Entanglement of distant atoms for quantum networks

Quantennetze versprechen viele revolutionäre Anwendungen, wie zum Beispiel sichere Quantenkommunikation und verteiltes Quantencomputing. Im Mittelpunkt dieser Netze steht die Fähigkeit, die Verschränkung zwischen weit entfernten Knoten über photonische Kanäle zu verteilen. Verschiedene physikalische Kandidaten werden aktiv erforscht, um als Quantensystem in den Knoten zu dienen. Hier verwenden wir neutrale Einzelatome, um eine Quantennetzwerkverbindung zwischen zwei unabhängigen Knoten zu realisieren, die sich in 400 m voneinander entfernten Gebäuden befinden. Diese Arbeit konzentriert sich auf zwei Themen, die Demonstration eines geräteunabhängigen Quantenschlüsselverteilungsprotokolls und die Verschränkungsverteilung zwischen den Knoten über Dutzende von Kilometern Telekommunikationsfaser mit Hilfe von Quantenfrequenzumwandlung. Die geräteunabhängige Quantenschlüsselverteilung ermöglicht die Erzeugung geheimer Schlüssel über einen nicht vertrauenswürdigen Kanal unter Verwendung nicht charakterisierter und potenziell nicht vertrauenswürdiger Geräte. Das ordnungsgemäße und sichere Funktionieren der Geräte kann durch einen statistischen Test mit einer Bell-Ungleichung bestätigt werden, so dass nur noch die Integrität der Nutzerstandorte mit anderen Mitteln garantiert werden muss. Die Realisierung geräteunabhängiger Protokolle stellt jedoch eine Herausforderung dar—hauptsächlich da es schwierig ist hochwertige verschränkte Zustände zwischen zwei entfernten Orten mit hoher Detektionseffizienz herzustellen. Hier stellen wir ein experimentelles System vor, das die Verteilung von Quantenschlüsseln in einem völlig geräteunabhängigen Szenario ermöglicht. Indem wir eine ereignisbereite Atom-Atom-Verschränkungstreue von F>0.892(23) erreichen, beobachten wir eine signifikante Verletzung der CHSH-Bell-Ungleichung von S=2.578(75)—oberhalb der klassischen Grenze von 2—und eine Quantenbitfehlerrate von 0.078(9). Für das implementierte Protokoll mit Zufallsschlüsseln ergibt sich daraus eine Geheimschlüsselrate von 0.07 Bit pro Verschränkungsereignis im asymptotischen Limit, was die Fähigkeit des Systems zur Erzeugung von Geheimschlüsseln in einem geräteunabhängigen Szenario demonstriert. Das zweite Thema ist die Verschränkungsverteilung über große Entfernungen mit optische Fasern, für die es unerlässlich ist, bei Telekommunikationswellenlängen zu arbeiten, um hohe Dämpfungsverluste zu überwinden. Die meisten Quantensysteme, die derzeit erforscht werden, arbeiten jedoch im sichtbaren Licht oder nahen Infrarot. Wir verwenden eine polarisationserhaltende Quantenfrequenzumwandlung in beiden Knotenpunkten, um die Wellenlänge der mit den Atomen verschränkten Photonen von 780 nm in das S-Band der Telekommunikation zu transformieren. Dank einer beispiellosen Effizienz der externen Konversion von 57% und minimalem induziertem Rauschen berichten wir über die Beobachtung von Atom-Photon- und angekündigter Atom-Atom-Verschränkung, die über Telekom-Glasfaserverbindungen mit einer Länge von bis zu 33 km erzeugt wurde. Wir analysieren die Verschränkungstreue für verschiedene Glasfaserverbindungslängen und zeigen, dass diese bei längeren Verbindungen hauptsächlich durch die atomare Dekohärenz begrenzt ist. Die Atom-Atom-Verschränkung wird erst nach dem Empfang des Ankündigungssignals analysiert, das eine Zeitverzögerung enthält, um die klassischen Kommunikationszeiten zwischen den Knoten zu berücksichtigen und ein realistisches Szenario einer Quantennetzwerkverbindung zu simulieren. Die in dieser Arbeit vorgestellten Ergebnisse ebnen den Weg zur ultimativen Form der sicheren Quantenkommunikation in zukünftigen Quantennetzwerken und sind ein Meilenstein auf dem Weg zur Realisierung von Quantennetzwerkverbindungen über große Entfernungen.Quantum networks promise many revolutionary applications, such as secure quantum communication and distributed quantum computing. Central to these networks is the ability to distribute entanglement between distant nodes using photonic channels. Various physical candidates are under active research to serve as quantum system in the nodes. Here, we employ neutral single-atoms to realise a quantum network link between two independent nodes located in buildings 400 m apart. This thesis focusses on two topics, namely, the demonstration of a device-independent quantum key distribution protocol and entanglement distribution between the nodes over tens of kilometres of telecom fibre employing quantum frequency conversion. Device-independent quantum key distribution enables the generation of secret keys over an untrusted channel using uncharacterized and potentially untrusted devices. The proper and secure functioning of the devices can be certified by a statistical test probing a Bell inequality, thereby leaving only the integrity of the users' locations to be guaranteed by other means. The realisation of device-independent protocols, however, is challenging—mainly because it is difficult to establish high-quality entangled states between two remote locations with high detection efficiency. Here we present an experimental system that allows for quantum key distribution in a fully device-independent setting. By achieving an event-ready atom-atom entanglement fidelity of F>0.892(23), we observe a significant violation of a CHSH Bell inequality of S=2.578(75)—above the classical limit of 2—and a quantum bit error rate of 0.078(9). For the implemented random key-bases protocol, this results in a secret key rate of 0.07 bits per entanglement generation event in the asymptotic limit, and thus demonstrates the system's capability to generate secret keys in a device-independent setting. The second topic is long-distance entanglement distribution over optical fibres, for which it is essential to operate at telecom wavelengths to overcome high attenuation losses. Most quantum system under active research, however, operate in the visible or near infrared. We employ polarization-preserving quantum frequency conversion in both nodes to transform the wavelength of the photons that are entangled with the atoms from 780 nm to the telecom S band. Enabled by an unprecedented external device conversion efficiency of 57% and minimal induced noise, we report on the observation of atom-photon and heralded atom-atom entanglement generated over telecom fibre links with a length up to 33 km. We analyse the entanglement fidelity for different fibre link lengths and show that for longer links the fidelity is mainly limited by atomic decoherence. The atom-atom entanglement is analysed only after receiving the heralding signal including a time delay to account for classical communication times between the nodes to simulate a realistic quantum network link scenario. The results presented in this thesis pave the way towards the ultimate form of quantum secure communications in future quantum networks and are a milestone on the road to realise long-distance quantum network links

Homophobia in Catholic schools: An exploration of teachers’ rights and experiences in Canada and Australia

Little is known about the experiences of non-heterosexual educators in Catholic schools. This international comparative analysis reveals previously unreported data from Australian and Canadian qualitative studies that examine the experiences of lesbian, gay, bisexual, transgender and intersex (LGBTI) teachers, and LGBTI Allies from Australia and Canada who are currently teaching or have taught in Catholic schools. Bringing their work together for the first time, the two lead researchers compare their investigations and reveal disheartening similarities with religiously inspired homophobia despite differing legal and policy contexts of the two countries. These two studies reveal that LGBTI teachers, and LGBTI Allies, rely on their personal beliefs and local school community culture and policies to understand their equality rights and this has significant implications for the field of education

Opting out increases HIV testing in a large sexually transmitted infections outpatient clinic

In January 2007, opt-out HIV testing replaced provider-initiated testing at the sexually transmitted infections (STI) outpatient clinic in Amsterdam, The Netherlands. The effect of the opt-out strategy on the uptake of HIV testing was studied and factors associated with refusal of HIV testing were identified. Data routinely collected at the STI clinic were analysed separately for men who have sex with men (MSM) and heterosexuals. Logistic regression analysis was used to identify factors associated with opting out. In 2007, 12% of MSM and 4% of heterosexuals with (presumed) negative or unknown HIV serostatus declined HIV testing. Refusals gradually decreased to 7% and 2% by the year end. In 2006, before the introduction of opt-out, 38% of MSM and 27% of heterosexuals declined testing. The proportion of HIV-positive results remained stable among MSM, 3.4% in 2007 versus 3.7% in 2006, and among heterosexuals, 0.2% in 2007 versus 0.3% in 2006. In both groups factors associated with opting out were: age >or=30 years, no previous HIV test, the presence of STI-related complaints and no risky anal/vaginal intercourse. Among heterosexuals, men and non-Dutch visitors refused more often; among MSM, those warned of STI exposure by sexual partners and those diagnosed with gonorrhoea or syphilis refused more often. An opt-out strategy increased the uptake of HIV testing. A sharp increase in testing preceeded a more gradual increase, suggesting time must pass to optimise the new strategy. A small group of visitors, especially MSM, still opt out. Counselling will focus on barriers such as fear and low risk perception among high-risk visitors considering opting ou

Long-lived quantum memory enabling atom-photon entanglement over 101 km telecom fiber

Long-distance entanglement distribution is the key task for quantum networks, enabling applications such as secure communication and distributed quantum computing. Here we report on novel developments extending the reach for sharing entanglement between a single $^{87}$Rb atom and a single photon over long optical fibers. To maintain a high fidelity during the long flight times through such fibers, the coherence time of the single atom is prolonged to 7 ms by applying a long-lived qubit encoding. In addition, the attenuation in the fibers is minimized by converting the photon's wavelength to the telecom S-Band via polarization-preserving quantum frequency conversion. This enables to observe entanglement between the atomic quantum memory and the emitted photon after passing 101 km of optical fiber with a fidelity better than 70.8$\pm$2.4%. The fidelity, however, is no longer reduced due to loss of coherence of the atom or photon but in the current setup rather due to detector dark counts, showing the suitability of our platform to realize city-to-city scale quantum network links.Comment: 11 pages, 8 figures, comments are welcom

Entangling single atoms over 33 km telecom fibre

Quantum networks promise to provide the infrastructure for many disruptive applications, such as efcient long-distance quantum communication and distributed quantum computing1,2 . Central to these networks is the ability to distribute entanglement between distant nodes using photonic channels. Initially developed for quantum teleportation3,4 and loophole-free tests of Bell’s inequality5,6 , recently, entanglement distribution has also been achieved over telecom fbres and analysed retrospectively7,8 . Yet, to fully use entanglement over long-distance quantum network links it is mandatory to know it is available at the nodes before the entangled state decays. Here we demonstrate heralded entanglement between two independently trapped single rubidium atoms generated over fbre links with a length up to 33 km. For this, we generate atom–photon entanglement in two nodes located in buildings 400 m line-of-sight apart and to overcome high-attenuation losses in the fbres convert the photons to telecom wavelength using polarization-preserving quantum frequency conversion9 . The long fbres guide the photons to a Bell-state measurement setup in which a successful photonic projection measurement heralds the entanglement of the atoms10. Our results show the feasibility of entanglement distribution over telecom fbre links useful, for example, for device-independent quantum key distribution11–13 and quantum repeater protocols. The presented work represents an important step towards the realization of large-scale quantum network links