In this work we focus on evaluating the effectiveness of two non-Gaussian operations, photon subtraction (PS) and quantum scissors (QS) in terms of Continuous Variable (CV)-Quantum Key Distribution (QKD) over lossy channels. Each operation is analysed in two scenarios, one with the operation applied transmitter-side to a Two-Mode Squeezed Vacuum (TMSV) state and a second with the operation applied to the TMSV state receiver-side. We numerically evaluate the entanglement and calculate the QKD key rates produced in all four possible scenarios. Our results show that for a fixed value of initial squeezing in the TMSV state, the states produced by the non-Gaussian operations are more robust to loss, being capable of generating higher key rates for a given loss. More specifically, we find that for values of initial TMSV squeezing below 1.5dB the highest key rates are obtained by means of transmitter-QS. On the other hand, for squeezing above 1.5dB we find that receiver-PS produces the best key rates. Our results will be important for future CV-QKD implementations over free-space channels, such as the satellite-Earth channel.

Free space Quantum Key Distribution (QKD) links between terminals that move relative to each other pose a number of challenges. The beam-steering components (typically mirrors) and the relative movement lead to variable birefringence, for particular configuration there will also be fixed birefringence. The paper proposes a polarization calibration scheme suitable for real-time operation. Simulation and experiment are performed to verify the proposed scheme.

We show that the Quantum Generative Adversarial Network (QGAN) paradigm can be employed by an adversary to learn generating data that deceives the monitoring of a Cyber-Physical System (CPS) and to perpetrate a covert attack. As a test case, the ideas are elaborated considering the navigation data of a Micro Aerial Vehicle (MAV). A concrete QGAN design is proposed to generate fake MAV navigation data. Initially, the adversary is entirely ignorant about the dynamics of the CPS, the strength of the approach from the point of view of the bad guy. A design is also proposed to discriminate between genuine and fake MAV navigation data. The designs combine classical optimization, qubit quantum computing and photonic quantum computing. Using the PennyLane software simulation, they are evaluated over a classical computing platform. We assess the learning time and accuracy of the navigation data generator and discriminator versus space complexity, i.e., the amount of quantum memory needed to solve the problem.

This paper proposes the use of photon-added coherent states (PACSs) in quantum pulse position modulation (PPM). PACSs are the most simple examples of non-Gaussian non-classical states of light, which cannot be described by the classical laws of physics. We show that employing a PACS in PPM modulation improves the quantum communication reliability with respect to using a coherent state with the same energy. We also evaluate the impact of thermal noise, phase and energy loss, on the system performance.

Quantum sensing using entangled photon pairs is gradually establishing itself as a cornerstone in modern communication networks. The unrivalled capability of quantum sensing techniques in distilling signals plagued by noise, renders them suitable for deployment in backscatter communication networks. Several attempts have recently been made to utilize pairs of entangled signal-idler photons to enhance the sensitivity of photo-detection in backscatter networks. However, these attempts always assumed the lossless retention of the idler mode, which is a challenging task from a practical perspective. In this study we examine the extent to which quantum correlations remain after retaining the idler mode in a lossy memory element, while the signal photon propagates through a lossy thermal channel as usual. We also examine briefly two different detection schemes and estimate the signal-to-noise ratio from their different perspectives. This new proposed model is one step further towards the realization of quantum backscattering for practical applications.

A semi-quantum key distribution (SQKD) protocol allows two users A and B to establish a shared secret key that is secure against an all-powerful adversary E even when one of the users (e.g., B) is semi-quantum or classical in nature while the other is fully-quantum. A mediated SQKD protocol allows two semi-quantum users to establish a key with the help of an adversarial quantum server. We introduce the concept of a multi-mediated SQKD protocol where two (or more) adversarial quantum servers are used. We construct a new protocol in this model and show how it can withstand high levels of quantum noise, though at a cost to efficiency. We perform an information theoretic security analysis and, along the way, prove a general security result applicable to arbitrary MM-SQKD protocols. Finally, a comparison is made to previous (S)QKD protocols.

The Schrodinger's cat state is created from the macroscopic superposition of coherent states and is well-known to be a useful resource for quantum information processing protocols. In order to extend such protocols to a global scale, we study the continuous variable (CV) teleportation of the cat state via a satellite in low-Earth-orbit (LEO). Past studies have shown that the quantum character of the cat state can be preserved after CV teleportation, even when taking into account the detector efficiency. However, the channel transmission loss has not been taken into consideration. Traditionally, optical fibers with fixed attenuation are used as teleportation channels. Our results show that in such setup, the quantum character of the cat state is lost after 5 dB of channel loss. We then investigate the free-space channel between the Earth and a satellite, where the loss is caused by atmospheric turbulence. For a down-link channel with 5 to 10 dB of loss, we find that the teleported state preserves higher fidelity relative to a fixed attenuation channel. The results in this work will be important for deployments over Earth-Satellite channels of protocols dependent on cat-state qubits.

Due to scalability issue in current quantum technologies, many quantum algorithms can only be implemented for small size problems. Scalability remains a bottleneck for current quantum technologies. For solving real-life size hard problems in the near-term, we explore classes of search graphs that can be efficiently reduced to a implementable scale for many quantum algorithms. The reduced Hamiltonian preserves the dynamics of the original Hamiltonian. One of the quantum algorithms we choose for the reduced Hamiltonian is continuous time quantum walk (CTQW). We further show how to determine the correct value of the coupling factor of the underlying CTQW. With wrong couple factor values, the optimality (quadratic speedup) from CTQW might be lost. In this work, we extend the class of reducible graphs to complete bipartite graphs with random k edges removed. We further demonstrate through mathematical proof and simulation experiment using IBM QISKIT to show that the quadratic speed-up is preserved for the CTQW.