We have improved the hardware and software of our autocompensating system for quantum key distribution by replacing bulk optical components at the end stations with fiber-optic equivalents and implementing software that synchronizes end-station activities, communicates basis choices, corrects errors, and performs privacy amplification over a local area network. The all-fiber-optic arrangement provides stable, efficient, and high-contrast routing of the photons. The low-bit error rate leads to high error-correction efficiency and minimizes data sacrifice during privacy amplification. Characterization measurements made on a number of commercial avalanche photodiodes are presented that highlight the need for improved devices tailored specifically for quantum information applications. A scheme for frequency shifting the photons returning from Alice's station to allow them to be distinguished from backscattered noise photons is also described.
We have improved the hardware and software of our autocompensating system for quantum key distribution by replacing bulk optical components at the end stations with fiber-optic equivalents and implementing software that synchronizes end-station activities, communicates basis choices, corrects errors, and performs privacy amplification over a local area network. The all-fiber-optic arrangement provides stable, efficient, and high-contrast routing of the photons. The low-bit error rate leads to high error-correction efficiency and minimizes data sacrifice during privacy amplification. Characterization measurements made on a number of commercial avalanche photodiodes are presented that highlight the need for improved devices tailored specifically for quantum information applications. A scheme for frequency shifting the photons returning from Alice's station to allow them to be distinguished from backscattered noise photons is also described.
We have developed a system for quantum key distribution (QKD), based on standard telecommunication lasers, detectors, and optical fiber, that passively compensates for time-dependent variations of the fiber-optic path due to stress, temperature changes, or birefringence. This approach allows information encoded in phase shifts imposed on single-photon-level pulses to be accurately read out after transmission over many kilometers of uncontrolled fiber. Cooled InGaAs avalanche photodiodes, pulse-biased using a special noise canceling circuit, are used to detect single 1.31-/spl mu/m infrared photons with a high efficiency, low dark-count rate, and subnanosecond time resolution. A single optical fiber carries both the quantum information and precise 1.55 /spl mu/m timing pulses between the two end stations. Overall synchronization of end-station activities, public discussion of basis choices, error correction, and privacy amplification have all been implemented over a local area network (LAN). The system at present generates raw, error-corrected, and privacy-amplified key data at rates of /spl sim/1000, 600, and 200 bits/s, respectively, over a 10-km single-mode fiber link.
A novel, user-friendly quantum key distribution (QKD) system operating at a wavelength of 1550nm and at a clock rate of 10MHz was constructed to explore the compatibility of this emerging technology with the optical fiber network environment. Custom circuit boards providing the low-level control and sensing functions for both the transmitter and receiver were developed, allowing software-based system reconfiguration via USB interface to personal computers. The computer control allowed the user to change operating parameters such as detector bias voltages and pulse delays and also allowed for self-tuning of the system. Epitaxx avalanche photodiodes, operated in Geiger mode, were used to detect the single photons. A complete QKD protocol stack incorporating the sifting, reconciliation, privacy amplification, authentication and key confirmation functions was implemented in software. The system was tested over twenty five kilometers of dark underground fiber, producing 18.6 million sifted bits, with a sifted bit error rate of 4.9% at an average number of photons per pulse of 0.2, during a continuous 12-hour period of self-sustaining operation: a small portion of the secret bits distilled from each session's sifted bits were used to authenticate the next session. A total of 6.8 million shared secret bits were produced.
In this thesis, several novel secure optical communication network models are presented. The security of these network models is achieved using quantum cryptographic-key distribution. Both point-to-point and multiple access broadcast networks are considered. Furthermore, in developing these models, the simplicity, generality and flexibility are highly maintained. Thus, it is possible to use these models efficiently for software simulation purposes. Each secure optical communication network model is assumed to be composed of N communication nodes (stations) that are connected by three types of fiber optic channels. These are the quantum, authenticated public, and timing channels.
Errors can occur in quantum transmission due to eavesdropping and system inefficiencies. Thus, an error elimination technique is implemented. In this technique, several rounds of random permutations and block parity comparisons are done between the transmitter and receiver. Then, another strategy of random subset parity comparisons is performed. Blocks or subsets of discordant parities are subjected to a bisective search to find error locations and eliminate them.
After all quantum transmission errors are discovered and discarded, an important mathematical technique called privacy amplification is implemented.
Using this technique, eavesdropper information about the final cryptographic key is reduced to an arbitrarily value smaller than one bit. This is essentially achieved by publicly choosing length-reducing transformations to apply to primary keys so that partial eavesdropper information about inputs conveys almost no knowledge of the outputs.
In the designing of secure models for multiple access broadcast networks, both of broadcasting and multicasting quantum transmission modes are enabled. This is done via introducing electro-optic switches in the implementation of quantum channels. Hence, the efficiency of quantum cryptography protocols can significantly be increased, in many situations.
All networks quantum cryptography protocols are simulated via the object oriented programming (OOP) approach using the Borland C ++ Compiler Version 4.5. Simulation results for both error elimination and privacy amplification stages are presented. It is important to mention that all the described protocols are secure even against an enemy possessing unlimited computing power.
Summary form only given. Quantum cryptography, or quantum key distribution (QKD), invented by Bennett and Brassard is a method for creating shared, assuredly secret, cryptographic key data over unsecured optical links. Security is guaranteed by the fundamental quantum properties of light. We describe our system in great detail, and report recent improvements and results obtained in generating error-corrected, privacy-amplified photographic key data over a 10-km single-mode fiber link. Bob transmits two orthogonally-polarized phase coherent pulses.
Quantum cryptographic key distribution (QKD) uses extremely faint light pulses to carry quantum information between two parties (Alice and Bob), allowing them to generate a shared, secret cryptographic key. Autocompensating QKD systems automatically and passively compensate for uncontrolled time-dependent variations of the optical fibre properties by coding the information as a differential phase between orthogonally polarized components of a light pulse sent on a round trip through the fibre, reflected at mid-course using a Faraday mirror. We have built a prototype system based on standard telecom technology that achieves a privacy-amplified bit generation rate of ~1000 bits s -1 over a 10 km optical fibre link. Quantum cryptography is an example of an application that, by using quantum states of individual particles to represent information, accomplishes a practical task that is impossible using classical means.
We present a communication system based on quantum degenerate four-wave mixing for achieving fully autocompensating high-dimensional quantum cryptography. We prove that random phase shifts and couplings (cross-talking) among 2N optical modes (spatial and polarization modes), represented by arbitrary SU(2N) transformations and due to mechanical and thermal perturbations, imperfections, and so on, are autocompensated after a single round trip between Alice and Bob. Bob uses a source of single photons or, alternatively, attenuated coherent states and thus autocompensated 1-qudit states are received by Bob for using them with high-dimensional quantum key distribution (QKD) protocols that provide larger secret key rates for large N. A security study is also presented for this autocompensating quantum cryptography system, where it is found that the secure key rate increases with respect to nonautocompensated links for decoy-state QKD in the case of collective attacks. This plug-and-play system can be used in both optical fiber and free-space communication systems.