It has long been known that quantum networks will enable a whole new range of communication tasks to be undertaken. The simplest is quantum key distribution (QKD) and are commercially available but currently only operate securely over distances around 100 km. A significant advance has been the development of mdiQKD, a scheme where Alice and Bob send one photon at a time to an intermediate node where a Bell measurement is performed. This Bell measurement can only succeed when both Alice and Bob photons arrive at the same time and so the key rate is limited by the exponential losses in both fibres. It limits the practical distance keys can be generated to less than 400km. Spatial or temporal multiplexing is a natural solution to this where one stores the photons that independently arrive from Alice and Bob. Only when the immediate node has both does it perform the Bell measurement. This means we are effectively only limited by fibres losses in one half of the channel. It however means one requires quantum memories at this immediate node, a technically challenging feat and one that changes the general resources used in QKD schemes. In our spatial multiplexed approach, we propose the use of an “all photonic non-destructive measurement (QND)” to herald whether the photon has arrived successfully from either Alice or Bob. Optical switches can them be used to route these photons to the Bell measurement, meaning that we are only limited by the channel loss between either Alice and the immediate node or Bob and the intermediate node, but not both. Further this can achieved without the use of quantum memories at all. Only optical switches, single-photon sources, photon detectors, and passive feed-forward techniques are required. Our approach can be applied naturally to entanglement distribution and so has applications beyond QKD.
It has long been known that quantum networks will enable a whole new range of communication tasks to be undertaken. The simplest is quantum key distribution (QKD) and are commercially available but currently only operate securely over distances around 100 km. A significant advance has been the development of mdiQKD, a scheme where Alice and Bob send one photon at a time to an intermediate node where a Bell measurement is performed. This Bell measurement can only succeed when both Alice and Bob photons arrive at the same time and so the key rate is limited by the exponential losses in both fibres. It limits the practical distance keys can be generated to less than 400km. Spatial or temporal multiplexing is a natural solution to this where one stores the photons that independently arrive from Alice and Bob. Only when the immediate node has both does it perform the Bell measurement. This means we are effectively only limited by fibres losses in one half of the channel. It however means one requires quantum memories at this immediate node, a technically challenging feat and one that changes the general resources used in QKD schemes. In our spatial multiplexed approach, we propose the use of an “all photonic non-destructive measurement (QND)” to herald whether the photon has arrived successfully from either Alice or Bob. Optical switches can them be used to route these photons to the Bell measurement, meaning that we are only limited by the channel loss between either Alice and the immediate node or Bob and the intermediate node, but not both. Further this can achieved without the use of quantum memories at all. Only optical switches, single-photon sources, photon detectors, and passive feed-forward techniques are required. Our approach can be applied naturally to entanglement distribution and so has applications beyond QKD.
Recent field demonstrations of quantum key distribution (QKD) networks hold promise for unconditionally secure communication. However, owing to loss in optical fibres, the length of point-to-point links is limited to a hundred kilometers, restricting the QKD networks to intracity. A natural way to expand the QKD network in a secure manner is to connect it to another one in a different city with quantum repeaters. But, this solution is overengineered unless such a backbone connection is intercontinental. Here we present a QKD protocol that could supersede even quantum repeaters for connecting QKD networks in different cities below 800 km distant. Nonetheless, in contrast to quantum repeaters, this protocol uses only a single intermediate node with optical devices, requiring neither quantum memories nor quantum error correction. Our all-photonic 'intercity' QKD protocol bridges large gaps between the conventional intracity QKD networks and the future intercontinental quantum repeaters, conceptually and technologically.
Quantum networks, integrating quantum communication, quantum metrology, and distributed quantum computing, could provide secure and efficient information transfer, high-resolution sensing, and an exponential speed-up in information processing. Deterministic entanglement distribution over long distances is a prerequisite for scalable quantum networks, enabling the utilization of device-independent quantum key distribution (DI-QKD) and quantum teleportation to achieve secure and efficient information transfer. However, the exponential photon loss in optical fibres prohibits efficient and deterministic entanglement distribution. Quantum repeaters, incorporating entanglement swapping and entanglement purification with quantum memories, offer the most promising means to overcome this limitation in fibre-based quantum networks. Despite numerous pioneering efforts toward realizing quantum repeaters, a critical bottleneck remains, as remote memory-memory entanglement suffers from decoherence more rapidly than it can be established and purified over long distances. We overcome this by developing long-lived trapped-ion memories, an efficient telecom interface, and a high-visibility single-photon entanglement protocol. This allows us to establish and maintain memory-memory entanglement over a 10 km fibre within the average entanglement establishment time for the same distance. As a direct application, we demonstrate metropolitan-scale DI-QKD, distilling 1,917 secret keys out of 4.05*10^5 Bell pairs over 10 km. We further report a positive key rate over 101 km in the asymptotic limit, extending the achievable distance by more than two orders of magnitude. Our work provides a critical building block for quantum repeaters and marks an important step toward scalable quantum networks.
Quantum Key Distribution (QKD) has opened a new avenue for secure communication, by allowing one to distribute a random secret key between two users that are connected through a public channel without revealing information to unauthorized parties. If a message is encrypted with the one time pad (a well-known crytography algorithm) using secret keys created by QKD, then the ciphertext is information-theoretically secure and thus unbreakable for adversaries. Despite its unprecedented security, loss in the physical channel has prevented QKD from being used over distances beyond a few hundred kilometers. Fortunately, quantum repeaters have opened a path for long-distance QKD by providing a means to establish entanglement between distant users. The goal of this thesis has been to develop a source of entangled photon pairs that is suitable for quantum repeaters and then use it to test some of the fundamental building blocks of a quantum repeater : the heralded creation of entangled photons by means of entanglement swapping with properties that allow interfacing with optical quantum memories; the reversible mapping of quantum states from members of entangled photon pairs in and out of solid-state quantum memories; two-photon interference and a Bell state measurement with photons recalled from separate quantum memories. The demonstration of these key ingredients of a quantum repeater constitutes a significant step towards the establishment of entanglement over hundreds of kilometer distance, and hence long-distance QKD.
Over the last few decades, quantum key distribution (QKD) has gained a lot of attention due to its promise of establishing secret keys between authenticated users even in the presence of an eavesdropper who is only confined by the laws of nature. Secure key established by QKD in conjunction with one-time pad (OTP) encryption thus promises to end the long standing battle between code-makers and code-breakers. Spurred by its great promise, QKD has been the first quantum information technology to be commercialized and QKD systems are available from a number of vendors.However, these systems are still vulnerable to side-channel attacks as the components used in these systems don’t necessarily conform to the idealistic assumptions made in security proofs of QKD. Of the many components, single photon detectors have been identified as the most vulnerable component allowing, for instance, so-called ‘blinding attacks.’ In light of this, measurement device independent quantum key distribution (MDIQKD) protocol was proposed as a means to make QKD inherently immune to all possible detector side channel attacks, due to the particular property of the so-called Bell state measurement. The aim of this thesis is to develop techniques that will allow developing a cost- effective MDIQKD system that is suitable for quantum networks and to use these techniques to perform quantum teleportation on a metropolitan scale for the first time. More precisely this thesis describes the assessment of performance of MDIQKD using differ- ent hardware; the development of cost-effective MDIQKD system for quantum networks; the building of a practical quantum random generator (QRNG) suitable for high speed QKD systems; the demonstration of quantum teleportation on a metropolitan scale and the realization of an efficient Bell state analyzer for time-bin qubits that improves the efficiency of all quantum information processing tasks including MDIQKD and quantum teleportation. The above demonstrations constitute an important step towards realizing practical quantum internet.
Quantum Key distribution (QKD) enables two parties, Alice and Bob, to share a sequence of random bit whose secrecy is guaranteed by the fondations of quantum physics. The natural counterpart of this genuine inviolability lies in moderate key generation rates and achievable distances, due to severe exper- imental constraints, such as photon generation probabilities, detector's inefficiencies, detector's dark counts, and losses in fiber distribution channels. As well as improving these exper- imental resources since the last couple of years, several strate- gies are developed for increasing this bit rate. One promising way consists in exploiting the spectral correlations of time- energy entangled photons, enabling higher information con- tent per paired photon. Our idea is to combine the maturity of QKD schemes with high performance standard telecom com- ponents to propose a practical scheme allowing the distribu- tion of secret bits over 200 km at unprecedented rates.
Progress in the development of techniques for the construction of multiuser quantum communications networks is reviewed in light of the plans for an EU quantum communications infrastructure (EU QCI). Quantum key distribution networks may be classified as trusted node; quantum repeater-entanglement-swapping based; point-to-multipoint based on passive beam splitters, active optical switchers or WDM; high dimensional-multipartite entanglement and flexible reconfigurable multipartite networks. Both satellite and terrestrial implementations are possible and ore both planned for EU QCI; their relative merits are discussed. Current technology falls short in several respects of what is required to address use cases well. Bit rates as a function of distance are currently limited by the characteristics of available devices and are an order of magnitude or more below the theoretical bounds. Non-ideal device behaviour can open loopholes for side-channel attacks. Limited range constrains network geometry. New techniques are being developed to overcome these problems. The more sophisticated schemes depend on ongoing advances in the generation and distribution of entanglement. Particularly promising are the measurement device independent protocol, which eliminates photon-detector related security weaknesses, and the twin-field protocol, which uses similar techniques to extend range, in principle even beyond previously accepted theoretical bounds. Agreement of keys between more than two users can be accomplished classically after pairwise quantum key exchange but direct quantum multipartite agreement using may offer advantages. The same applies to secret sharing. Practical quantum repeaters, to eliminate the need for trusted nodes, are closer to being realised due to recent progress in optical memories.