27 research outputs found
Developments towards practical free-space quantum cryptography
We describe a free space quantum cryptography system which is designed to allow continuous unattended key exchanges for periods of several days, and over ranges of a few kilometres. The system uses a four-laser faint-pulse transmission system running at a pulse rate of 10MHz to generate the required four alternative polarization states. The receiver module similarly automatically selects a measurement basis and performs polarization measurements with four avalanche photodiodes. The controlling software can implement the full key exchange including sifting, error correction, and privacy amplification required to generate a secure key
Parametric fluorescence in periodically poled silica fibres
We report the observation of quasiphase matched parametric fluorescence from a periodically poled silica fiber. A pair-photon production rate of more than 100 MHz around 1532 nm was achieved in second-order nonlinear gratings for 300 mW of pump power at 766 nm. These results are very promising for the realization of reliable all-fiber single-photon sources for quantum cryptography systems and metrology applications
Conditional generation of sub-Poissonian light from two-mode squeezed vacuum via balanced homodyne detection on idler mode
A simple scheme for conditional generation of nonclassical light with
sub-Poissonian photon-number statistics is proposed. The method utilizes
entanglement of signal and idler modes in two-mode squeezed vacuum state
generated in optical parametric amplifier. A quadrature component of the idler
mode is measured in balanced homodyne detector and only those experimental runs
where the absolute value of the measured quadrature is higher than certain
threshold are accepted. If the threshold is large enough then the conditional
output state of signal mode exhibits reduction of photon-number fluctuations
below the coherent-state level.Comment: 7 pages, 6 figures, REVTe
Quantum and Classical Noise in Practical Quantum Cryptography Systems based on polarization-entangled photons
Quantum-cryptography key distribution (QCKD) experiments have been recently
reported using polarization-entangled photons. However, in any practical
realization, quantum systems suffer from either unwanted or induced
interactions with the environment and the quantum measurement system, showing
up as quantum and, ultimately, statistical noise. In this paper, we investigate
how ideal polarization entanglement in spontaneous parametric downconversion
(SPDC) suffers quantum noise in its practical implementation as a secure
quantum system, yielding errors in the transmitted bit sequence. Because all
SPDC-based QCKD schemes rely on the measurement of coincidence to assert the
bit transmission between the two parties, we bundle up the overall quantum and
statistical noise in an exhaustive model to calculate the accidental
coincidences. This model predicts the quantum-bit error rate and the sifted key
and allows comparisons between different security criteria of the hitherto
proposed QCKD protocols, resulting in an objective assessment of performances
and advantages of different systems.Comment: Rev Tex Style, 2 columns, 7 figures, (a modified version will appear
on PRA