition, the feasibility of the protocol using the polarization maintaining fibers is investigated. \nThrough various studies, experiments, and component design, the feasibility of a pigtailed waveguide entangled photon source has been shown to need further investigation, while the feasibility of implementing polarization maintaining fibers to the ground station has been shown to be effective. It is particularly effective as a passive polarization compensation system that uses entanglement, however a similar concept is effective for non-entangled single photons. This work contributes to a long line of achievements leading towards satellite implementations of quantum key distribution for an eventual global quantum cryptographic network.