Security against eavesdropping in quantum cryptography

In this article we deal with the security of the BB84 quantum cryptography protocol over noisy channels using generalized privacy amplification. For this we estimate the fraction of bits needed to be discarded during the privacy amplification step. This estimate is given for two scenarios, both of which assume the eavesdropper to access each of the signals independently and take error correction into account. One scenario does not allow a delay of the eavesdropper's measurement of a measurement probe until he receives additional classical information. In this scenario we achieve a sharp bound. The other scenario allows a measurement delay, so that the general attack of an eavesdropper on individual signals is covered. This bound is not sharp but allows a practical implementation of the protocol.Comment: 11 pages including 3 figures, contains new results not contained in my Phys. Rev. A pape

Security against individual attacks for realistic quantum key distribution

I prove the security of quantum key distribution against individual attacks for realistic signals sources, including weak coherent pulses and downconversion sources. The proof applies to the BB84 protocol with the standard detection scheme (no strong reference pulse). I obtain a formula for the secure bit rate per time slot of an experimental setup which can be used to optimize the performance of existing schemes for the considered scenario.Comment: 10 pages, 4 figure

Quantum Cryptography with Coherent States

The safety of a quantum key distribution system relies on the fact that any eavesdropping attempt on the quantum channel creates errors in the transmission. For a given error rate, the amount of information that may have leaked to the eavesdropper depends on both the particular system and the eavesdropping strategy. In this work, we discuss quantum cryptographic protocols based on the transmission of weak coherent states and present a new system, based on a symbiosis of two existing ones, and for which the information available to the eavesdropper is significantly reduced. This system is therefore safer than the two previous ones. We also suggest a possible experimental implementation.Comment: 20 pp. Revtex, Figures available from the authors upon request, To be published in PRA (March 95

Design of a speed meter interferometer proof-of-principle experiment

The second generation of large scale interferometric gravitational wave detectors will be limited by quantum noise over a wide frequency range in their detection band. Further sensitivity improvements for future upgrades or new detectors beyond the second generation motivate the development of measurement schemes to mitigate the impact of quantum noise in these instruments. Two strands of development are being pursued to reach this goal, focusing both on modifications of the well-established Michelson detector configuration and development of different detector topologies. In this paper, we present the design of the world's first Sagnac speed meter interferometer which is currently being constructed at the University of Glasgow. With this proof-of-principle experiment we aim to demonstrate the theoretically predicted lower quantum noise in a Sagnac interferometer compared to an equivalent Michelson interferometer, to qualify Sagnac speed meters for further research towards an implementation in a future generation large scale gravitational wave detector, such as the planned Einstein Telescope observatory.Comment: Revised version: 16 pages, 6 figure

Entanglement of coherent states and decoherence

A possibility to produce entangled superpositions of strong coherent states is discussed. A recent proposal by Howell and Yazell [Phys. Rev. A 62, 012102 (2000)] of a device which entangles two strong coherent coherent states is critically examined. A serious flaw in their design is found. New modified scheme is proposed and it is shown that it really can generate non-classical states that can violate Bell inequality. Moreover, a profound analysis of the effect of losses and decoherence on the degree of entanglement is accomplished. It reveals the high sensitivity of the device to any disturbances and the fragility of generated states

Conditional beam splitting attack on quantum key distribution

We present a novel attack on quantum key distribution based on the idea of adaptive absorption [calsam01]. The conditional beam splitting attack is shown to be much more efficient than the conventional beam spitting attack, achieving a performance similar to the, powerful but currently unfeasible, photon number splitting attack. The implementation of the conditional beam splitting attack, based solely on linear optical elements, is well within reach of current technology.Comment: Submitted to Phys. Rev.

Local-Oscillator Noise Coupling in Balanced Homodyne Readout for Advanced Gravitational Wave Detectors

The second generation of interferometric gravitational wave detectors are quickly approaching their design sensitivity. For the first time these detectors will become limited by quantum back-action noise. Several back-action evasion techniques have been proposed to further increase the detector sensitivity. Since most proposals rely on a flexible readout of the full amplitude- and phase-quadrature space of the output light field, balanced homodyne detection is generally expected to replace the currently used DC readout. Up to now, little investigation has been undertaken into how balanced homodyne detection can be successfully transferred from its ubiquitous application in table-top quantum optics experiments to large-scale interferometers with suspended optics. Here we derive implementation requirements with respect to local oscillator noise couplings and highlight potential issues with the example of the Glasgow Sagnac Speed Meter experiment, as well as for a future upgrade to the Advanced LIGO detectors.Comment: 7 pages, 5 figure

Coherent pulse implementations of quantum cryptography protocols resistant to photon number splitting attacks

A new class of quantum cryptography (QC) protocols that are robust against the most general photon number splitting attacks in a weak coherent pulse implementation has been recently proposed. In this article we give a quite exhaustive analysis of several eavesdropping attacks on these schemes. The eavesdropper (Eve) is supposed to have unlimited technological power while the honest parties (Alice and Bob) use present day technology, in particular an attenuated laser as an approximation of a single-photon source. They exploit the nonorthogonality of quantum states for decreasing the information accessible to Eve in the multi-photon pulses accidentally produced by the imperfect source. An implementation of some of these protocols using present day technology allow for a secure key distribution up to distances of $\sim$ 150 km. We also show that strong-pulse implementations, where a strong pulse is included as a reference, allow for key distribution robust against photon number splitting attacks.Comment: 16 pages, 11 figure

Optimum detection for extracting maximum information from symmetric qubit sets

We demonstrate a class of optimum detection strategies for extracting the maximum information from sets of equiprobable real symmetric qubit states of a single photon. These optimum strategies have been predicted by Sasaki et al. [Phys. Rev. A{\bf 59}, 3325 (1999)]. The peculiar aspect is that the detections with at least three outputs suffice for optimum extraction of information regardless of the number of signal elements. The cases of ternary (or trine), quinary, and septenary polarization signals are studied where a standard von Neumann detection (a projection onto a binary orthogonal basis) fails to access the maximum information. Our experiments demonstrate that it is possible with present technologies to attain about 96% of the theoretical limit.Comment: 10 pages, 11 figures, to be submitted to Phys. Rev. A Converted to REVTeX4 format, and a few other minor modifications according to the comments from PRA referre

Theoretical efficient high capacity Quantum Key Distribution Scheme

A theoretical quantum key distribution scheme using EPR pairs is presented. This scheme is efficient in that it uses all EPR pairs in distributing the key except those chosen for checking eavesdroppers. The high capacity is achieved because each EPR pair carries 2 bits of key code.Comment: 3 pages and 1 figure, to appear in Physical Review