This dissertation investigates the potential of multi-photon tolerant protocols for satellite-aided global quantum key distribution (QKD). Recent investigations like braided single-stage protocol and the implementation of the three-stage protocol in fiber have indicated that multi-photon tolerant protocols have wide-ranging capabilities for increasing the distance and speed of quantum-secure communication. This dissertation proposes satellite-based network multicasting and its operation that can profitably use multi-photon tolerant protocols for quantum-secure global communication. \nWith a growingly interconnected world and an increasing need for security in communication, communication satellites at Lower Earth Orbits (LEO), Medium Earth Orbit (MEO) and Geostationary Earth Orbit (GEO) have a potential role in serving as a means to distribute secure keys for encryption among distant endpoints. This dissertation systematically evaluates such a role. The dissertation proposes a layered framework using satellites and fiber optic links that can form a composite system for carrying the information payload and distributing quantum-secure keys for encrypting information in transit. \nQuantum communications links are currently point-to-point. Considering the concept of global QKD network, there is need for multicast quantum links. Multi casting can be achieved in quantum networks by (a) using multiple wavelengths, or (b) using use specific set of bases. In efforts to develop a composite quantum secure global communication system; this dissertation also introduces the concept of multi-photon tolerant quantum threshold cryptography. The motivation for development of threshold cryptography is that a secret can be encrypted with multiple users and requires multiple users to decrypt. The quantum threshold cryptography is proposed by using idea of multiple bases. This can be considered as step forward towards multiparty quantum communication. This dissertation also proposed layered architecture for key distribution. \nConcisely, this dissertation proposes the techniques like multicasting in quantum scenario, quantum threshold cryptography to achieve the goal of secured global communication.
This dissertation investigates the potential of multi-photon tolerant protocols for satellite-aided global quantum key distribution (QKD). Recent investigations like braided single-stage protocol and the implementation of the three-stage protocol in fiber have indicated that multi-photon tolerant protocols have wide-ranging capabilities for increasing the distance and speed of quantum-secure communication. This dissertation proposes satellite-based network multicasting and its operation that can profitably use multi-photon tolerant protocols for quantum-secure global communication. \nWith a growingly interconnected world and an increasing need for security in communication, communication satellites at Lower Earth Orbits (LEO), Medium Earth Orbit (MEO) and Geostationary Earth Orbit (GEO) have a potential role in serving as a means to distribute secure keys for encryption among distant endpoints. This dissertation systematically evaluates such a role. The dissertation proposes a layered framework using satellites and fiber optic links that can form a composite system for carrying the information payload and distributing quantum-secure keys for encrypting information in transit. \nQuantum communications links are currently point-to-point. Considering the concept of global QKD network, there is need for multicast quantum links. Multi casting can be achieved in quantum networks by (a) using multiple wavelengths, or (b) using use specific set of bases. In efforts to develop a composite quantum secure global communication system; this dissertation also introduces the concept of multi-photon tolerant quantum threshold cryptography. The motivation for development of threshold cryptography is that a secret can be encrypted with multiple users and requires multiple users to decrypt. The quantum threshold cryptography is proposed by using idea of multiple bases. This can be considered as step forward towards multiparty quantum communication. This dissertation also proposed layered architecture for key distribution. \nConcisely, this dissertation proposes the techniques like multicasting in quantum scenario, quantum threshold cryptography to achieve the goal of secured global communication.
This paper proposes and analyzes the potential of a multi-photon tolerant quantum communication protocol to secure satellite communication. For securing satellite communication, quantum cryptography is the only known unconditionally secure method. A number of recent experiments have shown feasibility of satellite-aided global quantum key distribution (QKD) using different methods such as: Use of entangled photon pairs, decoy state methods, and entanglement swapping. The use of single photon in these methods restricts the distance and speed over which quantum cryptography can be applied. Contemporary quantum cryptography protocols like the BB84 and its variants suffer from the limitation of reaching the distances of only Low Earth Orbit (LEO) at the data rates of few kilobits per second. This makes it impossible to develop a general satellite-based secure global communication network using the existing protocols. The method proposed in this paper allows secure communication at the heights of the Medium Earth Orbit (MEO) and Geosynchronous Earth Orbit (GEO) satellites. The benefits of the proposed method are two-fold: First it enables the realization of a secure global communication network based on satellites and second it provides unconditional security for satellite networks at GEO heights. The multi-photon approach discussed in this paper ameliorates the distance and speed issues associated with quantum cryptography through the use of contemporary laser communication (lasercom) devices. This approach can be seen as a step ahead towards global quantum communication.
Single-photon quantum communications (QC) offers the attractive feature of 'future proof', forward security rooted in the laws of quantum physics. Ground based quantum key distribution (QKD) experiments in optical fiber have attained transmission ranges in excess of 200km, but for larger distances we proposed a methodology for satellite-based QC. Over the past decade we have devised solutions to the technical challenges to satellite-to-ground QC, and we now have a clear concept for how space-based QC could be performed and potentially utilized within a trusted QKD network architecture. Functioning as a trusted QKD node, a QC satellite ('QC-sat') could deliver secret keys to the key stores of ground-based trusted QKD network nodes, to each of which multiple users are connected by optical fiber or free-space QC. A QC-sat could thereby extend quantum-secured connectivity to geographically disjoint domains, separated by continental or inter-continental distances. In this paper we describe our system concept that makes QC feasible with low-earth orbit (LEO) QC-sats (200-km-2,000-km altitude orbits), and the results of link modeling of expected performance. Using the architecture that we have developed, LEO satellite-to-ground QKD will be feasible with secret bit yields of several hundred 256-bit AES keys per contact. With multiple ground sites separated by {approx} 100km, mitigation of cloudiness over any single ground site would be possible, potentially allowing multiple contact opportunities each day. The essential next step is an experimental QC-sat. A number of LEO-platforms would be suitable, ranging from a dedicated, three-axis stabilized small satellite, to a secondary experiment on an imaging satellite. to the ISS. With one or more QC-sats, low-latency quantum-secured communications could then be provided to ground-based users on a global scale. Air-to-ground QC would also be possible.
Information security is a growing concern in our modern world, where almost everything can be done online. To protect security, classical encryption protocols, such as RSA, are used. These encryption protocols are almost always based on mathematical problems that are computational difficult. Therefore, the security is only valid under limited computational resources, and therefore do not provide provable security. An exception is the one-time pad protocol, which is provably secure but requires an existing shared key that is as long as the information it must encrypt. The exchange of such a key can be challenging, often requiring the two partied to physically meet to exchange the key or the use a trusted courier to physically carry the key on a hard drive. \n \nQuantum key distribution (QKD) offers a solution by utilizing quantum mechanics to grow a secure cryptographic key shared between two distant parties. The quantum nature of the particles used in the exchange ensure that any eavesdropper would leave signs of their presence, allowing the users to precisely quantify the security of the key that is produced. This allows secure generation of a random key which can be used for the one-time pad encryption protocol which, unlike most encryption protocols, does not rely on computational assumptions, and is secure regardless of available computational power. \n \nCurrent implementations of QKD are limited to a single link distance of ≈200 km, preventing implementation of QKD on a global scale, or even between distant cities within a single country, without some additional techniques. One promising solution is the use of orbiting low Earth orbit satellite platforms as trusted nodes in a quantum communications network. The main purpose of this work has been to show the viability of this solution, trough theoretical performance modeling and experimental demonstrations. \n \nThorough numerical simulations have been developed to evaluate the performance and challenges in implementing QKD using a low Earth orbit satellite platform and determine an optimal approach to its implementation. The simulations include a realistic satellite orbit analysis, all expected loss mechanisms, estimates of background contributions and realistic simulation of quantum optical processes. This work addresses the questions of optimal wavelength and beam waist, the effect of the telescope design and pointing error, and the impact of detector degradation due to exposure to radiation in the space environment. The simulation is used to determine the length of secure key for QKD and the performance of fundamental quantum optics experiments such as Bell tests and quantum teleportation. We compare the advantages and disadvantages of uplink and dowlink scenarios, and show that an uplink, despite having reduced performance compared to a downlink, offers more scientific freedom, by allowing changes to the quantum source, and benefits from a simpler satellite design with reduced pointing requirement. This work provides a theoretical foundation for ongoing design and development of quantum systems for satellite implementation of QKD. \n \nIn addition to theoretical analyses, experiments where developed and conducted corresponding to the challenges likely to be faced by a satellite uplink. Two main challenges where identified and experimentally overcome. First, the difficulty of operating in the high loss regime of a satellite uplink, which typically experience losses in the range of 40-50 dB when above 40° of elevation from the horizon, an elevation only reached by half of the satellite passes. This challenge was addressed by demonstrating full QKD protocols at losses exceeding 50 dB. Also, the ability to perform full QKD during the short duration of a satellite pass was shown by replicating the varying loss of archetypal passes and successfully extracting secure keys. Secondly, the difficulty of accurately tracking and pointing to a receiver platform traveling at the high angular speed of a satellite was overcome by successfully exchange quantum signals to a truck traveling at angular speeds exceeding that of a low Earth orbit satellite. This required the design, construction, and implementation of a transmitter system and a quantum receiver system capable of active pointing using a custom pointing system. \n \nAn additional experiment was also performed where we experimentally investigate the feasibility of performing QKD using light scattered by a diffusive screen. A system capable of doing so could be used to create QKD hot-spots, where a QKD source could be aimed at a diffusive screen to allow multiple users to simultaneously exchange secure keys with the source without the need for high precision pointing. \n \nThrough these analyses and experimental demonstrations we evaluated the performance of QKD using a satellite and have shown its technological readiness to be implemented. Implementation of such a system would allow QKD to be performed on a global scale, enabling communications security that is not based on computational assumptions without the need to physically transport the key from one party to the other.
Quantum key distribution (QKD) has attracted much attention on secure communications across global networks. QKD over satellite networks can overcome the limitations of terrestrial optical networks, such as large attenuation over long distance fiber channel and difficulty of intercontinental domain communications. Different QKD networks (around the world) can intercommunicate through quantum satellites, leading to a global quantum network in near future. This raises a new resource allocation and management problem of QKD involving multiple satellite layers and distributed ground stations. Using existing schemes, a single satellite cannot perform QKD for ground stations for the whole day. Moreover, the research problem is more challenging due to limitations of satellite coverage: limited cover time of low earth orbit (LEO) satellite, high channel losses of geostationary earth orbit (GEO) satellite, etc. To overcome these limitations, our study proposes a double-layer quantum satellite network (QSN) implemented quantum key pool (QKP) to relay keys for ground stations. We propose a new architecture of trusted-repeater-based double-layer quantum satellite networks, comprising GEO and LEO satellites. We also address the routing and key allocation (RKA) problem for key-relay services over QSNs. We propose a novel joint GEO-LEO routing and key allocation (JGL-RKA) algorithm to solve the RKA problem. Simulative results show that the proposed scheme can increase success probability of key-relay services significantly. We also present the impact of different route selections mechanisms, number of satellite links, satellite node capability, and service granularity on network performance.
This paper presents an investigation of practical quantum secure communication using multi-photon tolerant protocols. Multi-photon tolerant protocols loosen the limit on the number of photons imposed by currently used quantum key distribution protocols. The multi-photon tolerant protocols investigated in this paper are multi-stage protocols that do not require any prior agreement between a sender Alice and a receiver Bob. The security of such protocols stems from the fact that the optimal detection strategies between the legitimate users and the eavesdropper are asymmetrical, allowing Bob to obtain measurement results deterministically while imposing unavoidable quantum noise to the eavesdropper Eve's measurement. Multi-photon tolerant protocols are based on the use of transformations known only to the communicating party applying them i.e. either Alice or Bob. In this paper multi-photon tolerant protocols are used in order to share a key or a message between a sender Alice and a receiver Bob. Thus such protocols can be either used as quantum key distribution (QKD) protocols or quantum communication protocols. In addition, multi-stage protocols can be used to share a key between Alice and Bob, followed by the shared key used as a seed key to a single-stage protocol, called the braiding concept. This paper presents a practical study of multi-photon tolerant multi-stage protocols. Security aspects as well as challenges to the practical implementation are discussed. In addition, secret raw key generation rates are calculated with respect to both losses and distances over a fiber optical channel. It is well-known that raw key generation rates decreases with the increase in channel losses and distances. In this paper, coherent non-decoying quantum states are used to transfer the encoded bits from Alice to Bob. Raw key generation rates are calculated for different average photon numbers µ and compared with the case of µ=0.1, which is the average number of photons used in most single-photon based QKD protocols. Furthermore, an optimal average number of photons to be used within the secure region of the multi-photon tolerant protocols is calculated. It is worth noting that, with the increased key generation rates and distances of communication offered by the multi-photon tolerant protocols, quantum secure communication need not be restricted to quantum key distribution; it can be elevated to attain direct quantum secure communication.
As more aspects of modern society depend on digital communication, we increasingly rely on infrastructure that ensures the privacy and security of this communication. Classically, this has been provided by cryptographic protocols such as public-key encryption, in which secrets called keys are exchanged between different parties to enable secure communication. The rapid development of quantum algorithms which violate the assumptions of these protocols, however, poses a security challenge to modern cryptography. \n \nQuantum resources can also be used to strengthen cryptographic security, particularly the security of key exchange protocols. This approach, QKD, can be implemented by encoding in quantum systems such as single photons sent through free-space or a fiber. Fiber based QKD devices are already commercially available, but are fundamentally limited to distributing keys over a few hundred kilometers. To address this distance limitation, research QKD systems are being developed to exchange keys through free-space to satellites. This work considers practical challenges to building and testing both types of QKD devices. \n \nFirstly, we consider modeling and mission analysis for airborne demonstrations of QKD to stratospheric balloons and aircraft to simulate a satellite. Based on the mission parameters available for both platforms, we found aircraft platforms were more promising for testing prototype QKD satellite systems. We developed a mission planning tool to help design the flight geometries for testing the device. \n \nNext, we developed three new components for a QKD satellite prototype. The requirements for electro-optical devices in orbit are very different from lab environments, mandating new approaches to designing QKD devices. We developed a quad single photon detector package to meet the requirements for free-space links to low earth orbit. Moreover, we designed and built optical systems for analyzing the polarization of photons and an adaptive optics unit to increase the efficiency of collecting the encoded photons. All three devices were tested in conditions that simulated the time and loss of a satellite pass. \n \nFinally, we demonstrated a laser damage attack on a live commercial QKD system. Our attack injected additional optical power into the sender device to modify security-critical components. Specifically, our attack damaged the PIN diodes which monitor the encoded photon number, reducing their sensitivity or completely blinding them. Our damage could compromise the entire key, and was performed during system operation while raising no alarms. \n \nIn summary, this work shows the trade-offs of testing QKD payloads on different airborne platforms, develops components for a satellite QKD payload, and demonstrates a security vulnerability in a commercial QKD system that can fully compromise the key. These results help address practical challenges to building QKD devices, improving the security of modern cryptography.