Quantum Technology: The Second Quantum Revolution

We are currently in the midst of a second quantum revolution. The first quantum revolution gave us new rules that govern physical reality. The second quantum revolution will take these rules and use them to develop new technologies. In this review we discuss the principles upon which quantum technology is based and the tools required to develop it. We discuss a number of examples of research programs that could deliver quantum technologies in coming decades including; quantum information technology, quantum electromechanical systems, coherent quantum electronics, quantum optics and coherent matter technology.Comment: 24 pages and 6 figure

Data security in photonic information systems using quantum based approaches

The last two decades has seen a revolution in how information is stored and transmitted across the world. In this digital age, it is vital for banking systems, governments and businesses that this information can be transmitted to authorised receivers quickly and efficiently. Current classical cryptosystems rely on the computational difficulty of calculating certain mathematical functions but with the advent of quantum computers, implementing efficient quantum algorithms, these systems could be rendered insecure overnight. Quantum mechanics thankfully also provides the solution, in which information is transmitted on single-photons called qubits and any attempt by an adversary to gain information on these qubits is limited by the laws of quantum mechanics. This thesis looks at three distinct different quantum information experiments. Two of the systems describe the implementation of distributing quantum keys, in which the presence of an eavesdropper introduces unavoidable errors by the laws of quantum mechanics. The first scheme used a quantum dot in a micropillar cavity as a singlephoton source. A polarisation encoding scheme was used for implementing the BB84, quantum cryptographic protocol, which operated at a wavelength of 905 nm and a clock frequency of 40 MHz. A second system implemented phase encoding using asymmetric unbalanced Mach-Zehnder interferometers, with a weak coherent source, operating at a wavelength of 850 nm and pulsed at a clock rate of 1 GHz. The system used depolarised light propagating in the fibre quantum channel. This helps to eliminate the random evolution of the state of polarisation of photons, as a result of stress induced changes in the intrinsic birefringence of the fibre. The system operated completely autonomously, using custom software to compensate for path length fluctuations in the arms of the interferometer and used a variety of different single-photon detector technologies. The final quantum information scheme looked at quantum digital signatures, which allows a sender, Alice, to distribute quantum signatures to two parties, Bob and Charlie, such that they are able to authenticate that the message originated from Alice and that the message was not altered in transmission

Semi-Quantum Conference Key Agreement (SQCKA)

A need in the development of secure quantum communications is the scalable extension of key distribution protocols. The greatest advantage of these protocols is the fact that its security does not rely on mathematical assumptions and can achieve perfect secrecy. In order to make these protocols scalable, has been developed the concept of Conference Key Agreements, among multiple users. In this thesis we propose a key distribution protocol among several users using a semi-quantum approach. We assume that only one of the users is equipped with quantum devices and generates quantum states, while the other users are classical, i.e., they are only equipped with a device capable of measuring or reflecting the information. This approach has the advantage of simplicity and reduced costs. We prove our proposal is secure and we present some numerical results on the lower bounds for the key rate. The security proof applies new techniques derived from some already well established work. From the practical point of view, we developed a toolkit called Qis|krypt⟩ that is able to simulate not only our protocol but also some well-known quantum key distribution protocols. The source-code is available on the following link: - https://github.com/qiskrypt/qiskrypt/.Uma das necessidades no desenvolvimento de comunicações quânticas seguras é a extensão escalável de protocolos de distribuição de chaves. A grande vantagem destes protocolos é o facto da sua segurança não depender de suposições matemáticas e poder atingir segurança perfeita. Para tornar estes protocolos escaláveis, desenvolveu-se o conceito de Acordo de Chaves de Conferência, entre múltiplos utilizadores. Nesta tese propomos um protocolo para distribuição de chaves entre vários utilizadores usando uma abordagem semi-quântica. Assumimos que apenas um dos utilizadores está equipado com dispositivos quânticos e é capaz de gerar estados quânticos, enquanto que os outros utilizadores são clássicos, isto é, estão apenas equipados com dispositivos capazes de efectuar uma medição ou refletir a informação. Esta abordagem tem a vantagem de ser mais simples e de reduzir custos. Provamos que a nossa proposta é segura e apresentamos alguns resultados numéricos sobre limites inferiores para o rácio de geração de chaves. A prova de segurança aplica novas técnicas derivadas de alguns resultados já bem estabelecidos. Do ponto de vista prático, desenvolvemos uma ferramenta chamada Qis|krypt⟩ que é capaz de simular não só o nosso protocolo como também outros protocolos distribuição de chaves bem conhecidos. O código fonte encontra-se disponível no seguinte link: - https://github.com/qiskrypt/qiskrypt/

Entropic uncertainty relations and their applications

© 2017 American Physical Society. Heisenberg's uncertainty principle forms a fundamental element of quantum mechanics. Uncertainty relations in terms of entropies were initially proposed to deal with conceptual shortcomings in the original formulation of the uncertainty principle and, hence, play an important role in quantum foundations. More recently, entropic uncertainty relations have emerged as the central ingredient in the security analysis of almost all quantum cryptographic protocols, such as quantum key distribution and two-party quantum cryptography. This review surveys entropic uncertainty relations that capture Heisenberg's idea that the results of incompatible measurements are impossible to predict, covering both finite- and infinite-dimensional measurements. These ideas are then extended to incorporate quantum correlations between the observed object and its environment, allowing for a variety of recent, more general formulations of the uncertainty principle. Finally, various applications are discussed, ranging from entanglement witnessing to wave-particle duality to quantum cryptography

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

No-signalling attacks and implications for (quantum) nonlocality distillation

The phenomenon of nonlocality, which can arise when entangled quantum systems are suitably measured, is perhaps one of the most puzzling features of quantum theory to the philosophical mind. It implies that these measurement statistics cannot be explained by hidden variables, as requested by Einstein, and it thus suggests that our universe may not be, in principle, a well-determined entity where the uncertainty we perceive in physical observations stems only from our lack of knowledge of the whole. Besides its philosophical impact, nonlocality is also a resource for information- theoretic tasks since it implies secrecy: If nonlocality limits the predictive power that any hidden variable (in the universe) can have about some observations, then it limits in particular the predictive power of a hidden variable held by an adversary in a cryptographic scenario. We investigate whether nonlocality alone can empower two parties to perform unconditionally secure communication in a feasible manner when only a provably minimal set of assumptions are made for such a task to be possible — independently of the validity of any physical theory (such as quantum theory). Nonlocality has also been of interest in the study of foundations of quantum theory and the principles that stand beyond its mathematical formalism. In an attempt to single out quantum theory within a broader set of theories, the study of nonlocality may help to point out intuitive principles that distinguish it from the rest. In theories where the limits by which quantum theory constrains the strength of nonlocality are surpassed, many “principles” on which an information theorist would rely on are shattered — one example is the hierarchy of communication complexity as the latter becomes completely trivial once a certain degree of nonlocality is overstepped. In order to study the structure of such super-quantum theories — beyond their aforementioned secrecy aspects — we investigate the phenomenon of distillation of nonlocality, the ability to distill stronger forms of nonlocality from weaker ones. By exploiting the inherent connection between nonlocality and secrecy, we provide a novel way of deriving bounds on nonlocality-distillation protocols through an ad versarial view to the problem

Storing, single photons in broadband vapor cell quantum memories

Single photons are an essential resource for realizing quantum technologies. Together with compatible quantum memories granting control over when a photon arrives, they form a foundational component both of quantum communication and quantum information processing. Quality solid-state single photon sources deliver on the high bandwidths and rates required for scalable quantum technology, but require memories that match these operational parameters. In this thesis, I report on quantum memories based on electromagnetically induced transparency and built in warm rubidium vapor, with such fast and high bandwidth interfaces in mind. I also present work on a heralded single photon source based on parametric downconversion in an optical cavity, operated in a bandwidth regime of a few 100s of megahertz. The systems are characterized on their own and together in a functional interface. As the photon generation process is spontaneous, the memory is implemented as a fully reactive device, capable of storing and retrieving photons in response to an asynchronous external trigger. The combined system is used to demonstrate the storage and retrieval of single photons in and from the quantum memory. Using polarization selection rules in the Zeeman substructure of the atoms, the read-out noise of the memory is considerably reduced from what is common in ground-state storage schemes in warm vapor. Critically, the quantum signature in the photon number statistics of the retrieved photons is successfully maintained, proving that the emission from the memory is dominated by single photons. We observe a retrieved single-photon state accuracy of $g_{c,\,\text{ret}}^{(2)}=0.177(23)$ for short storage times, which remains $g_{c,\,\text{ret}}^{(2)}<0.5$ throughout the memory lifetime of $680(50)\,$ns. The end-to-end efficiency of the memory interfaced with the photon source is $\eta_{e2e}=1.1(2)\,\%$, which will be further improved in the future by optimizing the operating regime. With its operation bandwidth of $370\,$MHz, our system opens up new possibilities for single-photon synchronization and local quantum networking experiments at high repetition rates