Quantum cryptography uses the quantum properties of individual photons to provide two or more users with means to communicate securely and efficiently. Specifically, quantum key distribution (QKD) focuses on the secure communication between two distant parties and guarantees the security of a transmitted message between them. Although QKD is the most mature application of quantum cryptography, applications of quantum mechanics in communication systems are not limited to QKD and other cryptographic protocols can be implemented as well. This thesis focuses on the implementation of quantum cryptographic systems over deployed fiber. A first QKD system was built and used to demonstrate the BB84 protocol over deployed fiber between the University of Calgary and SAIT. The system implemented a novel tool called quantum frames that allow for the quantum signals to be routed, for clock synchronization, and for channel stabilization in a network scenario. The same system was used to examine the scalability of the secret key generation rate in different steps of the process in order to detect potential bottlenecks for key generation rate of high speed systems. A second QKD system was developed to demonstrate the measurement-device independent QKD protocol. The system worked over deployed fiber between the University of Calgary, SAIT, and Foothills Hospital in the City of Calgary. The advantage of the measurement-device independent QKD protocol over previous QKD protocols is that it eliminates the possibility of side-channel attacks that target single photon detectors used in QKD systems. This demonstration also involved the first demonstration of a Bell state measurement over deployed fiber. Bell state measurements were further studied by using novel single photon detectors, which allowed demonstrating a highly efficient Bell measurement. Finally, a cryptographic protocol known as private queries was demonstrated between the University of Calgary and SAIT. Quantum private queries were demonstrated for the first time over deployed fiber by developing and implementing an error correction scheme to make the protocol noise and loss tolerant.
The information society that presides today is dependent on the communication industry to facilitate unintelligible data transfers between authenticated parties. Such requirements have, to date, taken advantage of security based on the mathematical complexities of certain algorithms. However, the advancement of computing power and the advent of the quantum computer together with the vulnerability of this scheme to mathematical progress have prompted the introduction of quantum cryptography. This process, through the laws of quantum physics, ensures provably secure data communication. Quantum cryptography provides physical protection to individual bits of information thus providing a hardware implemented solution. The implementation of this theoretical concept requires much practical innovation for transparent deployment into current cryptographic solutions. This thesis introduces the concept of quantum cryptography in a practical perspective. It raises a few core concerns with the present quantum cryptographic technology and provides some solutions towards the practical deployment of commercially feasible quantum cryptographic systems. The thesis commences with an introduction to classical cryptography focussing on key management protocols. This is followed by the presentation of the basic concepts of Quantum Key Distribution (QKD) together with an explanation of some QKD protocols and parameter required to classify such protocols. Chapter 2 discusses the theoretical and practical aspects of quantum channels in particular optical fibre. The primary challenges of transferring classical and quantum data along these channels are mentioned together with some solutions. A description of experimental usage with present QKD solutions is presented in Chapter 3. An investigation into highly efficient QKD protocols follows illustrating effective post-distribution processing for increasing the efficiency of the BB84 protocol. Chapter 4 begins with the limitations of present day QKD systems and explicates Quantum Networks as a possible solution. An introduction to classical networking theory is first presented after which some quantum network architectures based on passive optical networks are illustrated. Finally the proposed Quantum City project in conjunction with the eThekwini Municipality is explained. The realization of this project is intended to be complete by the third quarter of 2008 effectively making Durban into the first Quantum City in the world.
This doctoral thesis summarizes research in quantum cryptography done at the Department of Electronics and Telecommunications at the Norwegian University of Science and Technology (NTNU) from 1998 through 2007.\n\nThe opening parts contain a brief introduction into quantum cryptography as well as an overview of all existing single photon detection techniques for visible and near infrared light. Then, our implementation of a fiber optic quantum key distribution (QKD) system is described. We employ a one-way phase coding scheme with a 1310 nm attenuated laser source and a polarization-maintaining Mach-Zehnder interferometer. A feature of our scheme is that it tracks phase drift in the interferometer at the single photon level instead of employing hardware phase control measures. An optimal phase tracking algorithm has been developed, implemented and tested. Phase tracking accuracy of +-10 degrees is achieved when approximately 200 photon counts are collected in each cycle of adjustment. Another feature of our QKD system is that it uses a single photon detector based on a germanium avalanche photodiode gated at 20 MHz. To make possible this relatively high gating rate, we have developed, implemented and tested an afterpulse blocking technique, when a number of gating pulses is blocked after each registered avalanche. This technique allows to increase the key generation rate nearly proportionally to the increase of the gating rate. QKD has been demonstrated in the laboratory setting with only a very limited success: by the time of the thesis completion we had malfunctioning components in the setup, and the quantum bit error rate remained unstable with its lowest registered value of about 4%.\n\nMore than half of the thesis is devoted to various security aspects of QKD. We have studied several attacks that exploit component imperfections and loopholes in optical schemes. In a large pulse attack, settings of modulators inside Alice's and Bob's setups are read out by external interrogating light pulses, without interacting with quantum states and without raising security alarms. An external measurement of phase shift at Alice's phase modulator in our setup has been demonstrated experimentally. In a faked states attack, Eve intercepts Alice's qubits and then utilizes various optical imperfections in Bob's scheme to construct and resend light pulses in such a way that Bob does not distinguish his detection results from normal, whereas they give Bob the basis and bit value chosen at Eve's discretion. Construction of such faked states using several different imperfections is discussed. Also, we sketch a practical workflow of breaking into a running quantum cryptolink for the two abovementioned classes of attacks. A special attention is paid to a common imperfection when sensitivity of Bob's two detectors relative to one another can be controlled by Eve via an external parameter, for example via the timing of the incoming pulse. This imperfection is illustrated by measurements on two different single photon detectors. Quantitative results for a faked states attack on the Bennett-Brassard 1984 (BB84) and the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocols using this imperfection are obtained. It is shown how faked states can in principle be constructed for quantum cryptosystems that use a phase-time encoding, the differential phase shift keying (DPSK) and the Ekert protocols. Furthermore we have attempted to integrate this imperfection of detectors into the general security proof for the BB84 protocol. For all attacks, their applicability to and implications for various known QKD schemes are considered, and countermeasures against the attacks are proposed.\n\nThe thesis incorporates published papers [J. Mod. Opt. 48, 2023 (2001)], [Appl. Opt. 43, 4385 (2004)], [J. Mod. Opt. 52, 691 (2005)], [Phys. Rev. A 74, 022313 (2006)], and [quant-ph/0702262].
Cryptographic protocols can be classified by the type of security against eavesdropping which they provide. There exist mathematically secure schemes whose security relies on mathematical proofs (like the Vernam cipher discussed below) or conjectures (like public key RSA encryption) about the complexity of deciphering the message without possessing the correct key. The majority of current secure public Internet connections rely on such schemes. Alternatively, a cryptographic setup may provide a physically secure method for communicating. In these setups the security is provided by the physical laws governing the communication protocol. Here we first discuss a provably secure classical communication protocol and then quantum methods for distributing the necessary keys. The BB84 protocol relies on the impossibility of perfectly distinguishing non-orthogonal quantum states from one copy of the quantum system. The scheme was invented in 1984 and is the first of its kind. In contrast, the Ekert91 protocol makes use of Bell correlations between entangled pairs of photons. These correlations are destroyed when Eve attempts to make a measurement on one of the particles but also when imperfections, such as decoherence processes, affect the scheme. As long as no such “elements of reality” are introduced, and Bell correlations violating local realism can be generated by Alice and Bob, no eavesdropper can be present. One-time pads and the Vernam cipher The Vernam cipher is a cryptographic protocol which allows the encryption and decryption protocol to be publicly known. The security of the protocol relies entirely on the key which is private and not publicly known. Alice and Bob share identical n -bit secret key strings (one-time pad).
The work in this thesis can be divided into three parts. The first two parts deal with optical quantum communication protocols and the third part deals with quantum cryptography. The first part of the thesis is a step towards reformulating quantum protocols in terms of coherent states. Quantum communication protocols are typically formulated in terms of abstract qudit states and operations. This leaves the question of an experimental realization open. Direct translation of these protocols, say into single photons with some d-dimensional degree of freedom, are typically challenging to realize. Multi-mode coherent states, on the other hand, can be easily generated experimentally. Reformulation of protocols in terms of these states has been a successful strategy for implementation of quantum protocols. Quantum key distribution and the quantum fingerprinting protocol have both followed this route. In Chapter 3, we characterize the Gram matrices of multi-mode coherent states in an attempt to understand the class of communication protocols, which can be implemented using these states. We also derive the closure of the Gram matrices, which can be implemented in this way, so that we also characterize those matrices, which can be approximated arbitrarily well using multi-mode coherent states. \n \nIn the second part of the thesis, Chapter 4, we describe our collaboration with an experimental group to implement the quantum fingerprinting protocol and examine the tradeoffs between the resources required to implement such protocols. It is seen that it is difficult to implement the quantum fingerprinting protocol experimentally for large input sizes. This leads us to study the tradeoff between the two resources expended during optically implemented simultaneous message passing communication protocols: the duration of the protocol and the energy required to run it. We derive general bounds on the growth of these quantities which are valid for all optical protocols. We also develop tighter bounds for the growth of these resources for protocols implementing quantum fingerprinting with coherent states. \n \nFinally in Chapter 5, we venture into quantum cryptography. We introduce a new setting for two-party cryptography with temporarily trusted third parties. In this setting, in addition to Alice and Bob, there are third parties, which Alice and Bob both trust to be honest during the course of the protocol. However, once the protocol concludes, there is no guarantee over the behaviour of these third parties. It is possible that they collaborate and act adversarially against the honest parties. We implement a variant of bit commitment in this setting, which we call erasable bit commitment. In this primitive, Alice has the choice of either opening or erasing her commitment after the commit phase. The ability to ask for an erasure allows Alice to ask the trusted nodes to erase her commitment in case the trust period is about to expire. However, this ability also makes erasable bit commitment weaker than the standard version of bit commitment. In addition to satisfying the security requirements of bit commitment, our protocol also does not reveal any information about the commitment to the third parties. Lastly, our protocol for this primitive requires a constant number of trusted third parties and can tolerate a small number of dishonest trusted nodes as well as implementation errors.
In so-called private-key cryptographic systems, secure transmission of confi dential information through unprotected channels relies on the exchange of secret keys, known only to the sender, Alice, and the receiver, Bob.Quantum cryptography is based on the properties of quantum mechanics to obtain a provably secure key distribution.Most existing implement ations rely on either the polarization or the phase of very weak pulses of light sent through optical fibers.Recent results by research groups in British Telecom, Los Alamos National Laboratory and at the University of Geneva, have shown that quantum key distribution is possible over distances of tens of kilometers, using only standard telecom cables.However, due to birefringence and the effects of the environment, the polarization at the output of a fiber fluctuates randomly.Therefore, all existing systems need active polarization control.Systems based on phase also require interferometric detection, which necessitates active stabilization of the interferometers.To eliminate these problems, our team, led by Prof. N. Gisin in the Group of Applied Physics at the University of Geneva, is currently developing a new interferometric system implementing phase-encoded quantum key distribution (US patent pending).A schematic of the setup is given in Fig. 1.Its main features are:1) The system is based on time multiplexing, the interfering pulses following exactly the same spatial path, albeit with a small time delay.The interferometer is thus automatically aligned, and needs neither adjustments nor stabilization of the path lengths.2) Use of Faraday mirrors enables all birefringence effects in the fibers to be cancelled out.Therefore, the system does not require any polarization control.With this new system, Alice and Bob could exchange their cryptographic keys through standard telecom systems.They would be provided with a sending kit and