The use of polarized photons in cryptography holds the promise of secure cryptographic quantum key distribution schemes. In theory, quantum key distribution provides a key with unconditional security. In practice, however, the implemented schemes are often operated in a regime which excludes unconditional security, or even in a regime in which insecurity can be proven. We shall describe here the basics of quantum key distribution, provide an overview of the main ideas that lead to a proof of its unconditional security, and discuss the insecurity of practical quantum key distribution to long distances. The above review will be undertaken with a special emphasis on the role of the polarization of photons. Finally, we shall briefly touch on the importance of photons and their polarization degree of freedom in recent proposals for small-scale optical quantum computing.
The use of polarized photons in cryptography holds the promise of secure cryptographic quantum key distribution schemes. In theory, quantum key distribution provides a key with unconditional security. In practice, however, the implemented schemes are often operated in a regime which excludes unconditional security, or even in a regime in which insecurity can be proven. We shall describe here the basics of quantum key distribution, provide an overview of the main ideas that lead to a proof of its unconditional security, and discuss the insecurity of practical quantum key distribution to long distances. The above review will be undertaken with a special emphasis on the role of the polarization of photons. Finally, we shall briefly touch on the importance of photons and their polarization degree of freedom in recent proposals for small-scale optical quantum computing.
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
The use of quantum bits (qubits) in cryptography holds the promise of secure cryptographic quantum key distribution schemes. It is based usually on single-photon polarization states. Unfortunately, the implemented ``qubits'' in the usual weak pulse experiments are not true two-level systems, and quantum key distribution based on these imperfect qubits is totally insecure in the presence of high (realistic) loss rate. In this work, we investigate another potential implementation: qubits generated using a process of parametric downconversion. We find that, to first (two-photon) and second (four-photon) order in the parametric downconversion small parameter, this implementation of quantum key distribution is equivalent to the theoretical version. Once realistic measurements are taken into account, quantum key distribution based on parametric downconversion suffers also from sensitivity to extremely high (nonrealistic) losses. By choosing the small parameter of the process according to the loss rates, both implementations of quantum key distribution can in principle become secure against the attack studied in this paper. However, adjusting the small parameter to the required levels seems to be impractical in the weak pulse process. On the other hand, this can easily be done in the parametric downconversion process, making it a much more promising implementation.
The counter-intuitive features of quantum mechanics make it possible to solve problems and perform tasks that are beyond the abilities of non-quantum (classical) computers and communication devices. The field of quantum information processing studies how we can achieve such improvements by representing information as quantum states.One of the early achievements of quantum information processing is the development of quantum key distribution (QKD). QKD protocols allow two participants (Alice and Bob) to achieve the classically-impossible task of generating a secret shared key even if their adversary is computationally unlimited.Unfortunately, the security promises of QKD are true only in theory; practical implementations of QKD deviate from the theoretical protocols, and many of these deviations give rise to practical attacks. In this research thesis, we study the security properties of various QKD protocols in many practical settings:First, we study practical security of semiquantum key distribution (SQKD) protocols, where either Alice or Bob is non-quantum (classical). Following practical security problems in previous SQKD protocols, we suggest a new SQKD protocol (the "Mirror protocol") which can be securely implemented, and we prove it robust and secure against a wide range of attacks (the "uniform collective attacks").Then, we study "composable security" of the first QKD protocol created by Bennett and Brassard (BB84). BB84 has its unconditional security proved against adversaries performing the most general attacks in a theoretical (idealized) setting; however, some security approaches do not prove "composable security", which requires the secret key to remain secret even when Alice and Bob actually use it for cryptographic purposes. We generalize an algebraic security approach for BB84, making it prove composable security of BB84 (and many variants of BB84) against the most general attacks.Finally, we analyze an important practical attack (named "Bright Illumination"), showing how it can be modeled as a theoretical "Reversed-Space" attack.Overall, all results aim to enhance our understanding on how to bridge the gap between theory and practice in various sub-fields of QKD, and they may help solve a major open problem in the field of QKD: constructing a realistic QKD implementation that can be proved truly and unconditionally secure against any possible attack.
The use of quantum bits (qubits) in cryptography holds the promise of secure cryptographic quantum key distribution schemes. It is based usually on single-photon polarization states. Unfortunately, the implemented “qubits” in the usual weak pulse experiments are not true two-level systems, and quantum key distribution based on these imperfect qubits is totally insecure in the presence of high (realistic) loss rate. In this work, we investigate another potential implementation: qubits generated using a process of parametric downconversion. We find that, to first (two-photon) and second (four-photon) order in the parametric downconversion small parameter, this implementation of quantum key distribution is equivalent to the theoretical version.Once realistic measurements are taken into account, quantum key distribution based on parametric downconversion suffers also from sensitivity to extremely high (nonrealistic) losses. By choosing the small parameter of the process according to the loss rates, both implementations of quantum key distribution can in principle become secure against the attack studied in this paper. However, adjusting the small parameter to the required levels seems to be impractical in the weak pulse process. On the other hand, this can easily be done in the parametric downconversion process, making it a much more promising implementation.
Quantum cryptography is anew branch of physics and cryptography which exploit quantum mechanical phenomena to guarantee the secrecy of cryptographic keys. This paper introduces the basic concepts, problems and methods of quantum key distribution. It introduces some basic knowledge of quantum mechanics and explains how these physical laws can be used in cryptography. The basic protocols of quantum cryptographic used for key distribution based on entangled polarization photons pair are also presented and assessed. The well known concepts based on entangled pairs of photons (Ekert) schemes are presented. In these schemes Alice and Bob perform measurements on each photon along one of two, three or four directions given by unit vectors and (i, j = 1, 2, 3,4) respectively. Therefore the combinations are used from Alice and Bob is four, nine, or sixteen with respect to number of measurement directions (i = 2, 3 or 4) respectively. The analysis and comparative study between these versions are presented
Quantum cryptography is anew branch of physics and cryptography which exploit quantum mechanical phenomena to guarantee the secrecy of cryptographic keys. This paper introduces the basic concepts, problems and methods of quantum key distribution. It introduces some basic knowledge of quantum mechanics and explains how these physical laws can be used in cryptography. The basic protocols of quantum cryptographic used for key distribution based on entangled polarization photons pair are also presented and assessed. The well known concepts based on entangled pairs of photons (Ekert) schemes are presented. In these schemes Alice and Bob perform measurements on each photon along one of two, three or four directions given by unit vectors and (i, j = 1, 2, 3,4) respectively. Therefore the combinations are used from Alice and Bob is four, nine, or sixteen with respect to number of measurement directions (i = 2, 3 or 4) respectively. The analysis and comparative study between these versions are presented