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.
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.
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.
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.
Quantum Key Distribution (QKD) employs the laws of quantum mechanics for the purpose of cryptography. Two parties, commonly called Alice and Bob, are able to share a random key which is used to encrypt a message. Any eavesdropper trying to intercept their key will have to make measurements, thereby disturbing the system. This can be detected by Alice and Bob and they will then discard their key. Polarisation encoded QKD protocols use the polarisation of single photons as qubits to generate a cryptographic key. This can be implemented using a fibre optic link between Alice and Bob but the polarisation of light is altered when passed through a fibre due to birefringence caused by asymmetries in the fibre. This causes refractive differences for orthogonal components of the state of polarisation of light, so the polarisation is rotated as the photon is transmitted through the fibre. If the fibre is fixed, the change of polarisation will be unique and constant. This can be compensated by rotating each photon appropriately to its original state. Under typical environmental conditions, such as temperature changes and vibrations, the birefringence effects vary and should be compensated in real time. Therefore, an active polarisation controller is needed in order to maintain the state of polarisation of each qubit. An investigation was done to first track how the state of polarisation changes over time in a natural environment. Both wavelength-division multiplexing and time-division multiplexing were investigated as testing methods for the compensation system. A timedivision multiplexed system was developed to compensate the changes in polarisation. Since QKD protocols such as BB84 and B92 utilise two non-orthogonal bases, two polarisation controllers are usually used for compensation. However, by using a search algorithm, one polarisation controller was able to isolate the plane on the Poincare sphere that passes through both bases, thus compensating non-orthogonal states with one device.
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
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.