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As the vision of global-scale unconditional information security becomes gradually realized, the importance of inter-satellite quantum communications has been rapidly increasing. The recently proposed round-robin differential-phase-shift (RRDPS) quantum key distribution (QKD) protocol has attracted much attention not only due to its potential high error tolerance, but also due to its distinct feature that the information leakage can be bounded without monitoring signal disturbances. Despite many existing implementations over fiber-optic channels, the feasibility of RRDPS QKD over an inter-satellite channel is still unknown. Moreover, despite the current advances in orbital angular momentum (OAM) encoding and temporal mode (TM) encoding, most of the existing studies on RRDPS QKD are restricted to time-bin encoding. In this work, we remedy this situation by exploring the feasibility of performing RRDPS QKD using OAM encoding and TM encoding over an inter-satellite channel. Our results indicate that OAM encoding is preferable to time-bin encoding only under the circumstances where a low dimension and a large receiver aperture are used. However, we find that TM encoding is the best encoding scheme in RRDPS QKD over an inter-satellite channel. In particular, we show that TM encoding not only leads to the best performance and the largest feasible parameter range, but also, for the first time, enables all the theoretically available advantages of an increased dimension to be realized in the context of RRDPS QKD.

The Continuous-Variable (CV) quantum state of light allows for the teleportation of a Discrete-Variable (DV) photonic qubit. Such an operation is useful in the realm of hybrid quantum networks. However, it is known that the teleportation of a DV qubit via a Gaussian CV resource channel is severely limited under channel loss, with a teleportation fidelity beyond the classical limit unattainable for losses exceeding a small threshold of 0.5 dB. In this work, we present a new non-deterministic teleportation protocol that combines a Gaussian CV resource channel with a modified form of the Bell State Measurement that accommodates a DV-qubit fidelity beyond the classical limit for channel losses up to 20~dB. Beyond this orders of magnitude improvement, we also show how the use of non-Gaussian operations on the CV resource channel can lead to a DV-qubit fidelity approaching unity for any channel loss., Comment: 11 pages

In developing the global quantum Internet, quantum communication with low-earth-orbit satellites will play a pivotal role. Such communication will need to be two way: effective not only in the satellite-to-ground (downlink) channel but also in the ground-to-satellite channel (uplink). Given that losses on this latter channel are significantly larger relative to the former, techniques that can exploit the superior downlink to enhance quantum communication in the uplink should be explored. In this article, we do just that—exploring how continuous-variable entanglement in the form of two-mode-squeezed vacuum (TMSV) states can be used to significantly enhance the fidelity of ground-to-satellite quantum-state transfer relative to direct uplink-transfer. More specifically, through detailed phase-screen simulations of beam evolution through turbulent atmospheres in both the downlink and uplink channels, we demonstrate how a TMSV teleportation channel, created by the satellite, can be used to dramatically improve the fidelity of uplink coherent-state transfer relative to direct transfer. We then show how this, in turn, leads to the uplink-transmission of a higher alphabet of coherent states. Additionally, we show how non-Gaussian operations, acting on the received component of the TMSV state at the ground station, can lead to further enhancement of the fidelity. Since TMSV states can readily be produced in situ on a satellite platform and form a reliable teleportation channel for most quantum states, our work suggests that future satellites forming part of the emerging quantum Internet should be designed with uplink-communication via quantum teleportation in mind.

Anonymity networks are becoming increasingly popular in today's online world as more users attempt to safeguard their online privacy. Tor is currently the most popular anonymity network in use and provides anonymity to both users and services (hidden services). However, the anonymity provided by Tor is also being misused in various ways. Hosting illegal sites for selling drugs, hosting command and control servers for botnets, and distributing censored content are but a few such examples. As a result, various parties, including governments and law enforcement agencies, are interested in attacks that assist in de-anonymising the Tor network, disrupting its operations, and bypassing its censorship circumvention mechanisms. In this survey paper, we review known Tor attacks and identify current techniques for the de-anonymisation of Tor users and hidden services. We discuss these techniques and analyse the practicality of their execution method. We conclude by discussing improvements to the Tor framework that help prevent the surveyed de-anonymisation attacks., Comment: This work is published in IEEE Communications Surveys & Tutorials and is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/. Link to the article https://ieeexplore.ieee.org/abstract/document/9471821

High-purity single-photon sources (SPS) that can operate at room temperature are highly desirable for a myriad of applications, including quantum photonics and quantum key distribution. In this work, we realize an ultra-bright solid-state SPS based on an atomic defect in hexagonal boron nitride (hBN) integrated with a solid immersion lens (SIL). The SIL increases the source efficiency by a factor of six, and the integrated system is capable of producing over ten million single photons per second at room temperature. Our results are promising for practical applications of SPS in quantum communication protocols.

In practical Quantum Key Distribution (QKD) deployments we would like to design QKD solutions that provide for a target QKD key rate, in bits/pulse, at a specified upper-limit on the failure probability. However, in the finite-signalling setting, in which all real-world QKD systems exist, the common practice of achieving such a solution fails to deliver the maximum throughput rate of the classical decoder. This in turn means that the possibility that classical reconciliation becomes the bottleneck of the entire QKD protocol is not minimised. A design strategy that minimises this latter possibility, whilst achieving a target QKD rate with a target ceiling on the failure probability has not been developed – a situation we remedy here. Although our new design strategy detailed here is for LDPC codes and applied to two specific QKD protocols, the same strategy is generally applicable to all classical decoders and all QKD protocols. It is also deployable even in circumstances where the quantum bit error is variable, such as in satellite QKD systems.

The work reported here utilizes the best aspects of information theory and deep-learning concepts so as to provide, for the first time, a solution for a real-world location verification system (LVS) in the context of vehicular networks. it is well established that global positioning system coordinates supplied by vehicles will be a vital component of such emerging networks. This supplied location information, if erroneous and not verified, can seriously degrade the overall system performance and lead to significant safety issues. A number of location verification protocols and systems have been developed to address this important problem but all have operational constraints and performance limitations due to their requirement for ideal static channel conditions and assumed threat models. In this article, we remove such limitations by designing a neural-network-based LVS (NN-LVS) that can accommodate a priori unknown channel conditions and unknown threat models. Under most channel conditions, the NN-LVS shows a performance improvement of 50%, or more, relative to other LVSs. We also derive a new information-theoretic bound on the total error for an LVS and show how this new bound allows for a useful tradeoff in learning-time versus verification-performance for the NN-LVS. We demonstrate an improved performance for the NN-LVS within the context of vehicular networks using time of arrival measurements of the vehicles’ transmitted signals measured at multiple verifying base stations. The work reported here, we believe, paves the way to the actual deployment in real-world conditions of LVSs for emerging vehicular networks.

The photonic Temporal Mode (TM) represents a possible candidate for the delivery of viable multidimensional quantum communications. However, relative to other multidimensional quantum information carriers such as the Orbital Angular Momentum (OAM), the TM has received less attention. Moreover, in the context of the emerging quantum internet and satellite-based quantum communications, the TM has received no attention. In this work, we remedy this situation by considering the traversal through the satellite-to-Earth channel of single photons encoded in TM space. Our results indicate that for anticipated atmospheric conditions the photonic TM offers a promising avenue for the delivery of high-throughput quantum communications from a satellite to a terrestrial receiver. In particular, we show how these modes can provide for improved multiplexing performance and superior quantum key distribution in the satellite-to-Earth channel, relative to OAM single-photon states. The levels of TM discrimination that guarantee this outcome are outlined and implications of our results for the emerging satellite-based quantum internet are discussed., 7 pages, 3 figures. Comments are welcome

Ultra-reliable and low-latency communication (URLLC) is one category of service to be provided by next-generation wireless networks. Motivated by increasing security concerns in such networks, this article focuses on physical layer security (PLS) in the context of URLLC. The PLS technique mainly uses transmission designs based on the intrinsic randomness of the wireless medium to achieve secrecy. As such, PLS is of lower complexity and incurs less latency than traditional cryptography. In this article, we first introduce appropriate performance metrics for evaluating PLS in URLLC, illustrating the tradeoff between latency, reliability, and security. We then identify the key challenging problems for achieving PLS for URLLC, and discuss the role that channel state information can have in providing potential solutions to these problems. Finally, we present our recommendations on future research directions in this emerging area., Comment: 6 pages, 4 figures; Accepted by IEEE Wireless Communications

Heterogeneous quantum networks consisting of mixed-technologies - Continuous Variable (CV) and Discrete Variable (DV) - will become ubiquitous as global quantum communication matures. Hybrid quantum-entanglement between CV and DV modes will be a critical resource in such networks. A leading candidate for such hybrid quantum entanglement is that between Schr\"odinger-cat states and photon-number states. In this work, we explore the use of Two-Mode Squeezed Vacuum (TMSV) states, distributed from satellites, as a teleportation resource for the re-distribution of our candidate hybrid entanglement pre-stored within terrestrial quantum networks. We determine the loss conditions under which teleportation via the TMSV resource outperforms direct-satellite distribution of the hybrid entanglement, in addition to quantifying the advantage of teleporting the DV mode relative to the CV mode. Our detailed calculations show that under the loss conditions anticipated from Low-Earth-Orbit, DV teleportation via the TMSV resource will always provide for significantly improved outcomes, relative to other means for distributing hybrid entanglement within heterogeneous quantum networks., Comment: 15 pages; updated references and minor clarifications

Designing a practical Continuous Variable (CV) Quantum Key Distribution (QKD) system requires an estimation of the quantum channel characteristics and the extraction of secure key bits based on a large number of distributed quantum signals. Meeting this requirement in short timescales is difficult. On standard processors, it can take several hours to reconcile the required number of quantum signals. This problem is exacerbated in the context of Low Earth Orbit (LEO) satellite CV-QKD, in which the satellite flyover time is constrained to be less than a few minutes. A potential solution to this problem is massive parallelisation of the classical reconciliation process in which a large-code block is subdivided into many shorter blocks for individual decoding. However, the penalty of this procedure on the important final secured key rate is non-trivial to determine and hitherto has not been formally analysed. Ideally, a determination of the optimal reduced block size, maximising the final key rate, would be forthcoming in such an analysis. In this work, we fill this important knowledge gap via detailed analyses and experimental verification of a CV-QKD sliced reconciliation protocol that uses large block-length low-density parity-check decoders. Our new solution results in a significant increase in the final key rate relative to non-optimised reconciliation. In addition, it allows for the acquisition of quantum secured messages between terrestrial stations and LEO satellites within a flyover timescale even using off-the-shelf processors. Our work points the way to optimised global quantum networks secured via fundamental physics., Comment: Updated References

In this work, we explore the feasibility of performing satellite-to-Earth quantum key distribution (QKD) using the orbital angular momentum (OAM) of light. Due to the fragility of OAM states the conventional wisdom is that turbulence would render OAM-QKD non-viable in a satellite-to-Earth channel. However, based on detailed phase screen simulations of the anticipated atmospheric turbulence we find that OAM-QKD is viable in some system configurations, especially if quantum channel information is utilized in the processing of post-selected states. More specifically, using classically entangled light as a probe of the quantum channel, and reasonably-sized transmitter-receiver apertures, we find that non-zero QKD rates are achievable on sea-level ground stations. Without using classical light probes, OAM-QKD is relegated to high-altitude ground stations with large receiver apertures. Our work represents the first quantitative assessment of the performance of OAM-QKD from satellites, showing under what circumstances the much-touted higher dimensionality of OAM can be utilized in the context of secure communications., Comment: 15 pages, 8 figures. Manuscript improved (mainly some symbol changes) for better clarity. Comments are welcome

In this work we study the use of noiseless attenuation and noiseless amplification, in terms of multi-mode Continuous-Variable (CV) Quantum Key Distribution (QKD) over satellite-to-ground channels. We propose an improved multi-mode CV-QKD protocol where noiseless attenuation and noiseless amplification operations are applied at the transmitter and the receiver, respectively. Our results show that consistent with single-mode CV-QKD, the noiseless amplification operation, when applied at the receiver, can increase the transmission distance and the channel noise tolerance of multi-mode CV-QKD. Different from single-mode CV-QKD, in multi-mode CV-QKD the key rate improvement offered by noiseless amplification can be further enhanced by adding noiseless attenuation at the transmitter.

In this work we investigate, for the first time, the bandwidth cost of a Location Verification System (LVS) on the control overhead of a well-known geographical routing protocol for vehicular networks. In our new system, at the onset of a new connection, all vehicles in the selected route report their locations using the Global Positioning System (GPS) to an LVS which is installed at a base station. The LVS verifies the reported locations and broadcasts the decisions to all vehicles in the transmission range. Via local updating of all routing tables, all vehicles are notified of the decisions. This results in a geographical routing protocol that is significantly more secure compared to one with no LVS in place. Specifically, the likelihood of a malicious vehicle causing significant degradation to the network routing via location spoofing is significantly reduced. We demonstrate that the additional control overhead needed for this more secure routing protocol is manageable, and the protocol remains scalable provided the likelihood of a vehicle being malicious is below a determined value. Our work is of significance in that proves for the first time that advanced location verification techniques can be seamlessly integrated into the hybrid routing protocols embedded in vehicular networks.

In vehicular networks, vehicle claimed positions should be independently verified to help protect the wider network against location-spoofing attacks. In this work, we propose a new solution to the problem of location verification using the Cramer-Rao Bound (CRB) on location accuracy. Compared to known-optimal solutions, our technique has the advantage that it does not depend on a priori information on the probability of any vehicle being malicious. To analyze the performance of our new solution, we compare its operation, under Received Signal Strength (RSS) inputs, with a known optimal solution for this problem that assumes the probability of a vehicle being malicious is known. The results show that our new solution provides close to optimal performance over a wide range of anticipated channel conditions. Our solution is simple to deploy and can easily be integrated into a host of vehicular applications that use location information as an input.

Artificial Intelligence (AI) solutions for wireless location estimation are likely to prevail in many real-world scenarios. In this work, we demonstrate for the first time how the Cramer-Rao upper bound on localization accuracy can facilitate efficient neural-network solutions for wireless location estimation. In particular, we demonstrate how the number of neurons for the network can be intelligently chosen, leading to AI location solutions that are not time-consuming to run and less likely to be plagued by over-fitting. Experimental verification of our approach is provided. Our new algorithms are directly applicable to location estimates in many scenarios including the Internet of Things, and vehicular networks where vehicular GPS coordinates are unreliable or need verifying. Our work represents the first successful AI solution for a communication problem whose neural-network design is based on fundamental information-theoretic constructs. We anticipate our approach will be useful for a wide range of communication problems beyond location estimation., This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible

Orbital angular momentum (OAM) of light has been considered as a useful degree of freedom for high-capacity quantum communications. However, the practical usage of OAM within the atmospheric channel is known to be limited by turbulence-induced decoherence effects. In this work, we study the feasibility of utilizing the OAM of light in satellite-based entanglement distribution. In order to determine whether OAM entanglement could be distributed within the context of satellite-based quantum communications, we study the quantum state evolution of a pair of OAM-entangled photons that traverse independent and identically distributed satellite-to-ground atmospheric channels. Our calculations indicate the performance levels that any post-selection, or distillation, scheme must have in order to achieve useful OAM entanglement distribution via a low-Earth-orbit satellite.

Non-Gaussian operations have been studied intensively in recent years due to their ability to increase the secret key rate for certain CV-QKD protocols. However, most previous studies on such protocols are carried out in a single-mode setting, even though in reality any quantum state contains multi-mode components in frequency space. In this work we investigate the use of non-Gaussian operations in a multi-mode CV-QKD system. Our main finding is that, contrary to single-mode CV-QKD systems, in generic multi-mode CV-QKD systems it is possible to use non-Gaussian operations to increase the optimized secret key rate. More specifically, we find that at losses of order 30dB, which represents a distance of order 160km and the effective maximum distance for CV-QKD, the key rate for multi-mode non-Gaussian operations can be orders of magnitude higher than single-mode operations. Our results are important for real-world CV-QKD systems especially those dependent on quantum error correction - a process that requires non-Gaussian effects.

Satellite-based quantum cryptography has already been demonstrated using discrete variable technology. Nonetheless, there is great interest in using weak coherent pulses to perform quantum key distribution (QKD) in the continuous variable (CV) paradigm. In this work, we study the feasibility of performing coherent-state CV-QKD via the satellite-to-ground channel. We use numerical methods to simulate atmospheric turbulence and compare the results with ground-based experimental data so as to confirm the validity of our approach. We find the results obtained from the numerical simulations agree well with the experimental data and represent an improvement over the state-of-the-art analytical models. Using the simulation results we then derive QKD key rates and find that useful non-zero key rates can be found over a limited range of zenith angles. Determination of QKD key rates using experimentally validated simulations of low-zenith-angle atmospheric channels represents an important step towards proving the feasibility of real-world satellite-to-Earth CV-QKD.

Continuous variable quantum key distribution (CV-QKD) offers information-theoretic secure key sharing between two parties. The sharing of a phase reference frame is an essential requirement for coherent detection in CV-QKD. Due to the potential attacks related to transmitting the local oscillator (LO) alongside quantum signals, there has been a focus on using local LOs (LLOs) to establish a shared phase reference. In this work, we develop a new noise model of a current state-of-the-art LLO scheme in the context of the satellite-to-Earth channel. In doing this, we encapsulate detailed phase-screen calculations that determine the coherent efficiency - a critical parameter in free-space CV-QKD that characterizes the wavefront aberrations caused by atmospheric turbulence. Using our new noise model we then determine the CV-QKD key rates for the satellite-to-Earth channel, secure under general attacks in the finite-size regime of the LLO scheme. Our results are of practical importance for next-generation quantum-enabled satellites that utilize multi-photon technology as opposed to single-photon technology.

Currently, quantum key distribution (QKD) using continuous variable (CV) technology has only been demonstrated over short-range terrestrial links. Here we attempt to answer whether CV-QKD over the much longer satellite-to-Earth channel is feasible. To this end, we first review the concepts and technologies that will enable CV-QKD over the satellite-to-Earth channels. We then consider, in the infinite key limit, the simplest-to-deploy QKD protocols, the coherent state (CS) QKD protocol with homodyne detection and the CS-QKD protocol with heterodyne detection. We then focus on the CS-QKD protocol with heterodyne detection in the pragmatic setting of finite keys, where complete security against general attacks is known. We pay particular attention to the relevant noise terms in the satellite-to-Earth channel and their impact on the secret key rates. In system set-ups where diffraction dominates losses, we find that the main components of the total excess noise are the intensity fluctuations due to scintillation, and the time-of-arrival fluctuations between signal and local oscillator. We conclude that for a wide range of pragmatic system models, CS-QKD with information-theoretic security in the satellite-to-Earth channel is feasible., Comment: 16 pages, 6 figures, 4 tables, Closest to Published Version

Location information claimed by devices will play an ever-increasing role in future wireless networks such as 5G, the Internet of Things (IoT). Against this background, the verification of such claimed location information will be an issue of growing importance. A formal information-theoretic Location Verification System (LVS) can address this issue to some extent, but such a system usually operates within the limits of idealistic assumptions on a-priori information on the proportion of genuine users in the field. In this work we address this critical limitation by using a Neural Network (NN) showing how such a NN based LVS is capable of efficiently functioning even when the proportion of genuine users is completely unknown a-priori. We demonstrate the improved performance of this new form of LVS based on Time of Arrival measurements from multiple verifying base stations within the context of vehicular networks, quantifying how our NN-LVS outperforms the stand-alone information-theoretic LVS in a range of anticipated real-world conditions. We also show the efficient performance for the NN-LVS when the users' signals have added Non-Line-of-Site (NLoS) bias in them. This new LVS can be applied to a range of location-centric applications within the domain of the IoT., 6 Pages, 5 figures

The Schr\"odinger'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. 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 a setup, the quantum character of the cat state is lost after 5dB 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 of less than 500km and 30dB 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., Comment: \c{opyright} 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works

We examine the secrecy performance of three artificial-noise-aided secure transmission schemes, namely, the partially adaptive, fully adaptive, and on–off schemes. To this end, we provide new analysis to facilitate the optimization of the fraction $\phi$ of the transmit power allocated to the useful signal and redundancy rate $R_{\rm{E}}$ . Surprisingly, our examination indicates that the partially adaptive scheme, in which only the codeword rate $R_{\rm{B}}$ varies with the instantaneous channel gains, significantly outperforms the on–off scheme, in which both $R_{\rm{B}}$ and $R_{\rm{E}}$ vary. This performance gain can be characterized in terms of a higher average secrecy rate, subject to an upper bound on the secrecy outage probability. Furthermore, our results also demonstrate that the partially adaptive scheme can achieve almost the same secrecy performance as the fully adaptive scheme, which is of a much higher complexity, where $\phi$ , $R_{\rm{B}}$ , and $R_{\rm{E}}$ all vary with the instantaneous channel gains.

In the context of location verification systems (LVSs), this letter proves that the knowledge of a legitimate user’s transmit power has no effect on the optimal performance of a received signal strength (RSS)-based LVS. Specifically, we prove that the detection performance of a generalized likelihood ratio test (GLRT), where the unknown transmit power is estimated, is identical to that of a differential likelihood ratio test (D-LRT). Our analysis also proves the asymptotic optimality of D-LRT for an RSS-based LVS with unknown transmit power. These results are important for real-world deployments of LVSs, since D-LRT incurs a significantly lower implementation cost relative to GLRT.

I explore the use of quantum information as a security enabler for the future driverless vehicle. Specifically, I investigate the role combined classical and quantum information can have on the most important characteristic of the driverless vehicle paradigm - the vehicle location. By using information-theoretic verification frameworks, coupled with emerging quantum-based location-verification procedures, I show how vehicle positions can be authenticated with a probability of error simply not attainable in classical-only networks. I also discuss how other quantum applications can be seamlessly encapsulated within the same vehicular communication infrastructure required for location verification. The two technology enablers required for the driverless quantum vehicle are an increase in current quantum memory timescales (likely) and wide-scale deployment of classical vehicular communication infrastructure (underway). I argue the enhanced safety features delivered by the `Quantum Car' mean its eventual deployment is inevitable., Comment: 4 pages, 1 figure

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 higher key rates. Our results will be important for future CV-QKD implementations over free-space channels, such as the Earth-satellite channel. Accepted for Publication in Quantum Communications and Information Technology Workshop, IEEE Globecom, HI, USA, December 2019., Copyright 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works

In this work we review recent research in some select topics in quantum communications. In each of the topics chosen we highlight how the techniques presented are of direct interest to quantum protocols operating over macro-scales (inter-continental ranges) as well as to quantum protocols operating over nano-scales (inter-chip ranges).

Satellite-based quantum communications enable a bright future for global-scale information security. However, the spin orbital momentum of light, currently used in many mainstream quantum communication systems, only allows for quantum encoding in a two-dimensional Hilbert space. The orbital angular momentum (OAM) of light, on the other hand, enables quantum encoding in higher-dimensional Hilbert spaces, opening up new opportunities for high-capacity quantum communications. Due to its turbulence-induced decoherence effects, however, the atmospheric channel may limit the practical usage of OAM. In order to determine whether OAM is useful for satellite-based quantum communications, we numerically investigate the detection likelihoods for OAM states that traverse satellite-to-ground channels. We show that the use of OAM through such channels is in fact feasible. We use our new results to then investigate design specifications that could improve OAM detection - particularly the use of advanced adaptive optics techniques. Finally, we discuss how our work provides new insights into future implementations of space-based OAM systems within the context of quantum communications., 7 pages, 7 figures

Quantum signal processing invokes the injection of abstract quantum mechanical frameworks into classical signal processing problems. In this work we apply this idea to the notion of optimal likelihood ratio tests within the context of the location verification problem. We first draw parallels with quantum mechanical measurements and the notion of generalized likelihood ratio measurements. As we show, these quite different measurement frameworks are mathematically similar since both can be described in the language of projections into subspaces - the projections removing the nuisance parameters of the underlying system in the latter case. We then show how the imposition of an ‘artificial’ mathematical constraint, borrowed from a similar constraint imposed on quantum mechanics by the uncertainty principle, is likely to assist in machine-learning solutions of the location verification problem - such solutions being more useful in real-world deployments.

Terahertz (THz) communication is a topic of much research in the context of high-capacity next-generation wireless networks. Quantum communication is also a topic of intensive research, most recently in the context of space-based deployments. In this work we explore the use of THz frequencies as a means to achieve quantum communication within a constellation of micro-satellites in Low-Earth-Orbit (LEO). Quantum communication between the micro-satellite constellation and high-altitude terrestrial stations is also investigated. Our work demonstrates that THz quantum entanglement distribution and THz quantum key distribution are viable deployment options in the micro-satellite context. We discuss how such deployment opens up the possibility for simpler integration of global quantum and wireless networks. The possibility of using THz frequencies for quantum-radar applications in the context of LEO deployments is briefly discussed., Comment: 7 pages, 6 figures

Hybrid entanglement swapping supports the teleportation of any arbitrary states, regardless of whether the quantum information in the state is encoded in Discrete Variables (DV) or Continuous Variables (CV). In this work, we study the CV teleportation channel created between two ground receivers via direct lossy-distribution from a low-Earth-orbit (LEO) satellite. Such a flexible teleportation protocol has the potential to interconnect a global array of quantum-enabled devices regardless of the different intrinsic technology upon which the devices are built. However, past studies of hybrid entanglement swapping have not accounted for channel transmission loss. Here we derive the general framework for teleporting an arbitrary input mode over a lossy CV teleportation channel. We investigate the specific case where the input modes are part of DV states entangled in the photon number basis, then identify the optimal teleportation strategy. Our results show that, relative to DV photon-number entanglement sourced directly from the satellite, there are circumstances where our teleported DV states retain higher entanglement quality. We discuss the implications of our new results in the context of generating a global network of ultra-secure communications between different quantum-enabled devices which possess line-of-sight connections to LEO satellites. Specifically, we illustrate the impact the teleportation process has on the key rates from a Quantum Key Distribution protocol., Comment: copyright 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works

Location information will play a very important role in emerging wireless networks such as Intelligent Transportation Systems, 5G, and the Internet of Things. However, wrong location information can result in poor network outcomes. It is therefore critical to verify all location information before further utilization in any network operation. In recent years, a number of information-theoretic Location Verification Systems (LVSs) have been formulated in attempts to optimally verify the location information supplied by network users. Such LVSs, however, are somewhat limited since they rely on knowledge of a number of channel parameters for their operation. To overcome such limitations, in this work we introduce a Machine Learning based LVS (ML-LVS). This new form of LVS can adapt itself to changing environments without knowing the channel parameters. Here, for the first time, we use real-world data to show how our ML-LVS can outperform information-theoretic LVSs. We demonstrate this improved performance within the context of vehicular networks using Received Signal Strength (RSS) measurements at multiple verifying base stations. We also demonstrate the validity of the ML-LVS even in scenarios where a sophisticated adversary optimizes her attack location., Comment: 5 pages, 3 figures

Non-Gaussian operations have been studied intensively in recent years due to their ability to enhance the entanglement of quantum states. However, most previous studies on such operations are carried out in a single-mode setting, even though in reality any quantum state contains multi-mode components in frequency space. Whilst there have been general frameworks developed for multi-mode photon subtraction (PS) and photon addition (PA), an important gap exists in that no framework has thus far been developed for multi-mode photon catalysis (PC). In this work we close that gap. We then apply our newly developed PC framework to the problem of continuous variable (CV) entanglement distribution via quantum-enabled satellites. Due to the high pulse rate envisioned for such systems, multi-mode effects will be to the fore in space-based CV deployments. After determining the entanglement distribution possible via multi-mode PC, we then compare our results with the entanglement distribution possible using multi-mode PS and PA. Our results show that multi-mode PC carried out at the transmitter is the superior non-Gaussian operation for certain regions of the parameter space. The size of this region is highly dependent on the squeezing of the initial state. When carried out at the receiver, multi-mode PC is again found to be the superior non-Gaussian operation for certain regions of the parameter space. In this latter region, the region size is more dependent on the channel loss. Our new results should prove valuable for next-generation deployments of CV quantum-enabled satellites.

Quantum Key Distribution (QKD) via satellite offers up the possibility of unconditionally secure communications on a global scale. Increasing the secret key rate in such systems, via photonic engineering at the source, is a topic of much ongoing research. In this work we investigate the use of photon-added states and photon-subtracted states, derived from two mode squeezed vacuum states, as examples of such photonic engineering. Specifically, we determine which engineered-photonic state provides for better QKD performance when implemented over channels connecting terrestrial receivers with Low-Earth-Orbit satellites. We quantify the impact the number of photons that are added or subtracted has, and highlight the role played by the adopted model for atmospheric turbulence and loss on the predicted key rates. Our results are presented in terms of the complexity of deployment used, with the simplest deployments ignoring any estimate of the channel, and the more sophisticated deployments involving a feedback loop that is used to optimize the key rate for each channel estimation. The optimal quantum state is identified for each deployment scenario investigated., Comment: Updated reference list

Quantum Key Distribution (QKD) has been regarded as a novel approach to establish a secure communication between two legitimate partners. With the recent success of space based experimental QKD systems, researchers are now focussing on implementing more practical and efficient QKD systems that meet the challenges faced in space-based implementations. One pressing problem is the need for efficient and easy to implement reconciliation schemes for practical QKD systems. Rate-adaptive schemes based on LDPC codes constitute an appealing implementation of the key reconciliation process since they cover the entire range of the channel parameter space with a limited set of pre-defined Mother codes. In this paper, we investigate in detail some rate- adaptive reconciliation schemes based on LDPC codes, illustrating how their performance compares to other set-ups in which fixed-rate non- adaptive LDPC codes optimised for different channel conditions are adopted. In particular, the impact rate-adaptive codes have on decoding complexity, and subsequently the overall secure key throughput, is quantified through full blown simulations of an entanglement-based version of the QKD protocol within the context of an entanglement source onboard a satellite. Our work highlights that even though rate adaptive schemes schemes for QKD reconciliation appear ideal for future space-based implementations of single- photon QKD, identification of ideal Mother codes for such schemes remains to be resolved.

We design a new secure transmission scheme in the relay wiretap channel where a source communicates with a destination through a decode-and-forward relay in the presence of spatially random-distributed eavesdroppers. For the sake of practicality, we consider a general antenna configuration in which the source, relay, destination, and eavesdroppers are equipped with multiple antennas. In order to confuse the eavesdroppers, we assume that both the source and the relay transmit artificial noise signals in addition to information signals. We first derive a closed-form expression for the transmission outage probability and an easy-to-compute expression for the secrecy outage probability. Notably, these expressions are valid for an arbitrary number of antennas at the source, relay, and destination. We then derive simple yet valuable expressions for the asymptotic transmission outage probability and the asymptotic secrecy outage probability, which reveal the secrecy performance when the number of antennas at the source grows sufficiently large. Using our expressions, we quantify a practical performance metric, namely the secrecy throughput, under a secrecy outage probability constraint. We further determine the system and channel parameters that maximize the secrecy throughput, leading to analytical security solutions suitable for real-world deployment., 12 pages, 7 figures