600 km repeater-like quantum communications with dual-band stabilisation

Twin-field (TF) quantum key distribution (QKD) could fundamentally alter the rate-distance relationship of QKD, offering the scaling of a single-node quantum repeater. Although recent experiments have demonstrated the potential of TF-QKD, formidable challenges remain for its real world use. In particular, new methods are needed to extend both the distance beyond 500 km and key rates above current milli-bit per second values. Previous demonstrations have required intense stabilisation signals at the same wavelength as the quantum channel, thereby unavoidably generating noise due to Rayleigh scattering that limits the distance and bit rate. Here, we introduce a novel dual band stabilisation scheme based on wavelength division multiplexing that allows us to circumvent past limitations. An intense stabilisation signal that is spectrally isolated from the quantum channel is used to reduce the phase drift by three orders of magnitude, while a second, much weaker reference at the quantum wavelength locks the channel phase to a predetermined value. With this strategy, we realise a low noise implementation suitable for all the variants of TF-QKD protocols proposed so far and capable of generating real strings of bits for the first time. The setup provides repeater-like key rates over record communication distances of 555 km and 605 km in the finite-size and asymptotic regimes, respectively, and increases the secure key rate at long distance by two orders of magnitude to values of practical significance.Comment: 14 pages, 5 figures. Methods and supplementary materials are include

600-km repeater-like quantum communications with dual-band stabilization

Twin-field (TF) quantum key distribution (QKD) fundamentally alters the rate-distance relationship of QKD, offering the scaling of a single-node quantum repeater. Although recent experiments have demonstrated the new opportunities for secure long-distance communications allowed by TF-QKD, formidable challenges remain to unlock its true potential. Previous demonstrations have required intense stabilization signals at the same wavelength as the quantum signals, thereby unavoidably generating Rayleigh scattering noise that limits the distance and bit rate. Here, we introduce a dual-band stabilization scheme that overcomes past limitations and can be adapted to other phase-sensitive single-photon applications. Using two different optical wavelengths multiplexed together for channel stabilization and protocol encoding, we develop a setup that provides repeater-like key rates over communication distances of 555 km and 605 km in the finite-size and asymptotic regimes respectively and increases the secure key rate at long distance by two orders of magnitude to values of practical relevance

Coherent phase transfer for real-world twin-field quantum key distribution

Quantum mechanics allows distribution of intrinsically secure encryption keys by optical means. Twin-field quantum key distribution is one of the most promising techniques for its implementation on long-distance fiber networks, but requires stabilizing the optical length of the communication channels between parties. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical stabilization frames. In this approach, longer duty cycles for the key streaming come at the cost of a looser control of channel length, and a successful key-transfer using this technique in real world remains a significant challenge. Using interferometry techniques derived from frequency metrology, we develop a solution for the simultaneous key streaming and channel length control, and demonstrate it on a 206 km field-deployed fiber with 65 dB loss. Our technique reduces the quantum-bit-error-rate contributed by channel length variations to <1%, representing an effective solution for real-world quantum communications

Damaged Intestinal Epithelial Integrity Linked to Microbial Translocation in Pathogenic Simian Immunodeficiency Virus Infections

The chronic phase of HIV infection is marked by pathological activation of the immune system, the extent of which better predicts disease progression than either plasma viral load or CD4+ T cell count. Recently, translocation of microbial products from the gastrointestinal tract has been proposed as an underlying cause of this immune activation, based on indirect evidence including the detection of microbial products and specific immune responses in the plasma of chronically HIV-infected humans or SIV-infected Asian macaques. We analyzed tissues from SIV-infected rhesus macaques (RMs) to provide direct in situ evidence for translocation of microbial constituents from the lumen of the intestine into the lamina propria and to draining and peripheral lymph nodes and liver, accompanied by local immune responses in affected tissues. In chronically SIV-infected RMs this translocation is associated with breakdown of the integrity of the epithelial barrier of the gastrointestinal (GI) tract and apparent inability of lamina propria macrophages to effectively phagocytose translocated microbial constituents. By contrast, in the chronic phase of SIV infection in sooty mangabeys, we found no evidence of epithelial barrier breakdown, no increased microbial translocation and no pathological immune activation. Because immune activation is characteristic of the chronic phase of progressive HIV/SIV infections, these findings suggest that increased microbial translocation from the GI tract, in excess of capacity to clear the translocated microbial constituents, helps drive pathological immune activation. Novel therapeutic approaches to inhibit microbial translocation and/or attenuate chronic immune activation in HIV-infected individuals may complement treatments aimed at direct suppression of viral replication

Experimental implementation of Twin-Field Quantum Key Distribution Protocols

Quantum Key Distribution allows two distant users to establish a common secret string of bits by sending photons across a communication line, often an optical fibre. The photons, however, are scattered by the propagation medium and have only a small probability of reaching the end of the line, which limits the QKD key rate and its transmission range. A rigorous theorem limits the number of secure bits delivered by a point-to-point QKD link to 1.44η, with η being the channel transmission probability. This is known as the ‘repeaterless secret key capacity’, or the PLOB bound. The key question at the core of this thesis is to design and implement QKD systems that surpass the PLOB bound. Until very recently, this task was believed to be impossible with today’s technology. This changed with the introduction of the ‘Twin-Field’ (TF) QKD protocol, which features a key rate that scales proportionally to the square root of η and therefore offers a way to extend the current range of QKD. This work provides a contextualisation and description of the TF-QKD protocol and its variants. The experimental challenges for its implementation are considered and followed by the development of experimental techniques, setups and analysis frameworks necessary to implement the protocol. As a result, the first proof-of-principle demonstration of the protocol over highly attenuated channels is obtained and described. In this experiment, a secure key could be distributed in excess of 90 dB channel loss and the PLOB bound could be exceeded for the first time. The setup implemented for this experiment is currently considered the first realisation of an effective quantum repeater. A second experiment, which exploits a novel dual-band phase stabilisation technique, is also developed. In this experiment, TF-QKD is performed over long communication channels that reached over 600 km of fibre length and 100 dB of channel loss. This experiment represents today’s longest fibre-based quantum communication system

Background discrimination techniques using Artificial Neural Networks for the GERDA experiment

The aim of this thesis is to perform an analysis of Signal recognition and Background rejection to outline a framework for data analysis based on Artificial Neural Network (ANN) suitable to the discrimination of samples from the GERDA experiment. More precisely, this work will focus on setting up an ANN that, through Pulse Shape Analysis techniques, can recognize a possible 0vbb decay from background events

Three-observer Bell inequality violation on a two-qubit entangled state

Bipartite Bell inequalities can be simultaneously violated by two different pairs of observers when weak measurements and signaling is employed. Here we experimentally demonstrate the violation of two simultaneous CHSH inequalities by exploiting a two-photon polarization maximally entangled state. Our results demonstrate that large double violation is experimentally achievable. Our demonstration may have impact for Quantum Key Distribution or certification of Quantum Random Number generators based on weak measurements.Comment: Revtex, 5 pages + appendi