Quantum and Classical Noise in Practical Quantum Cryptography Systems based on polarization-entangled photons

摘要

Quantum-cryptography key distribution (QCKD) experiments have been recentlyreported using polarization-entangled photons. However, in any practicalrealization, quantum systems suffer from either unwanted or inducedinteractions with the environment and the quantum measurement system, showingup as quantum and, ultimately, statistical noise. In this paper, we investigatehow ideal polarization entanglement in spontaneous parametric downconversion(SPDC) suffers quantum noise in its practical implementation as a securequantum system, yielding errors in the transmitted bit sequence. Because allSPDC-based QCKD schemes rely on the measurement of coincidence to assert thebit transmission between the two parties, we bundle up the overall quantum andstatistical noise in an exhaustive model to calculate the accidentalcoincidences. This model predicts the quantum-bit error rate and the sifted keyand allows comparisons between different security criteria of the hithertoproposed QCKD protocols, resulting in an objective assessment of performancesand advantages of different systems.