Entanglement is one of the most striking phenomena in quantum physics, playing important roles in fundamental physics and quantum information science. It enables a secure means of communication—quantum cryptography—and builds up the foundation of its unconditional security. Entanglement-based quantum cryptography has received great attention from the early demonstrations to the recent remarkable achievements. In a practical scenario, although entanglement-based quantum cryptography can provide inherent source-independent security, its detection side has been shown to be vulnerable to external probing attacks. Here we show that entanglement swapping can effectively solve this critical issue, enabling a side-channel-free quantum cryptography.Entanglement swapping allows each user’s quantum state preparation and detection in a completely private station, which is immune to any external probing side channels. We demonstrate the entanglement-swapping quantum cryptography scheme in the field based on two independent entanglement photon sources. Based on the remote entangled photon pairs, we implement the Ekert-1991 protocol under a channel attenuation equivalent to 100 km of standard optical fiber, achieving a Bell violation value of S=2.659±0.092 and a secret key rate of 0.0163 bit/s. While recent device-independent QKD demonstrations have reached 100 km using atoms or ions, our photonic ES-QKD offers a complementary, all-optical pathway that is directly compatible with existing fiber networks and quantum repeaters.
Entanglement is one of the most striking phenomena in quantum physics, playing important roles in fundamental physics and quantum information science. It enables a secure means of communication—quantum cryptography—and builds up the foundation of its unconditional security. Entanglement-based quantum cryptography has received great attention from the early demonstrations to the recent remarkable achievements. In a practical scenario, although entanglement-based quantum cryptography can provide inherent source-independent security, its detection side has been shown to be vulnerable to external probing attacks. Here we show that entanglement swapping can effectively solve this critical issue, enabling a side-channel-free quantum cryptography.Entanglement swapping allows each user’s quantum state preparation and detection in a completely private station, which is immune to any external probing side channels. We demonstrate the entanglement-swapping quantum cryptography scheme in the field based on two independent entanglement photon sources. Based on the remote entangled photon pairs, we implement the Ekert-1991 protocol under a channel attenuation equivalent to 100 km of standard optical fiber, achieving a Bell violation value of S=2.659±0.092 and a secret key rate of 0.0163 bit/s. While recent device-independent QKD demonstrations have reached 100 km using atoms or ions, our photonic ES-QKD offers a complementary, all-optical pathway that is directly compatible with existing fiber networks and quantum repeaters.
Establishing entanglement between two remote systems by the method of entanglement swapping is an essential step for a long-distance quantum network. Here we report a field-test entanglement swapping experiment with two independent telecommunication band entangled photon-pair sources over an optical fiber network in Hefei. The two sources are located at two nodes that are 12.5km apart and the Bell-state measurement is performed at a third location which is connected to the two source nodes with 14.7-km and 10.6-km optical fibers, respectively. The observed average visibility is $79.9\ifmmode\pm\else\textpm\fi{}4.8%$, which is sufficient for the violation of Bell inequalities. Furthermore, with the swapped entanglement, we demonstrate a source-independent quantum key distribution, which is also immune to any detection attacks at the measurement site.
Quantum cryptography uses the quantum properties of individual photons to provide two or more users with means to communicate securely and efficiently. Specifically, quantum key distribution (QKD) focuses on the secure communication between two distant parties and guarantees the security of a transmitted message between them. Although QKD is the most mature application of quantum cryptography, applications of quantum mechanics in communication systems are not limited to QKD and other cryptographic protocols can be implemented as well. This thesis focuses on the implementation of quantum cryptographic systems over deployed fiber. A first QKD system was built and used to demonstrate the BB84 protocol over deployed fiber between the University of Calgary and SAIT. The system implemented a novel tool called quantum frames that allow for the quantum signals to be routed, for clock synchronization, and for channel stabilization in a network scenario. The same system was used to examine the scalability of the secret key generation rate in different steps of the process in order to detect potential bottlenecks for key generation rate of high speed systems. A second QKD system was developed to demonstrate the measurement-device independent QKD protocol. The system worked over deployed fiber between the University of Calgary, SAIT, and Foothills Hospital in the City of Calgary. The advantage of the measurement-device independent QKD protocol over previous QKD protocols is that it eliminates the possibility of side-channel attacks that target single photon detectors used in QKD systems. This demonstration also involved the first demonstration of a Bell state measurement over deployed fiber. Bell state measurements were further studied by using novel single photon detectors, which allowed demonstrating a highly efficient Bell measurement. Finally, a cryptographic protocol known as private queries was demonstrated between the University of Calgary and SAIT. Quantum private queries were demonstrated for the first time over deployed fiber by developing and implementing an error correction scheme to make the protocol noise and loss tolerant.
Recently, a quantum key distribution (QKD) scheme based on entanglement swapping, called measurement-device-independent QKD (mdiQKD), was proposed to bypass all measurement side-channel attacks. While mdiQKD is conceptually elegant and offers a supreme level of security, the experimental complexity is challenging for practical systems. For instance, it requires interference between two widely separated independent single-photon sources, and the secret key rates are dependent on detecting two photons—one from each source. Here, we demonstrate a proof-of-principle experiment of a QKD scheme that removes the need for a two-photon system and instead uses the idea of a two-qubit single-photon to significantly simplify the implementation and improve the efficiency of mdiQKD in several aspects.
Quantum entanglement, one of the most intriguing phenomena in quantum mechanics, forms the cornerstone of emerging technologies in quantum information science. It describes the non-classical correlations between spatially separated particles, whereby the measurement outcome of one particle instantaneously determines the state of the other, regardless of the distance between them. This paper explores the fundamental principles of quantum entanglement and its critical role in advancing quantum cryptography and quantum computing. In cryptography, entanglement enables protocols such as Ekert’s E91 and BBM92 quantum key distribution schemes, which leverage nonlocal correlations to ensure unconditional security based on the violation of Bell’s inequalities. In quantum computing, entanglement serves as a powerful computational resource, facilitating speedups in algorithms, enabling quantum teleportation, and supporting robust quantum error correction techniques. The discussion integrates both theoretical underpinnings and experimental realizations, highlighting recent advances in photonic entanglement sources, superconducting qubits, and trapped-ion systems. Challenges related to decoherence, scalability, and entanglement distribution in quantum networks are examined, along with potential solutions. By bridging foundational theory with practical applications, this study underscores the pivotal role of entanglement in shaping the future of secure communications and computational paradigms beyond the capabilities of classical systems.
Key negotiation plays an important role in ensuring secure communication channel, and establishes confidentiality between parties. Quantum Key Distribution (QKD) represents an advanced and highly secure method for distributing cryptographic keys between involved parties. Multiple QKD implementation possibilities have led to the upgrading of various QKD protocols. Quantum key distribution involves two types of protocols approaches prepare-and-measure and entanglement based, this paper focuses entanglement-based protocol. Prepare-and-measure protocols are vulnerable to certain attacks and have lower key generation rate as compared to entanglement based protocol. Entanglement is certified as a key element in the arena of quantum information processing, contributing in applications like quantum cryptography, quantum computation and quantum communication. Entanglement leads to the phenomenon known as entanglement swapping, entanglement swapping phenomenon involves parties getting entangled without interacting with each other. This research focuses on the advantages and the security provided by entanglement swapping in quantum key distribution. Blockchain also utilizes key distribution process with the involvement of classical cryptography. Classical cryptography blockchain are not quantum-resistant, blockchain with quantum computing infrastructure are more advance and secure. Implementing entanglement swapping in the infrastructure of blockchain can make the blockchain quantum-resistant to secure communication and transactions. Within this manuscript, we theoretically study the entanglement swapping of Bell states in quantum key distribution and propose an idea of blockchain with entanglement swapping to enhance quantum resistance. This study also explores the employing of entanglement swapping in cloud infrastructure.
The impending realization of fault-tolerant quantum computers threatens the foundational public-key cryptography underpinning global telecommunications and financial infrastructure. While algorithmic Post-Quantum Cryptography (PQC) provides mathematical computational hardness against quantum cryptanalysis, Quantum Key Distribution (QKD) establishes an alternative information-theoretically secure physical paradigm based on the immutable laws of quantum mechanics: the No-Cloning theorem and Heisenberg's uncertainty principle. However, deploying scalable, wide-area quantum networks introduces formidable physical and systems engineering challenges: exponential optical fiber attenuation that restricts direct point-to-point QKD to beneath 150 km, high dark-count rates in superconducting nanowire single-photon detectors (SNSPDs), non-deterministic Bell-state entanglement swapping across quantum repeaters, and intense Raman scattering cross-talk when multiplexing quantum channels alongside multi-terabit classical DWDM optical networks. This paper delivers an exhaustive, mathematically formal technical investigation and empirical architectural evaluation of quantum networking and QKD systems. We formalize the asymptotic secret key rate bounds under the Decoy-State BB84 and Measurement-Device-Independent (MDI-QKD) protocols, model continuous entanglement routing over quantum repeaters utilizing bipartite Bell-pair purification, and establish optical co-existence bounds under Dense Wavelength Division Multiplexing (DWDM). Furthermore, we construct an empirical optical testbed evaluating an 8-node metropolitan quantum network co-propagating QKD over 100 Gbps classical optical traffic, paired with hybrid PQC-QKD key derivation functions (ML-KEM + QKD). Our empirical findings demonstrate that decoy-state BB84 achieves a sustainable secret key rate of 1.42 kbps across 100 km of standard single-mode fiber (SMF-28), while hybrid PQC-QKD integration eliminates single-point cryptographic failures with less than 2.4 ms handshake latency overhead. Finally, we formulate open challenges in memory-assisted atomic quantum repeaters, satellite-to-ground freespace entanglement links, and quantum internet transport protocols.