The Networked Quantum Technologies (NQIT) Hub is one of four Quantum Hubs funded by the UK government. These are tasked with the technology development for a range of applications of quantum phenomena. The NQIT hub is a partnership of nine UK universities, and more than 30 institutions worldwide, and is focused on building scalable quantum computing devices and subsystems. The 'flagship' goal is the fabrication of an information processing network consisting of 20 processing modes with 20 qubits in each. To implement this processing nodes use ion-traps to store and process information, and the interconnection network between the traps uses photonic links. In this presentation an outline of the architecture will be presented, together with the engineering approaches we are using achieve this goal, together with the challenges. Potential applications will also be discussed.

It seems impossible to endow opportunistic characteristic to quantum network on the basis that quantum channel cannot be overheard without disturbance. We propose an opportunistic quantum network coding scheme by taking full advantage of channel characteristic of quantum teleportation. On one hand, it utilizes quantum channel for the secure transmission of quantum states and can detect eavesdroppers by means of quantum channel verification. On the other hand, it utilizes classical channel for opportunistic listening and can acquire coding opportunity from neighbors. Analysis results show that our scheme can reduce the times of transmissions for relay nodes and can effectively defend against classical passive attack and quantum active attack.

In this paper, we consider the problem of how to efficiently and perfectly transmit unknown quantum states in muti-unicast networks. We propose a nonlinear quantum network coding scheme with classical communication resource. On the conditions of allowance of free classical communication between adjacent nodes in a network, as well as unlimited capacities of classical channels, perfect quantum network coding is available. By means of phase deviation correction, we design the encoding procedure of nonlinear coding schemes. Moreover, we also provide the upper bound on the total number of classical communication which is required to achieve perfect quantum communication, as well as analysis of the scheme's property.

Quantum entanglement among remote agents is a key enabler for many important quantum communication operations. To establish entanglement between two remote agents, one agent can locally generate entangled qubit pairs and transmit one of the qubits to other agents through quantum channels. However, the transmitted qubits will inevitably be contaminated by the noise inherent in the quantum channels. To address this challenge, quantum entanglement distillation (QED) algorithms have been developed to generate highly-entangled qubit pairs from many contaminated ones. Nevertheless, existing algorithms either have low efficiency or require large-scale quantum circuits. This paper focuses on phase-damping channel and proposes an efficient QED algorithm that only require small-scale quantum circuit.

We will review INRIM efforts to produce a metrology for quantum communication purposes, ranging from the establishment of measurement procedures for specific quantities related to QKD components, namely pseudo single-photon sources, and detectors , to the implementation of novel QKD protocol based on paradigm other than non-commuting observables, to the realization of an almost noiseless single-photon source, to the development of quantum tomographic techniques.

Eavesdropper tapping communication links is a common problem encountered in communication systems. The famous no cloning theorem and the uncertainity principle are instrumental in ensuring secrecy of classical information when transmitted using quantum states. With Wyner's wiretap channel as motivation, we study the effect of a unitarily interacting probe on the extraction of information by an eavesdropper. We motivate the theoretical results by considering a practical example of a unitary interaction between the transmitted qubit and the probe used by an eavesdropper. The role played by entanglement is also studied in the setting of single shot communication between sender and receiver nodes.

This work presents a secret key agreement (SKA) protocol with two-way training scheme for a base station (BS) equipped with a large antenna array in a multiuser time-division duplex system in the presence of an active eavesdropper. In particular, this work focuses the pilot contamination attack (PCA) in which the eavesdropper transmits its target user's training sequence in hopes of acquiring possible information leak by steering beam towards the eavesdropper when the BS transmits random sequences to legitimate users for sharing common randomness as a source of secret key. To nullify the PCA and enhance the secret key generation efficiency, we propose a secret key agreement protocol with two-way training scheme. As an essential part of the protocol, we first derive an estimator for channel gain from the BS to the eavesdropper. The protocol will also be analyzed in terms of secrecy outage probability and average secret key rate to see how the two-way training scheme affects on the security performances. Our study shows that the two-way training scheme in the SKA protocol improves the security performances especially in a fast fading environment.

We propose two constructions, namely Construction 1 and Construction 2, for the design of proto-graph quantum LDPC codes by exploiting conventional quasi cyclic proto-graph LDPC codes with lift operation, and tensor product operation, respectively. We show that by carefully designing the proto-matrix using quadratic residue sets of parameters $p = 4n\pm 1$, the proposed Construction 1 yields proto-graph quantum LDPC codes of various rates. Furthermore, by performing tensor product between two non-binary parity-check matrices obtained from idempotent polynomials of a quadratic residue set, the proposed Construction 2 yields a class proto-graph quantum LDPC codes with rate at least 0.9. The performance of the proposed codes over quantum depolarizing channel with iterative sum-product decoding algorithm is shown.

We propose and study the feasibility of wireless quantum key distribution (QKD) in indoor environments. Such systems are essential in providing wireless access to the developing quantum communications networks. We find a practical regime of operation, where, in the presence of background noise and loss, secret keys can be exchanged. Our findings identify the trade-off between the acceptable amount of background noise and the receiver field of view, where the latter determines the accessibility of the QKD system. In particular, we show that, with a proper setting, we can provide mobility for the QKD users without imposing stringent conditions on beam steering.