Quantum Key Distribution With an Integrated Photonic Receiver

Photonic integrated circuits (PICs) are key in advancing quantum technologies for secure communications. They offer inherent stability, low losses and compactness compared to standard fiber-based and free-space systems. Our reasearch demonstrates PIC's effectivness in enhancing quantum communications, implementing a three-state BB84 protocol with decoy-state method. We employ an integrated receiver and superconducting nanowire single photon detectors (SNSPDs) to achieve technological advancements. One of the most notable results is the extraction of a secret key over a record-breaking 45 dB channel attenuation. Our results demonstrate a remarkable 220% boost in key rate compared to our prototype fiber-based receiver over a 10 dB channel attenuation. This improvement in the secret key rate (SKR) signifies the potential of integrated photonics to advance the field of quantum communication

Deploying an Inter‐European Quantum Network

Around 40 years have passed since the first pioneering works introduced the possibility of using quantum physics to enhance communications safety. Nowadays, quantum key distribution (QKD) exited the physics laboratories to become a mature technology, triggering the attention of States, military forces, banks, and private corporations. This work takes on the challenge of bringing QKD closer to a consumer technology: deployed optical fibers by telecommunication companies of different States have been used to realize a quantum network, the first-ever connecting three different countries. This work also emphasizes the necessity of networks where QKD can come up besides classical communications, whose coexistence currently represents the main limitation of this technology. This network connects Trieste to Rijeka and Ljubljana via a trusted node in Postojna. A key rate of over 3 kbps in the shortest link and a 7-hour-long measurement demonstrate the system's stability and reliability. The network has been used to present the QKD at the G20 Digital Ministers' Meeting in Trieste. The experimental results, together with the interest that one of the most important events of international politics has attracted, showcase the maturity of the QKD technology bundle, placing it in the spotlight for consumer applications in the near term

Nonlocal chromatic dispersion compensation of optical long-distance fibers for quantum communication

I recenti sviluppi nel campo della computazione quantistica rappresentano una minaccia concreta per la sicurezza informatica basata sulla crittografia classica. Se è vero che gli apparati di comunicazione quantistica sono già realtà, bisogna tuttavia considerare che al fine di rendere la quantum key distribution (QKD) un’alternativa competitiva è necessario che sia, oltre che economicamente accessibile, utilizzabile su grandi distanze e con velocità adeguate. Questo lavoro di tesi si pone l’obiettivo di adoperarsi al miglioramento della comunicazione quantistica su fibra ottica. È stata costruita una sorgente di coppie di fotoni entangled adoperati per la codifica di una chiave quantistica distribuita verso due parti connesse in fibra con attenuazione di -30 dB. I fotoni sono stati prodotti mediante down conversion parametrica a una lunghezza d’onda di 1550 nm, tipicamente impiegata nell’ambito delle telecomunicazioni, e sono soggetti a dispersione cromatica durante la propagazione in un mezzo dispersivo proporzionalmente alla larghezza del loro spettro. La dispersione è stata cancellata in entrambi i rami agendo su uno solo dei due fotoni, sfruttando l’anticorrelazione spettrale dei due fotoni entrangled. Si è dimostrato come seguendo questo procedimento il rate della chiave quantistica sia aumentato di 37 volte, spalancando le porte della QKD anche a scenari in cui è attualmente praticabile solo con rate bassi, quando non del tutto impraticabile. The recent development of quantum computation represents a real threat for cybersecurity relying on classical cryptography. If it is true that quantum communication devices are already a reality, it is necessary to consider that in order to make quantum key distribution (QKD) a good competitor it has to be, in addition to economically affordable, usable over long distances and with appropriate velocities. This master thesis aims to work in the improvement of quantum communication in optical fiber. An entangled photons source has been built and the photons have been used to codify a quantum key distributed over two parties connected by a fiber with an attenuation of -30 dB. The photons have been produced by spontaneous parametric down conversion at a wavelength of 1550 nm, usually employed in telecommunication, and are subject to chromatic dispersion during their propagation in a dispersive medium proportionally to their spectral width. The dispersion has been canceled in both arms acting just on one photon, exploiting the spectral anticorrelation of the two entangled photons. Has been proved that thanks to this procedure the quantum key rate has increased by 37 times, opening the doors of QKD to scenarios in which is currently possible just with low rate, if not impossible at all

Quantum key distribution over 100 km of underwater optical fiber assisted by a fast-gated single-photon detector

Nowadays quantum key distribution (QKD) represents the most mature quantum technology, and multiple countries as well as private institutions are building their quantum network. However, QKD devices are still far from representing a product within everyone’s reach. Indeed, limitations in terms of compatibility with existing telecom infrastructure and limited performances in terms of secret key rate, using noncryogenic detection systems, are still critical. In this work, we implemented a quantum key distribution link between Sicily (Italy) and Malta utilizing two different single-photon avalanche diode (SPAD) detectors. The performances of a standard commercial SPAD have been compared with the results achieved with an alternative prototype of fast-gated system in a package (SIP) SPAD; the SIP detector has shown to be able to accomplish a 14 times higher key rate compared with the commercial device over the channel showing 20 dB of losses.Nowadays Quantum Key Distribution represents the most mature quantum technology, and multiple countries as well as private institutions are building their quantum network. However, QKD devices are still far from representing a product within everyone's reach. Indeed, limitations in terms of compatibility with existing telecom infrastructure and limited performances in terms of secret key rate, using non-cryogenic detection systems, are still critical. In this work, we implemented a quantum key distribution link between Sicily (Italy) and Malta utilizing two different Single-Photon Avalanche Diode (SPAD) detectors. The performances of a standard commercial SPAD have been compared with the results achieved with a new prototype of fast-gated System in a Package (SiP) SPAD; the SiP detector has shown to be able to accomplish a fourteen times higher key rate compared with the commercial device over the channel showing 20 dB of losses

Practical high-dimensional quantum key distribution protocol over deployed multicore fiber

Abstract Quantum key distribution (QKD) is a secure communication scheme for sharing symmetric cryptographic keys based on the laws of quantum physics, and is considered a key player in the realm of cyber-security. A critical challenge for QKD systems comes from the fact that the ever-increasing rates at which digital data are transmitted require more and more performing sources of quantum keys, primarily in terms of secret key generation rate. High-dimensional QKD based on path encoding has been proposed as a candidate approach to address this challenge. However, while proof-of-principle demonstrations based on lab experiments have been reported in the literature, demonstrations in realistic environments are still missing. Here we report the generation of secret keys in a 4-dimensional hybrid time-path-encoded QKD system over a 52-km deployed multicore fiber link forming by looping back two cores of a 26-km 4-core optical fiber. Our results indicate that robust high-dimensional QKD can be implemented in a realistic environment by combining standard telecom equipment with emerging multicore fiber technology