In this work, we present two new methods for Variational Quantum Circuit (VQC) Process Tomography onto n qubits systems: PT_VQC and U-VQSVD.Compared to the state of the art, PT_VQC halves in each run the required amount of qubits for process tomography and decreases the required seed states from 4nto 2n, ensuring high-fidelity reconstruction of the targeted unitary U. It is worth noting that, for a fixed reconstruction accuracy, PT_VQC achieves faster convergence per iteration compared to Quantum Deep Neural Network (QDNN) and tensor network schemes.U-VQSVD utilizes variational singular value decomposition to extract eigenvectors (up to a global phase) and their associated eigenvalues from an unknown unitary representing a general channel. We assess the performance of U-VQSVD by executing an attack on a non-unitary channel Quantum Physical Unclonable Function (QPUF), outperforming an uninformed impersonation attack by a factor of 2 to 5, depending on the qubit dimension.For the two presented methods, we propose a new approach to calculate the complexity of the displayed VQC, based on what we denote as optimal depth.

Physical Unclonable Functions (PUFs) are hardware devices with the assumption of possessing inherent, non-clonable physical randomness which leads to unique pairs of inputs and outputs that provide a secure fingerprint for cryptographic protocols like Authentication. In the case of quantum PUFs (QPUFs), the input-output pairs consists of quantum states instead of classical bitstrings, offering advantages over classical PUFs (CPUFs) such as challenge reusability via public channels and non-reliance over any trusted party due to the no-cloning theorem. In recent literature, a generalized mathematical frame-work for studying QPUFs was developed, which paved the way for having QPUF models with provable security. It was proved that existential unforgeability against Quantum Polynomial Time (QPT) adversaries cannot be achieved by any random unitary QPUF. Since measurements are non-unitary quantum processes, we define a QPUF based on random von Neumann measurements. We prove that such a QPUF is existentially unforgeable. Thus, we introduce the first model in existing literature that depicts such a high level of provable security. We also prove that the Quantum Phase Estimation (QPE) protocol applied on a Haar random unitary serves as an approximate implementation for this kind of QPUF as it approximates a von Neumann measurement on the eigenbasis of the unitary.

Nearest-neighbour clustering is a powerful set of heuristic algorithms that find natural application in the decoding of signals transmitted using the$M$-Quadrature Amplitude Modulation ($M$-QAM) protocol. Lloyd et al. proposed a quantum version of the algorithm that promised an exponential speedup. We analyse the performance of this algorithm by simulating the use of a hybrid quantum-classical implementation of it upon 16-QAM and experimental 64-QAM data. We then benchmark the implementation against the classical k-means clustering algorithm. The choice of quantum encoding of the classical data plays a significant role in the performance, as it would for the hybrid quantum-classical implementation of any quantum machine learning algorithm. In this work, we use the popular angle embedding method for data embedding and the swap test for overlap estimation. The algorithm is emulated in software using Qiskit and tested on simulated and real-world experimental data. The discrepancy in accuracy from the perspective of the induced metric of the angle embedding method is discussed, and a thorough analysis regarding the angle embedding method in the context of distance estimation is provided. We detail an experimental optic fibre setup as well, from which we collect 64-QAM data. This is the dataset upon which the algorithms are benchmarked. Finally, some promising current and future directions for further research are discussed.

An upper bound to the identification capacity of discrete memoryless wiretap channels is derived under the requirement of semantic effective secrecy, combining semantic secrecy and stealth constraints. A previously established lower bound is improved by applying it to a prefix channel, formed by concatenating an auxiliary channel and the actual channel. The bounds are tight if the legitimate channel is more capable than the eavesdropper’s channel. An illustrative example is provided for a wiretap channel that is composed of a point-to-point channel, and a parallel, reversely degraded wiretap channel. A comparison with results for message transmission and for identification with only secrecy constraint is provided.

Deterministic identification over K-input multiple-access channels with average input cost constraints is considered. The capacity region for deterministic identification is determined for an average-error criterion, where arbitrarily large codes are achievable. For a maximal-error criterion, upper and lower bounds on the capacity region are derived. The bounds coincide if all average partial point-to-point channels are injective under the input constraint, i.e. all inputs at one terminal are mapped to distinct output distributions, if averaged over the inputs at all other terminals. The achievability is proved by treating the MAC as an arbitrarily varying channel with average state constraints. For injective average channels, the capacity region is a hyperrectangle. The modulo-2 and modulo-3 binary adder MAC are presented as examples of channels which are injective under suitable input constraints. The binary multiplier MAC is presented as an example of a non-injective channel, where the achievable identification rate region still includes the Shannon capacity region.

We study the problem of identification over a DMC wiretap channel under effective secrecy. In identification, due to the fact that single messages are compared to each other, all conditions are inherently semantic, and thus we are forced to consider semantic effective secrecy. We show that even effective secrecy “comes for free” by giving an achievability theorem for stealth identification. We use two concatenated transmission codes, the first one is a resolvability transmission code. The second code is an effective-secrecy transmission code. An achievable rate is derived for the problem.

Secret-sharing building blocks based on quantum broadcast communication are studied. The confidential capacity region of the pure-loss bosonic broadcast channel is determined with key assistance, under the assumption of the long-standing minimum output-entropy conjecture. If the main receiver has a transmissivity of $\eta\lt\frac{1}{2}$, then confidentiality solely relies on the key-assisted encryption of the one-time pad. We also address conference key agreement for the distillation of two keys, a public key and a secret key. A regularized formula is derived for the key-agreement capacity region. In the pure-loss bosonic case, the key-agreement region is included within the capacity region of the corresponding broadcast channel with confidential messages. We then consider a network with layered secrecy, where three users with different security ranks communicate over the same broadcast network. We derive an achievable layered-secrecy region for a pure-loss bosonic channel that is formed by the concatenation of two beam splitters.

We determine the semantic security capacity for quantum wiretap channels. We extend methods for classical channels to quantum channels to demonstrate that a strongly secure code guarantees a semantically secure code with the same secrecy rate. Furthermore, we show how to transform a non-secure code into a semantically secure code by means of biregular irreducible functions (BRI functions). We analyze semantic security for classical-quantum channels and for quantum channels.

The study of properties of randomly chosen quantum states has in recent years led to many insights into quantum entanglement. In this work, we study private quantum states from this point of view. Private quantum states are bipartite quantum states characterised by the property that carrying out simple local measurements yields a secret bit. This feature is shared by the maximally entangled pair of quantum bits, yet private quantum states are more general and can in their most extreme form be almost bound entangled. In this work, we study the entanglement properties of random private quantum states and show that they are hardly distinguishable from separable states and thus have low repeatable key, despite containing one bit of key. The technical tools we develop are centred around the concept of locally restricted measurements and include a new operator ordering, bounds on norms under tensoring with entangled states and a continuity bound for a relative entropy measure.

The study of properties of randomly chosen quantum states has in recent years led to many insights into quantum entanglement. In this work, we study private quantum states from this point of view. Private quantum states are bipartite quantum states characterized by the property that carrying out simple local measurements yields a secret bit. This feature is shared by the maximally entangled pair of quantum bits, yet private quantum states are more general and can in their most extreme form be almost bound entangled. In this work, we study the entanglement properties of random private quantum states and show that they are hardly distinguishable from separable states and thus have low repeatable key, despite containing one bit of key. The technical tools we develop are centered around the concept of locally restricted measurements and include a new operator ordering, bounds on norms under tensoring with entangled states and continuity bounds for relative entropy measures. A full version of this paper is accessible at: http://arxiv.org/abs/1801.2861 [1].