John Rarity

QUANTUM RANDOM-NUMBER GENERATION AND KEY SHARING

Abstract We review the status of interferometry-based quantum cryptography and compare photon-pair and faint-pulse schemes. The key technical limitations in both cases are the propagation losses and detector performance. We also discuss a simple approach to generating the random measurement bases used in quantum cryptography systems which exploits random partition at a beam splitter. This removes the need for active components in the receiver, reducing system complexity and losses.

Photon statistics of pulsed parametric light

Abstract We calculate and measure the photon statistics of weak light pulses emitted in the process of pulsed parametric down-conversion. We find photon bunching as expected from a chaotic light source when viewed through a narrow-band filter.

Photon counting with passively quenched germanium avalanche

We demonstrate photon counting in germanium avalanche photodiodes biased beyond breakdown and quenched with a simple series resistance circuit. The devices show moderate (> 7%) quantum efficiency with limited afterpulsing and dark counts and subnanosecond jitter.

Secure free-space key exchange to 1.9 km and beyond

Abstract We have designed and built a free space quantum cryptography system. It operates using weak laser pulses with polarization modulation by acousto-optic switching. The system also incorporates full key sifting and error correction between computers connected by a serial port. We have used this system to exchange keys over ranges up to 1.9km with absolute security. The system is robust against transmission (and diffraction) losses up to 20 dB. Building from this initial result we analyse the feasibility of exchanging keys to a low earth orbit satellite.

Bragg scattering from an obliquely illuminated photonic crystal fiber

Scattering at visible frequencies from a two-dimensional silica/air photonic crystal material in the form of a fine fiber reveals the hexagonal crystal structure of the material. Oblique illumination allows the observation of first-order Bragg conditions even for a crystal structure with a pitch several times the wavelength of light. These scattering measurements demonstrate the feasibility of a low-loss waveguide based on photonic bandgap effects.

Quantum correlated twin beams

We discuss recent progress in the study of the non-classical properties of light beams generated by non-degenerate parametric splitting in χ(2) nonlinear birefringent crystals, with special emphasis on their quantum correlation (“twin beams”). We describe experimental results using successively pure parametric fluorescence, parametric amplification of a weak signal beam pumped by a pulsed laser, and parametric oscillation in a cavity pumped by a cw laser. In this review, we compare the respective advantages and drawbacks of the different approaches.

Characterization of novel active area silicon avalanche photodiodes operating in the Geiger mode

We present a method for characterizing avalanche photodiode (APD) photon-counting detector efficiency as a function of active area. Various shallow-junction silicon APDs having a novel active area were manufactured and tested. We show that cylindrical and checkquerboard-shaped active areas have dark counts two orders of magnitude lower than standard circular devices with an equivalent active area. A parallel implementation of small active areas creates gettering sites for defects to migrate to, which is believed to create relatively defect-free active areas as the perimeter-to-area ratio is increased. However, a compromise between a large perimeter-to-area ratio and a structure useful for practical applications must be considered to optimize the detector.

NUMBER FLUCTUATIONS OF POLYDISPERSE SCATTERERS SINGLE-INTERVAL STATISTICS AND EXPERIMENTAL ERRORS

The intensity statistics of incoherently detected light scattered from fluctuating numbers of polydisperse scatterers undergoing brownian motion are discussed. General formulae are developed for the normalized moments of the intensity distribution and normalized intensity correlation functions. A relationship between the variance of any finite duration measurement of a normalized moment and the higher-order correlation functions is also obtained. This error is evaluated up to the fourth normalized moment. Theoretical moments and errors are then compared with measurements on polydisperse polystyrene latex samples. Although agreement between theory and experiment is good, it is clear that any measurement of polydispersity by this technique can be only relative.

Quantum cryptography to satellites for global secure key distribution

We have designed and built a free space secure key exchange system using weak laser pulses with polarisation modulation by acousto-optic switching. We have used this system to exchange keys over a 1.2km ground range with absolute security. Building from this initial result we analyse the feasibility of exchanging keys to a low earth orbit satellite.