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Ubiquitous Quantum Communications

Periodic Reporting for period 2 - QuantCom (Ubiquitous Quantum Communications)

Reporting period: 2019-12-01 to 2021-05-31

There is tangible evidence that every Euro invested in telecommunications on average results in 40 percent wealth-creation benefit, because a vibrant economy critically hinges on the availability of flawless services. This has become quite plausible during COVID pandemic, because the entire global economy has been totally restructured with the aid of seamless networking.

THE PROBLEM: Fuelled by the spectacular multimedia electronics revolution facilitated by advances in both signal processing and nano-technology during the past few decades, semi-conductor technology has reached nano-scale integration density and, as a result, science and engineering only has two potential avenues for its continued evolution. Option one is to make the semiconductor chips larger for accommodating more powerful signal processing, but this results in excessively large chips, which easily break up, hence reducing the fraction of faultless chips. Alternatively, we have to conceive techniques of dealing with the quantum effects.

ITS IMPORTANCE TO SOCIETY AS A WHOLE: Hence this project aims for contributing to the 'quantum jig-saw puzzle', with special emphasis on the enabling techniques of ubiquitous quantum communications, potentially leading to job- and wealth-creation on a similar scale to the economic benefits of flawless classic wireless communications.

THE OVERALL OBJECTIVES: The ultimate objective of the project is to build bridges across the exciting fields of quantum physics, mathematics, computer science and hardware aspects of quantum communications. In this context the radical goal of this multi-disciplinary project is to conceive new enabling techniques both for classical-domain as well as for quantum-domain communications, whilst paving the way for the introduction of perfectly secure quantum-communications solutions.
WP 1: Quantum Decoherence Mitigation and Quantum Codes

A suite of quantum error correction codes (QECC) was designed for mitigating the decoherence and was disseminated in leading-edge IEEE journals, as seen in the list of publications. The most inventive solutions are our short topological block codes, which correct the errors before the decoherence results in catastrophic error-proliferation.

WP 2: QUANTUM KEY DISTRIBUTION

QKD constitutes one of the most well-known applications of quantum communication but, in reality, it only represents a secret key negotiation protocol. The seminal QKD proposal is commonly referred to as the Bennett-Brassard protocol (BB84), which is based on the so-called 'prepare-and-measure' protocol, while the E91 protocol is based on pre-shared entanglement. As a further development, in 2003 Deng and Long conceived the radical concept of quantum secure direct-communication (QSDC), which constitutes a fully-fledged confidential quantum communications protocol, rather than being a pure secret key negotiation procedure. However, there are still numerous open challenges in the way of wide-spread QSDC, such as its limited attainable rate and distance, as well as its reliance on quantum memory. These challenges were solved by designing several QSDC schemes, which no longer rely on quantum memory. This was achieved by communicating using so-called Einstein, Podelski, Rosen (EPR) pairs amalgamated with sophisticated error correction codes, as detailed in the related IEEE Transactions papers.

WP 3: ENTANGLEMENT SWAPPING

QUANTUM TELEPORTATION is the key communication functionality of the Quantum Internet, allowing the "transmission'' of qubits without the physical transfer of the particle storing the qubit. Quantum teleportation is facilitated by the action of quantum entanglement, a somewhat counter-intuitive physical phenomenon with no direct counterpart in the classical world. As a consequence, the very concept of the classical communication system model has to be redesigned to account for the peculiarities of quantum teleportation. This re-design is a crucial prerequisite for constructing any effective quantum communication protocols. The aim of this WP is to shed light on this key concept, with the objective of highlighting the fundamental differences between the transmission of classical information versus the teleportation of quantum information. This allowed us to investigate quantum teleportation and to tackle some of the challenges in the design of practical quantum teleportation in the face of the ubiquitous quantum decoherence, which has no direct counterpart in the classical world.

WP 4: THE PRACTICAL GRAND CHALLENGE: FLYING AERONAUTICAL AD HOC NETWORKS

Relying on multi-hop communication techniques, aeronautical ad hoc networks (AANETs) seamlessly integrate ground base stations (BSs) and satellites into aircraft communications for enhancing the on-demand connectivity of planes in the air. In this context we have quantified the performance of the classic shortest-path routing algorithm in the context of the real flight data collected in the North-Atlantic Region. Specifically, in this integrated AANET context we have investigated the shortest-path routing problem, with the objective of minimizing the total delay of the in-flight connection from the ground BS, subject to certain minimum-rate constraints for all selected links, in support of low-latency and high-speed services. Inspired by the best-first search and priority queue concepts, we modelled the problem formulated by a weighted digraph and found the optimal route based on the shortest-path algorithm. The results demonstrate that aircraft-aided multi-hop communications are capable of reducing the total delay of satellite communications, when relying on real historical flight data.
WP 1: DESIGNING IMPROVED QUANTUM CODES

At the time of writing most quantum circuits have a shorter coherence time than the time required for carrying out the decoding of long QECCs. Hence low-complexity yet powerful short codes will be designed for mitigating the effects of short coherence times.

WP 2/WP 3: DESIGNING IMPROVED QKD/QSDC AND ENTANGLEMENT SWAPPING SCHEMES

The availability of pre-shared entanglement amongst remote quantum nodes is required for facilitating quantum communications even in the face of quantum decoherence. Conventionally, the quantum decoherence in quantum communications is mitigated by performing the consecutive steps of quantum entanglement distillation followed by quantum teleportation. However, this conventional approach imposes a long delay. This will be circumvented by direct quantum communication schemes capable of flawless operation in the face of realistic noisy pre-shared entanglement. We will study the qubit error ratio, yield and goodput.

WP 4: THE PRACTICAL GRAND CHALLENGE: FLYING AERONAUTICAL AD HOC NETWORKS Relying on Quantum-Search Aided Communications

Finally, in the second half of the project there will be an increased emphasis on investigating the grand challenge set out in the proposal in the context of aeronautical ad hoc networks (AANET) and their Pareto optimization. Packet routing in aeronautical ad hoc networks is challenging due to their high-dynamic topology. In this context we will invoke deep reinforcement learning for routing in AANETs, aiming for minimizing the end-to-end (E2E) delay as our first objective. Specifically, a deep Q-network (DQN) will be conceived for capturing the relationship between the optimal routing decision and the local geographic node-topology observed by the forwarding node.
The overall system concept of QuantCom